acorn6 tut 080101 - 國立臺灣大學homepage.ntu.edu.tw/~choying/acorn_tut.pdf · 2017-02-16 ·...

ACORN6 Tutorial

© 2004-08 ImSpec LLC. All rights reserved. http://www.imspec.com

ACORN is an ImSpec LLC product with MODTRAN licensed technology. [email protected] ENVI is a registered trademark of Research Systems Inc. Fax: 206-666-3098 Version: 080101

2

Contents Chapter 1: ACORN Tutorial Overview……………………………….…. 4 Chapter 2: Introduction to Atmospheric Correction……………….... 7 Chapter 3: Mode 1. Radiative transfer atmospheric correction of calibrated hyperspectral data……………………………... 18 Chapter 4: Mode 1pb. Mode 1 for pushbroom imaging spectrometers with cross-track spectral calibration variation……………………………………...….. 34 Chapter 5 Mode 1.5. Radiative transfer atmospheric correction of calibrated hyperspectral data with water vapor and liquid water spectral fitting………………………………… 50 Chapter 6 Mode 1.5pb. Mode 1.5 Radiative transfer atmospheric correction of calibrated hyperspectral….. 62 Chapter 7 Mode 2. Single spectrum enhancement of a hyperspectral atmospheric correction…………………… 75

3

Chapter 8 Mode 3. Atmospheric correction using the empirical line method for hyperspectral data……….….. 84 Chapter 9 Mode 4. Convolution of hyperspectral data to multispectral data……………………………………...…….. 94 Chapter 10 Mode 5. Radiative transfer atmospheric correction of calibrated multispectral data………………………...…. 101 Chapter 11 Mode 5.5. Radiative transfer atmospheric correction of calibrated multispectral data with water vapor image………………………………………...…. 114 Chapter 12 Mode 6. Single spectrum enhancement of a multispectral atmospheric correction…………….……… 125 Chapter 13 Mode 7. Atmospheric correction by the empirical line method for multispectral data………...…. 134

4

Chapter 1:

ACORN Tutorial Overview The following topics are covered in this chapter:

Introduction………………………………………………………….…………………………...… 5

Organization………………………………………………………………………………………... 6

Input Image Data Format……………………………………………………...…………………… 6

5

Introduction

This tutorial is designed to introduce you to basic concept of atmospheric correction and the full capabilities of

the Atmospheric CORrection Now (ACORN) software. Data sets acquired by the Airborne Visible/Infrared

Imaging Spectrometer (AVIRIS) from Cuprite, Nevada, and Jasper Ridge, California are used to explore the

modes of atmospheric correction offered by ACORN. These example data sets are provided with the ACORN

software as well as an example data set acquired by the HYMAP sensor over Cuprite Nevada. In addition,

portions of two data sets acquired by the Hyperion pushbroom satellite imaging spectrometer are provided to

demonstrate the pushbroom atmospheric correction capabilities of ACORN. All the files necessary to run these

tutorial examples are provided with the ACORN software. The tutorial uses the ENVI software for display and

extraction of spectra. If other image processing software is used, similar display and data extraction capabilities

will be required.

The Atmospheric CORrection Now (ACORN) software has been developed to offer a range of atmospheric

correction capabilities for both hyperspectral and multispectral data sets:

Mode 1 Radiative transfer atmospheric correction of calibrated hyperspectral data.

Mode 1pb Mode 1 for pushbroom imaging spectrometer with cross-track spectral calibration variation.

Mode 1.5 Radiative transfer atmospheric correction of calibrated hyperspectral data with water vapor and

liquid water spectral fitting

Mode 1.5pb Mode 1.5 for pushbroom imaging spectrometer with cross-track spectral calibration variation.

Mode 2 Single spectrum enhancement of a hyperspectral atmospheric correction.

Mode 3 Atmospheric correction using the empirical line method for hyperspectral data.

Mode 4 Convolution of hyperspectral data to multispectral data.

Mode 5 Radiative transfer atmospheric correction of calibrated multispectral data.

Mode 5.5 Radiative transfer atmospheric correction of calibrated multispectral data with independently

know water vapor image for spatially varying water vapor correction

Mode 6 Single spectrum enhancement of a multispectral atmospheric correction.

Mode 7 Atmospheric correction by the empirical line method for multispectral data.

This tutorial goes through step-by-step examples of each of the ACORN modes of atmospheric correction.

Because the different ACORN modes require some of the same input parameters and input files there is

considerable overlap in aspects of the descriptions of the different modes. Each chapter is designed to be

complete. When working through this tutorial, it may be useful to refer to the ACORN Users Manuel which

contains complementary information.

Note: ACORN is not an image processing software package. File editing, image processing, and image

viewing capabilities are required to create ACORN support files and to view and assess ACORN results.

For this tutorial the ENVI software as well as standard text editing and spreadsheet software is used.

6

Organization This ACORN tutorial includes a brief introductory chapter and then a chapter presenting an overview of some of

the factors involved in atmospheric correction. The remaining chapters give step-by-step tutorial examples of

each of the ACORN modes.

Each of the ACORN mode tutorial chapters is designed to stand alone and not require review of other chapters to

work through the example. This leads to overlap of information and descriptions where ACORN modes require

similar files and parameters.

Input Image Data Format The input image data for ACORN must be stored as 16 bit integers. The conversion factors between the stored

integer value and the corresponding calibrated image radiance value must be known. The ACORN user is

responsible to obtain the image format data and calibration data from the image data provider.

7

Chapter 2:

Introduction to Atmospheric Correction

The following topics are covered in this chapter:

Radiance and Reflectance…………………………….……………………………………….…… 8

Components of the Measured Radiance………………………………………………...………… 10

Atmospheric Correction Options and Methodology………………………………….……….…… 13

Multispectral and Hyperspectral Measurements…...………………………………..…..…….…… 14

8

Radiance and Reflectance All airborne and spaceborne remote sensing instruments measure the upwelling radiance that arrives at the sensor.

The upwelling radiance results from the solar irradiance, the two-way transmittance of the atmosphere, the

scattering of the atmosphere, and the reflectance of the surface. Figure 2-1 shows a radiance image of the Cuprite,

Nevada data set. Figure 2-2 shows the measured radiance spectrum extracted from Stonewall Playa at the right

central edge of the image.

Figure 2-1. Image of Cuprite, Nevada acquired by the AVIRIS hyperspectral sensor. Wavelengths 655.8, 1483.3,

and 2268.4 nm are displayed as blue, green, and red respectively.

9

0

50

100

150

200

250

300

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Rad

iance

(W

/m^2

/um

/sr)

Stonewall Playa

Figure 2-2. Spectrum extracted from Stonewall playa located on the right central edge of the Cuprite, Nevada

radiance image.

This radiance spectrum is dominated by effects of the solar spectrum, the scattering of the atmosphere, and the

two-way transmittance of the atmosphere. The effect of the surface reflectance is not apparent in the radiance

spectrum. Figure 2-3 shows the surface reflectance of stonewall playa.

For the purposes of this tutorial and ACORN software, reflectance is a unitless parameter corresponding to the

ratio of the upwelling radiance from a surface to the expected upwelling radiance from a perfectly reflecting

perfectly Lambertian surface.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Ref

lect

ance

.

Stonewall Playa

Figure 2-3. Reflectance spectrum from Stonewall Playa. The x-axis is in units of wavelength in nanometers. The

y-axis is in units of reflectance.

10

The shape of the surface reflectance is related to the molecular absorption and scattering composition of the

surface material. To pursue surface related research and/or applications with remote sensing data, atmospheric

correction is a critical step. This is true for both hyperspectral and multispectral remote sensing measurements.

ACORN is designed to provide this atmospheric correction capability.

Components of the Measured Radiance The radiance measured by a remote sensing instrument begins with the energy from the sun. Figure 2-4 shows a

spectrum of the solar irradiance arriving at the top of the atmosphere. This spectrum has the approximate overall

form of a Planck function at 6000 degrees K temperature. Superimposed on this general form are the effects of

the solar atmosphere and range of temperatures in the upper zones of the sun.

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

1400.0

1600.0

1800.0

2000.0

2200.0

400.0 700.0 1000.0 1300.0 1600.0 1900.0 2200.0 2500.0

Wavelength (nm)

Irra

dia

nce

(W

/m^2/u

m)

Top of the AtmosphereIrradiance

Figure 2-4 Spectrum of the exo-atmospheric solar irradiance

In the passage from the top of the atmosphere to the surface and then to the remote sensing instrument energy is

absorbed by the molecules and particles of the atmosphere. Figure 2-5 shows a transmittance spectrum of the

atmosphere for a path from the top of the atmosphere to the surface and up to a remote sensing instrument.

11

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

400.0 700.0 1000.0 1300.0 1600.0 1900.0 2200.0 2500.0

Wavelength (nm)

Tra

nsm

itta

nce

Transmittance

Figure 2-5. Example transmittance spectrum of the atmosphere. The attenuation is due to the absorption and

scattering of energy by the molecules and particles of the atmosphere.

The dominant absorber in the atmosphere is water vapor. Figure 2-6 shows a series of transmittance spectra for

different amounts of water vapor in the atmosphere. Only the transmittance due to water vapor is shown. The

amount of water vapor is given in units of precipitable millimeters of water vapor. This corresponds to the depth

of the layer of liquid water if the water vapor in the atmosphere was completely condensed to liquid.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

400.0 700.0 1000.0 1300.0 1600.0 1900.0 2200.0 2500.0

Wavelength (nm)

Tra

nsm

itta

nce

.

1 mm Water Vapor

5 mm Water Vapor

10 mm Water Vapor

25 mm Water Vapor

50 mm Water Vapor

Figure 2-6. Transmittance of the atmosphere with different amounts of water vapor in units of precipitable

millimeters.

The transmittance effects of the other components of the atmosphere for standard atmospheric conditions are

shown in Figure 2-7. These include molecular absorption due to oxygen, carbon dioxide, ozone, and methane as

well as scattering effects of the molecules and aerosol particles in the atmosphere.

12

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

400.0 700.0 1000.0 1300.0 1600.0 1900.0 2200.0 2500.0

Wavelength (nm)

Tra

nsm

itta

nce

Transmittance

Scattering

O3

O2

O2CO2 CO2

CO2

CH4O2

Figure 2-7. Transmittance of the atmosphere due to constituents other than water vapor.

In addition to the atmospheric transmittance effects on the radiance measured by a remote sensing instrument,

some energy is scattered directly from the atmosphere towards the sensor. This is generally referred to as

atmospheric path radiance or path radiance. Figure 2-8 shows an example spectrum of path radiance. The

aerosol, molecular, and total path radiance are shown. The total path radiance is the radiance that would be

measured by a remote sensing instrument if the surface reflectance was zero.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

400.0 700.0 1000.0 1300.0 1600.0 1900.0 2200.0 2500.0

Wavelength (nm)

Rad

iance

(W

/m^2

/um

/sr)

Aerosol Scattered

Molecular Scattered

Path Radiance

Figure 2-8. Example spectrum of the atmospheric path radiance scattered in the direction of a remote sensing

instrument.

13

The total radiance measured by a remote sensing instrument includes both the path radiance and the surface

reflected radiance. The surface reflected radiance is the solar energy that has been reflected by the surface in the

direction of the sensor. The reflected radiance plus the path radiance is the total radiance measured by the remote

sensing instrument. Figure 2-9 shows the total radiance, path radiance, and reflected radiance for a 0.25 constant

reflectance surface.

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

400.0 700.0 1000.0 1300.0 1600.0 1900.0 2200.0 2500.0

Wavelength (nm)

Rad

iance

(W

/m^2/u

m/s

r) Path Radiance

Surface Reflected

Total

Figure 2-9. The total, reflected, and path radiance for a 0.25 constant reflectance surface.

Atmospheric Correction Options and Methodology ACORN offers a range of strategies for atmospheric correction. These include both empirical and radiative

transfer code based methods for atmospheric correction of both hyperspectral and multispectral data sets.

ACORN also offers several artifact suppression options as well as single spectrum enhancement options to

improve atmospheric correction results. The conversion of hyperspectral data to multispectral data for

investigation of multispectral solutions to remote sensing problems is also one of the capabilities of ACORN.

The most advanced form of atmospheric correction offered in ACORN is radiative transfer based. Radiative

transfer atmospheric correction of calibrated data uses both the calibrated data and additional specified parameters

to derive and model the absorption and scattering characteristics of the atmosphere. These modeled atmospheric

characteristics are then used to invert the radiance to apparent surface reflectance. This approach for atmospheric

correction is based in the following equations that can be found or derived from the information in the text

Radiative Transfer by Chandrasekhar (December 1960, Dover, ISBN: 0486605906). In simplified terms, Equation

2-1 gives the relationship between contributions between the solar source, atmosphere, and surface to the radiance

measured by an earth-looking sensor for a homogeneous plane parallel atmosphere.

Lt(λ) = F0(λ){ρa(λ) + Td(λ)ρ(λ)Tu(λ)/(1-s(λ)ρ(λ))}/π Equation 2-1

In this equation, Lt is the total radiance arriving at the sensor. F0 is the top of the atmospheric solar irradiance. ρa

is the reflectance of the atmosphere. Td is the downward transmittance of the atmosphere. ρ is the spectral

reflectance of the surface. Tu is the upward transmittance of the atmosphere. s may be thought of as the

downward reflectance of the atmosphere. λ is spectral wavelength. The solution of this equation for apparent

surface reflectance is given in Equation 2-2.

14

ρ(λ) = 1/[{F0(λ)Td(λ)Tu(λ)/π)/(Lt(λ)-F0(λ)ρa(λ)/π)}+s(λ)] Equation 2-2

These and other mathematical equations are used in ACORN for each of the radiative transfer based atmospheric

correction modes.

Multispectral and Hyperspectral Measurements

The two dominant types of data measured by remote sensing instruments today are multispectral and

hyperspectral. Multispectral instruments and measurement were developed first. Hyperspectral instruments were

conceived and developed only in the 1980s. Figure 2-10 shows the spectral response functions for the Landsat

multispectral sensor. Landsat has 6 multispectral bands in the solar reflected portion of the electromagnetic

spectrum. These response functions describe how the upwelling radiance arriving at Landsat is partitioned into

each of the six solar reflected spectrum multispectral band. Figure 2-11 shows an image of the Jasper Ridge,

California in the southern San Francisco Bay area. Figure 2-12 shows an example of upwelling radiance

measured through the Landsat spectral response functions for several surface targets. Figure 2-13 shows the

corresponding reflectance for these targets after ACORN atmospheric reflectance.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

No

rmal

ized

Res

po

nse

Response

Figure 2-10. Spectral response functions of the Landsat remote sensing instrument.

15

Figure 2-11. Image of Jasper Ridge, California.

0

20

40

60

80

100

120

140

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Rad

ian

ce (

W/m

^2

/um

/sr)

Target 1

Target 2

Target 3

Target 4

Figure 2-12. Multispectral radiance measurements from Jasper Ridge through the Landsat spectral response

functions.

16

0

2000

4000

6000

8000

10000

12000

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Ref

lect

ance

/10

000

.

Target 1

Target 2

Target 3

Target 4

Figure 2-13. Multispectral derived reflectance from Jasper Ridge after atmospheric correction with ACORN

To compare and contrast multispectral and hyperspectral measurement types the corresponding plots for the same

data set are given at the full spectral resolution of the AVIRIS hyperspectral instrument. Figure 2-14 shows the

spectral response function for the AVIRIS sensors from 400 to 1000 nm. The full range, 400 to 2500 nm, is not

shown because the plot becomes too compressed. Figure 2-15 shows the upwelling radiance spectra for the

targets in the Jasper Ridge, California data set. Figure 2-16 shows the corresponding reflectance spectra for these

targets following atmospheric correction with ACORN. The hyperspectral data show the full spectral signatures

across the wavelength range measured.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

400 500 600 700 800 900 1000

Wavelength (nm)

Norm

aliz

ed R

esp

on

se

Response

Figure 2-13. Spectral response functions for the AVIRIS hyperspectral remote sensing instrument over the range

from 400 to 1000 nm. The full range is not shown due to plot compression.

17

0

20

40

60

80

100

120

140

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Rad

iance

(W

/m^2

/um

/sr)

Target 1

Target 2

Target 3

Target 4

Figure 2-14. Measured hyperspectral radiance spectra from the Jasper Ridge, California AVIRIS data set.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Ref

lect

ance/

10

00

0.

Dry Grassland

Healthy grassland

Soil

Forest

Figure 2-15. Hyperspectral derived reflectance spectra after use of ACORN atmospheric correction.

18

Chapter 3 Mode 1. Radiative transfer atmospheric correction of calibrated hyperspectral data. The following topics are covered in this chapter:

Description…………………………….…………………………………….……………………... 19

Input Image Data………………...…………………………………….……………………………19

Starting ACORN………………………………………………………………………….……..…. 21

Selecting the Mode of Atmospheric Correction…………………………………………………… 22

Reviewing and Editing the Control File……………………………………….………….……..… 23

Running ACORN Mode 1…………………………………………………………………...………………. 27

ACORN Mode 1 Results…...……………………………………………………………………… 27

Additional Output Files…...…………………………………………………...……..…….……… 28

Artifact Suppression Type 1…...……………………………………………………..…….……… 29

Artifact Suppression Type 1 and 2…...……………………………….……………..………...……30

Artifact Suppression Type 1, 2, and 3…...…………………………………………..…….……..…31

Visibility Estimation…...………………………………………………………….…..…….…...… 31

Other Tutorial Examples…...…………………………………………..……………..…….……… 33

19

Description

ACORN mode 1 uses radiative transfer calculations and the measured, calibrated hyperspectral data to deduce a

subset of the atmospheric effects present in the hyperspectral data set. These derived atmospheric properties are

used in conjunction with modeled atmospheric properties to correct for the atmosphere in the hyperspectral data

set. With an input of calibrated hyperspectral radiance data, ACORN produces an output of apparent surface

reflectance.

Note: The hyperspectral data must be spectrally and radiometrically calibrated and stored as 16 bit

integers to use ACORN.

The ACORN user controls the strategy for water vapor estimation, artifact suppression and visibility constraint

and estimation.

The quality of the ACORN atmospheric correction is closely tied to the quality of the calibration of the image

data. For all of the modes of ACORN, the spectral and radiometric calibration of the data must be accurate. Partial

exceptions to this rule exist and are indicated in the description of each mode in the ACORN User’s Manual.

At present, perfect calibration and perfect knowledge of the atmosphere are not achievable. Some artifacts will be

present in every atmospheric correction. The strength of the artifacts will be related to the quality of the

calibration, the knowledge of the atmosphere, and the ability to model the atmosphere. Several options and modes

are offered in ACORN to help suppress artifacts in the atmospheric correction.

Input Image Data

For this example of ACORN mode 1 the AVIRIS data set acquired over Jasper Ridge, California is used. Begin

by examining the Jasper Ridge calibrated radiance data set provided with the ACORN software.

1. Start ENVI software on your computer

2. With ENVI, open the Jasper Ridge image data provide with ACORN. The default installed location is

c:\program files\ACORN6\Data\Jasper\jsp0rdn.

3. Select bands 30, 40, 20 to display as red, green, blue. This is done with in the Available Bands List window by

clicking the RGB Color option and then scrolling and clicking on band 30, 40 and 20 in that order. Then click the

Load Band button. The resulting image is shown in Figure 3-1. Figure 3-2 shows a set of extracted spectra from

the Jasper Ridge data. Table 3-1 gives the site, and location of these extracted spectra.

20

Figure 3-1. Jasper Ridge radiance image bands 30, 40 and 20 as red, green and blue.

4. Extract and examine the radiance spectra with ENVI Z profiles.

a) This is done in the ENVI image window by selecting Functions>Profiles>Z Profiles.

b) To move the cursor to a specific location, from the ENVI image window select Functions>Interactive

Analysis>Pixel Locator

c) In the pixel locator window, enter the X and Y location and click Apply.

d) To extract and save spectra in the z profile window select Option> New Window:Blank.

e) In the Z profile window click the right mouse button to show the X, Y location of the displayed spectrum.

f) To save a spectrum to the blank window drag X,Y location label to the new window.

g) Repeat this until you have all the Z profiles in the new window that you wish.

h) These extracted spectra may be saved to an ASCII file in the plot window by selecting File>Output

Data>ASCII.

i) To copy the plot window, click in the plot window, then press (<alt>+<prnt scrn>) then paste the window to

the destination document.

Table 3-1. Locations of Extracted Spectra

Site Sample Line

Stanford Golf Course 504 226

Jasper Ridge Grassland 314 159

Plowed Field 402 242

Redwood Forest 102 250

Felt Lake 398 355

21

Figure 3-2. Extracted spectra from Jasper Ridge radiance image.

These image radiance spectra contain the effects of the solar illumination, two-way transmittance and scattering of

the atmosphere, as well as reflectance of the surface. These image extracted spectra are also scaled so that the

radiance values fall in a range of efficiently stored integers. ACORN mode 1 is designed to use input and derived

parameter to assess and correct for these atmospheric effects.

Starting ACORN

In this section ACORN Mode 1 will be used to atmospherically correct the Jasper Ridge calibrated radiance data

set.

1. Start ACORN by selecting Start>Programs>ACORN6>ACORN6.

The ACORN user interface will appear as shown in Figure 3-3.

22

Figure 3-3. ACORN user software interface

Selecting the Mode of Atmospheric Correction

1. Select Mode 1

The Mode 1 Control file editor will be displayed as shown in Figure 3-4.

23

Figure 3-4. ACORN Mode 1 control file parameter entry window.

Reviewing and Editing the Control File

1. Select Open

2. Select the Jasper Ridge mode 1 control file. The default location for this file is

C:\Program Files\ACORN6\Data\Jasper\jsp1.in.

3. Examine the completed ACORN Mode 1 control file parameter entry panel shown in Figure 3-5. These are

the files and parameters necessary to perform a Mode 1 atmospheric correction. Each parameter and file is

described for the Jasper Ridge tutorial example.

24

Figure 3-5. Completed ACORN Mode 1 control file parameter entry window.

Note: Except for the examples provided with ACORN you must obtain the appropriate calibration files and

data parameters from the data provider.

Input Image File Name: This is the name of the input calibrated radiance image file. For this example, the

default location is

C:\Program Files\ACORN6\Data\Jasper\jsp0rdn.

Output Reflectance Image File Name: This is the name of the output image to be generated by ACORN. For

this example, the default location is

C:\Program Files\ACORN6\Data\Jasper\jsp1rfl.

Image Spectral Calibration file: This is the spectral calibration file for the input image. The format is ASCII.

The default location is

C:\Program Files\ACORN6\Data\Jasper\avr97.wvl.

25

The spectral calibration is a two column ASCII text file. The first column is the center position of each spectral

band in nanometers. The second column is the full-width-at-half-maximum (FWHM) of the appropriate Gaussian

function for that spectral band in nanometers. Table 3-2 shows a subset of this file.

Table 3-2. Jasper Ridge spectral calibration file.

369.85 9.61

379.69 9.58

389.53 9.55

… …

2486.99 10.07

2496.90 10.05

2506.81 10.03

Gain File: This is the file that converts the image file integer values to radiance in units of (W/m^2/um/sr). The

default location for this example is

C:\Program Files\ACORN6\Data\Jasper\avr97.gain.

The gain file is a one column file that is used by ACORN to convert the 16 bit integers image file data to the

correct radiance units. Table 3-3 shows a subset of the gain file for this example.

Note: The gain file values should be provided or calculated from information provided by the data

provider.

Table 3-3. Contents of the Jasper Ridge gain file.

0.2

0.2

0.2

…

0.1

0.1

0.1

Offset File: This is a file that corrects for any offset in the image file integer values in the conversion to radiance

in units of (W/m^2/um/sr). For this example, the default location is

C:\Program Files\ACORN6\Data\Jasper\avr97.gain.

The offset file is a one column file that is added to the radiance values. Table 3-4 shows a subset of this file.

Table 3-4. Contents of the Jasper Ridge offset file.

0.0

0.0

0.0

…

0.0

0.0

0.0

Image Dimension: These are the dimension of the input image in terms of bands, lines, samples, and offset. The

offset parameter is the number of bytes used to skip any embedded image headers. The dimensions of the Jasper

Ridge data set are 224 bands, 512 lines, and 614 samples with 0 byte offset.

26

Integer Format: This parameter specifies whether the image data are stored as big endian (host, intel) or little

endian (network or IEEE). The Jasper Ridge integer format is big endian.

Input File Format: This parameter specifies whether the input image data are band-interleaved-by-line (BIL) or

band-interleave-by-pixel (BIP). The Jasper Ridge data set is BIP file format

Image Latitude: These are the degrees, minute, seconds of the image data set latitude location. Each parameter

may be integer or decimal. If decimal degrees are used the minutes and second field should be zero. North

latitude is positive. The approximate latitude of the Jasper Ridge data set is 37.18.

Image Longitude: These are the degrees, minute, seconds of the image data set longitude location. Each

parameter may be integer or decimal. If decimal degrees are used the minutes and second field should be zero.

The approximate longitude of the Jasper Ridge data set is -122.3. East longitude is positive.

Image Date: This is the date of image acquisition in day, month, year format. This example data set was

acquired on the 3rd

of April, 1997.

Image Time: This is the nominal time of acquisition in hours, minutes, and seconds. The time must be

Greenwich Mean Time (GMT). Each parameter may be integer or decimal. The example Jasper Ridge data set

was acquired at 19:57:23 GMT.

Image Surface Elevation: This is the average elevation of the surface in the input image. The nominal elevation

of the Jasper Ridge example data set is 200 m.

Image Acquisition Altitude: This is the altitude of the instrument that acquired the image data in kilometers.

The altitude of acquisition of this example Jasper Ridge data set is 20 km.

Atmospheric Model: This option selects the appropriate atmospheric model to be used for atmospheric

correction. The options are mid-latitude-summer, mid-latitude-winter, and tropical. For this example the mid-

latitude-summer atmospheric model is selected.

Derive Water Vapor: This option selects the type of water vapor derivation for ACORN Mode 1. ACORN

derives water vapor from the strength of the water vapor absorption band expressed in the radiance spectrum.

i) none

ii) 820 water vapor band

iii) 940 water vapor band

iv) 1140 water vapor band

v) 940 and 1140 water vapor band

For this example the option to use both the 940 and 1140 nm water vapor bands is selected.

Fixed Water Vapor: If water vapor is not derived from the data themselves then a fixed water vapor amount is

required in units of precipitable mm of water vapor. A value of 10 precipitable mm is entered, but this parameter

will not be used because the derive water vapor option is selected.

Image Atmospheric Visibility: This parameter controls the visibility of the atmospheric model used in the

atmospheric correction. A visibility of 60 km is entered for this example.

ACORN Estimated Visibility: If selected ACORN will attempt to estimate the visibility from the data

themselves. This option is not selected for this example.

27

Artifact Suppression: This option allows selection of artifact suppression options.

i) Artifact suppression type 1 attempts to address spectral calibration mismatches between the image data

set and the radiative transfer code model of the atmosphere.

ii) Artifact suppression type 2 attempts to address radiometric calibration mismatches between the image

data set and the radiative transfer code model of the atmosphere.

iii) Artifact suppression type 3 attempts to address artifacts due to low measured signal.

For this example no artifact suppression options are selected.

4. Click Save As to save the ACORN mode 1 parameter file.

Running ACORN Mode 1

To execute ACORN mode 1, click Save & Run on the ACORN control panel. A processing status box will

appear to indicate the progress of the atmospheric correction. When the atmospheric correction is complete the

program will report the time required for the atmospheric correction. Click Okay to return to the control file

editor.

ACORN Mode 1 Results

The primary result of ACORN Mode 1 is the atmospherically corrected image.

1. With ENVI open the Jasper Ridge reflectance image. The location specified in the tutorial control file is

c:\program files\ACORN6\Data\Jasper\jsp1rfl.

Figure 3-6 shows the reflectance image with bands 30, 40, 20 displayed as Red, Green, Blue respectively.

2. Extract the atmospherically corrected spectra from the sites in table 3-1. The extracted spectra are shown in

Figure 3-7. These spectra are stored as 16 bit integers of reflectance multiplied by 10000. This preserves the

precision of the measurement and allows the data to be stored efficiently. Using 4 byte real numbers would

require twice the disk space.

3. Compare the extracted reflectance spectra with the input radiance spectra. The effects of the solar source and

atmosphere have been compensated. Residual spectral artifacts are present at spectral regions of strong

atmospheric absorption. Suppression strategies for these artifacts are discussed further along in this chapter.

Also, the water vapor is not completely compensated over vegetation sites. This is addressed in the chapter on

ACORN mode 1.5.

28

Figure 3-6. Atmospherically corrected Jasper Ridge image following implementation of ACORN mode 1.

Figure 3-7. Extracted spectra from Jasper Ridge data set after mode 1 atmospheric correction.

Additional Output Files

In addition to the output image file there are several other files created by ACORN.

29

.in This is the control file that provides the files and parameters that are used by ACORN. The file jsp1.in was

created in the default location for this tutorial example.

.eco In the same file location of the jsp1.in file will be a jsp1.in.eco. This file is an echo of the input file as it is

being interpreted by the ACORN software. If problems are detected interpreting the control file these will be

indicated in this file.

.diag1 In the same directory as the output reflectance file will be a jsp1rfl.diag1. The diag1 file confirms the

input parameters used in the atmospheric correction and indicates problems encountered during the run.

.diag2 In the same directory as the output reflectance the jsp1rfl.diag2 file will be created. This file contains

selected spectral outputs that may be helpful to understand problems with the atmospheric correction.

Artifact Suppression Type 1

Artifact suppression type 1 attempts to address spectral calibration mismatches between the image data set and the

radiative transfer code model of the atmosphere.

For this example from the ACORN Control Panel select Open and open file jsp1a1.in. The default installation

location for this file is

c:\Program Files\ACORN6\Data\Jasper\jsp1a1.in.

Notice that in the control file editor artifact type 1 is selected.

Click Save & Run

When the run is complete open the output image file with ENVI. The default location and name for this file is

c:\Program Files\ACORN6\Data\Jasper\jsp1a1rfl.

With ENVI extract the spectra from the five sites given in Table 3-1. Figure 3-8 shows these extracted spectra.

In comparison to the spectra shown in Figure 3-7 many of the spectral artifacts have been suppressed.

30

Figure 3-8. Atmospherically corrected spectra from Jasper Ridge with ACORN Mode 1 and artifact suppression

type 1.

Artifact Suppression Type 1 and 2 Artifact suppression type 2 attempts to address radiometric calibration mismatches between the image data set and

the radiative transfer code model of the atmosphere.




Notice that in the control file editor artifact type 1 and 2 are selected.

Click Save & Run

When the run is complete open the output image file with ENVI. The location and name for this file specified in

the tutorial example control file is



In comparison to the spectra shown in Figure 3-7 and Figure 3-8 additional spectral artifacts have been

suppressed.


type 1 and 2.

31

Artifact Suppression Type 1, 2, and 3 Artifact suppression type 3 attempts to address artifacts due to low measured signal.




Notice that in the control file editor artifact type 1, 2 and 3 are selected.

Click Save & Run





In comparison to the spectra shown in Figure 3-7, Figure 3-8, and Figure 3-9 the low signal, noise dominated

portions of the spectrum have been suppressed.


type 1, 2 and 3.

Visibility Estimation

ACORN will attempt to estimate the visibility from the data themselves.

For this example from the ACORN Control Panel select Open and open file jsp1a3v.in. The default location for

this file in the tutorial is

c:\Program Files\ACORN6\Data\Jasper\jsp1a3v.in.

32

Notice that in the control file editor artifact type 1, 2, 3 and estimate visibility are selected.

Click Save & Run

When the run is complete open the output image file with ENVI. The location and name for this file in the

tutorial is

c:\Program Files\ACORN6\Data\Jasper\jsp1a3vrfl.

With ENVI extract the spectra from the five sites given in Table 3-1.

For comparison spectra are shown in Figure 3-11 where ACORN was run with a fixed visibility of 20 km. 20 km

is too hazy an atmosphere for this data set and results in negative reflectance values in the visible portion of the

spectrum. The ACORN estimated visibility spectra in Figure 3-12 give a better result.

Figure 3-11. Atmospheric correction with fixed 20 km visibility used. 20 km is excessively haze and causes

negative reflectance results in the visible portion of the spectrum.

33

Figure 3-12. Atmospheric correction with ACORN visibility estimation used.

Other Tutorial Examples Additional tutorial examples are provided with ACORN for the Cuprite Nevada AVIRIS and Hymap data. The

default location for these files is

c:\Program Files\ACORN6\Data\Cuprite\cup1.in.

and

c:\Program Files\ACORN6\Data\Cupriteh\cuph1.in.

You may run these examples and examine the input and output files to gain additional experience with ACORN

mode 1.

34

Chapter 4 Mode 1pb. Mode 1 for pushbroom imaging spectrometers with cross-track spectral calibration variation. The following topics are covered in this chapter:

Description…………………………….…………………………………………………………… 35

Input Image Data………………...…………………………………………………………….……35

Starting ACORN……………………………………………………………...………………...….. 37

Selecting the Mode of Atmospheric Correction………………………………………………...…. 38

Reviewing and Editing the Control File…………………………………….…………………...… 39

Running ACORN Mode 1pb…...……………………………..…….………………………..….… 43

ACORN Mode 1pb Results…...…………………..…………………………………….…….…… 43

Additional Output Files…...…………………………………………...………………..…….…… 45

Artifact Suppression Type 1…………………………………………………………….…………. 45

Artifact Suppression Type 1 and 2………………………………………………………………….46

Artifact Suppression Type 1, 2 and 3……………………………………………………………,…47

Visibility Estimation……………………………………………………,……………….………… 48

35

Description ACORN mode 1pb is a new mode introduced in ACORN version 5 that allows use of a spectral calibration file

that accounts for the cross track spectral variation in pushbroom imaging spectrometers such as Hyperion. Figure

4-1 shows the cross-track variation in spectral calibration for spectral channel 40 of Hyperion. Across the 256

spatial elements of Hyperion the spectral calibration varies by ~3 nm. This is a 30% variation with respect to the

10 nm full-width-at-half-maximum of the Hyperion instrument. Accounting for this cross-track spectral variation

is essential for accurate atmospheric correction of spectra measured by pushbroom imaging spectrometers.

Note: Because considerable file reformatting is required to utilize the full cross-track spectral calibration

the pushbroom Mode requires longer computation time that the non pushbroom mode.

749

750

751

752

753

754

0 32 64 96 128 160 192 224 256

Cross-Track Sample (#)

Wav

elen

gth

(n

m)

Channel 40

Figure 4-1. Cross-track spectral calibration variation of the Hyperion pushbroom imaging spectrometer.

Input Image Data

For this example of ACORN mode 1pb a Hyperion data set acquired south of Jasper Ridge that includes the

Stanford University campus in California is used. Begin by examining the Stanford calibrated radiance data set

provided with the ACORN software.


2. With ENVI open the Stanford Hyperion data provide with ACORN. The default installed location is

c:\program files\ACORN6\Data\stanford\stnhyp_rdn.

3. Select bands near wavelengths 650, 550, 450 nm to display as red, green, blue. This is done with in the

Available Bands List window by clicking the RGB Color option and then scrolling and clicking on the

corresponding bands in that order. Then click the Load Band button. This image is shown in Figure 4-2. Figure

4-3 shows a set of extracted spectra from the Stanford data. Table 4-1 gives the site, and location of these

extracted spectra.

36

Figure 4-2. Stanford radiance image bands near 650, 550, 450 nm as red, green and blue.











Data>ASCII.



37


Site Sample Line

Golf Course 58 272

Red roof 36 94

Dry Grass 39 195

Felt Lake 13 227


These image radiance spectra contain the effects of the solar illumination, two-way transmittance and scattering of


radiance values fall in a range of efficiently stored integers. ACORN mode 1pb is designed to use input and

derived parameter to assess and correct for these atmospheric effects while accounting for the variation in cross-

track spectral calibration of a pushbroom sensor.

Starting ACORN

In this section ACORN Mode 1pb will be used to atmospherically correct the Jasper Ridge calibrated radiance

data set.


The ACORN user software interface will appear as shown in Figure 4-4.

38

Figure 4-4. ACORN user software interface

Selecting the Mode of Atmospheric Correction

1. Select Mode 1pb

The Mode 1pb Control file editor will be displayed as shown in Figure 4-5.

39

Figure 4-5. ACORN mode 1pb control file editor window.


1. Select Open

2. Select the Jasper Ridge mode 1 control file. The default location for this file is

C:\Program Files\ACORN6\Data\Stanford\stnhyp1.in.

3. Examine the ACORN Mode 1pb control file parameter entry panel shown in Figure 4-6. These are the files

and parameters necessary to perform a Mode 1pb atmospheric correction. Each parameter and file is described

for the Jasper Ridge example.



40

Figure 4-6. ACORN Mode 1pb control file parameter entry window.

Input Image File Name: This is the name of the input calibrated radiance image file. For this example, the

default location is

C:\Program Files\ACORN6\Data\Stanford\stnhyp_rdn.


this example, the default location is

C:\Program Files\ACORN6\Data\Stanford\stnhyp_rfl.

Cross Track Wavelength Calibration: This is the spectral calibration file for the input image. The format is

ASCII. The default location is C:\Program Files\ACORN6\Data\Stanford\hyp_wvlpb.txt

The cross-track wavelength calibration file is a two dimensional file. Each row corresponds to the cross-track

sample (1 to 256 for Hyperion). Each column is the wavelength position for the corresponding spectral channel

(1-242 for Hyperion). Table 4-2 shows a subset of this file.

41

Table 4-2. Stanford cross-track wavelength spectral calibration file.

355.37 365.56 375.75 . . . 2557.01 2567.01 2577.11

355.4 365.59 375.77 . . . 2557.01 2567.01 2577.11

355.43 365.61 375.8 . . . 2557.02 2567.02 2577.12

. . . . . .

. . . . . .

. . . . . .

352.83 363.02 373.21 . . . 2556.6 2566.6 2576.7

352.79 362.98 373.17 . . . 2556.59 2566.59 2576.69

352.74 362.93 373.12 . . . 2556.58 2566.58 2576.68

Cross Track FWHM Calibration: This is the spectral calibration file for the input image. The format is ASCII.

The default location is C:\Program Files\ACORN6\Data\Stanford\hyp_fwhmpb.txt

The full-width-at-half-maximum file is a two dimensional file. Each row corresponds to the cross-track sample (1

to 256 for Hyperion). Each column is the full-width-at-half-maximum for the corresponding spectral channel (1

to 242 for Hyperion). Table 4-3 shows a subset of this file.

Table 4-3. Stanford cross-track FWHM spectral calibration file.

11.23 11.23 11.23 . . . 10.44 10.44 10.44

11.23 11.23 11.23 . . . 10.44 10.44 10.44

11.23 11.23 11.23 . . . 10.44 10.44 10.44

. . . . . .

. . . . . .

. . . . . .

11.25 11.25 11.25 . . . 10.19 10.19 10.19

11.25 11.25 11.25 . . . 10.19 10.19 10.19

11.25 11.25 11.25 . . . 10.19 10.19 10.19



C:\Program Files\ACORN6\Data\Stanford\hyp_gain.txt.

The gain file is a one column file that is use by ACORN to convert the image file integers to the correct radiance

units. Table 4-4 shows a subset of this file.

Note: The gain file values should be provided or calculated from information provided by the data

provider.

Table 4-4. Contents of the Stanford gain file.

0.025

0.025

0.025

…

0.0125

0.0125

0.0125

42


in units of (W/m^2/um/sr). For this example, the default location is C:\Program Files\ACORN6\Data\Stanford\hyp_offset.txt


Table 4-5. Contents of the Stanford offset file.

0.0

0.0

0.0

…

0.0

0.0

0.0





band-interleave-by-pixel (BIP). The Stanford data set is BIL file format


endian (network or IEEE). The Stanford integer format is big endian.

Image Latitude: These are the degrees, minute, seconds of the image data set. Each parameter may be integer or

decimal. If decimal degrees are used the minutes and second field should be zero. North latitude is positive. The

approximate latitude of the Stanford data set is 37.47.

Image Longitude: These are the degrees, minute, seconds of the image data set. Each parameter may be integer

or decimal. If decimal degrees are used the minutes and second field should be zero. The approximate longitude

of the Stanford data set is -122.13. East longitude is positive.


acquired on the 31st of July, 2002.




Image Elevation: This is the average elevation of the surface in the input image. The nominal elevation of the

Stanford example data set is 200 m.


The altitude of acquisition of this example Stanford data set is 700 km.




43

Derive Water Vapor: This option selects the type of water vapor derivation for ACORN Mode 1pb. ACORN


i) none


iii) 940 water vapor band

iv) 1140 water vapor band

v) 940 and 1140 water vapor band

For this example the option to use the 1140 nm water vapor bands is selected.

Note: The Hyperion signal-to-noise ratio is generally poor in the 940 nm spectral region.

Fixed Water Vapor: If water vapor is not derived from the data themselves then a fixed water vapor amount is

required in units of precipitable mm of water vapor. A value of 10 precipitable mm is entered, but this parameter

will not be used because the derive water vapor option is selected.



ACORN Estimated Visibility: If selected ACORN will attempt to estimate the visibility from the data

themselves. This option is not selected for this example.







For this example no artifact suppression options are selected.

4. Click Save As to save the ACORN mode 1pb parameter file. The parameter file may be saved as the same

name or a new name selected.

Running ACORN Mode 1pb To execute ACORN mode 1pb, click Save & Run on the ACORN control panel. A processing status box will


program will return to the ACORN Control Panel. Click Okay to return to the control file editor.

ACORN Mode 1pb Results

The primary result of ACORN Mode 1pb is the atmospherically corrected image.

1. Open the Jasper Ridge reflectance image with the available image processing software. The location specified

in the tutorial control file is

c:\program files\ACORN6\Data\Stanford\stnhyp_rfl.

44



Figure 4-8. These spectra are stored as 2 byte integers of reflectance multiplied by 10000. This preserves the

precision of the measurement and allows the data to be stored efficiently. Using 4 byte real numbers would

require twice the disk space.


atmosphere have been compensated. Residual spectral artifacts are present at spectral regions of strong

atmospheric absorption. Suppression strategies for these artifacts are discussed further along in this chapter.

Also, the water vapor is not completely compensated over vegetation sites. This is addressed in the chapter on

ACORN mode 1.5.

Figure 4-6. Atmospherically corrected Jasper Ridge image following implementation of ACORN mode 1pb.

45

Figure 4-8. Extracted spectra from Stanford data set after mode 1pb atmospheric correction.



.in This is the control file that provides the files and parameters that are used by ACORN. The file stnhyp1pb.in

was created in the default location for this tutorial example.

.eco In the same file location of the jsp1.in file will be a stnhyp1pb.in.eco. This file is an echo of the input file as

it is being interpreted by the ACORN software. If problems are detected interpreting the control file these will be


.diag1 In the same directory as the output reflectance file will be a stnhyp1pb.diag1. The diag1 file confirms the


.diag2 In the same directory as the output reflectance the stnhyp1pb.diag2 file will be created. This file contains


Artifact Suppression Type 1

Artifact suppression type 1 attempts to address spectral calibration mismatches between the image data set and the

radiative transfer code model of the atmosphere.

For this example from the ACORN Control Panel select Open and open file stnhyp1pb_a100.in. The default

installation location for this file is

c:\Program Files\ACORN6\Data\Stanford\stnhyp1pb_a100.in.

Notice that in the control file editor artifact type 1 is selected.

46

Click Save & Run

When the run is complete open the output image file. The default location and name for this file is

c:\Program Files\ACORN6\Data\Stanford\stnhyp_a100_rfl.

Extract the spectra from the four sites given in Table 4-1. Figure 4-9 shows these extracted spectra. In

comparison to the spectra shown in Figure 4-8 many of the spectral artifacts have been suppressed.

Figure 4-9. Atmospherically corrected spectra from the Stanford image with ACORN Mode 1pb and artifact

suppression type 1.

Artifact Suppression Type 1 and 2 Artifact suppression type 2 attempts to address radiometric calibration mismatches between the image data set and

the radiative transfer code model of the atmosphere.

For this example from the ACORN control file editor window select Open. The default location for this file is


In the control file editor artifact type 1 and 2 are selected.

Click Save & Run

When the run is complete open the output image file with ENVI. The location and name for this file specifed in


c:\Program Files\ACORN6\Data\Stanford\stnhyp1pb_a110_rfl.

Extract the spectra from the five sites given in Table 4-1. Figure 4-10 shows these extracted spectra. In

comparison to the spectra shown in Figure 4-8 and Figure 4-9 additional spectral artifacts have been suppressed.

47


suppression type 1 and 2.

Artifact Suppression Type 1, 2, and 3 Artifact suppression type 3 attempts to address artifacts due to low measured signal.

For this example from the ACORN control file editor window select Open. The default installation location for

this file is


In the control file editor artifact type 1, 2 and 3 are selected.

Click Save & Run



c:\Program Files\ACORN6\Data\Stanford\stnhyp_a111_rfl.


In comparison to the spectra shown in Figure 4-8, Figure 4-9, and Figure 4-10 the low signal, excessive noise

portions of the spectrum have been suppressed.

48


suppression type 1, 2 and 3.

Visibility Estimation

ACORN will attempt to estimate the visibility from the data themselves.

For this example from the ACORN control file editor window select Open. The default location for this file in the

tutorial is

c:\Program Files\ACORN6\Data\Stanford\stnhyp1pb_a111v.in.

In the control file editor artifact type 1, 2, 3 and estimate visibility are selected.

Click Save & Run

When the run is complete open the output image file with ENVI. The location and name for this file in the

tutorial is

c:\Program Files\ACORN6\Data\Stanford\stnhyp_a111v_rfl.

Extract the spectra from the five sites given in Table 4-1.

For comparison spectra are shown in Figure 4-12 where ACORN was run with a fixed visibility of 20 km. 20 km

is too hazy an atmosphere for this data set and results in negative reflectance values in the visible portion of the

spectrum. The ACORN estimated visibility spectra in Figure 4-13 give a better result.

49

Figure 4-12. Atmospheric correction with fixed 10 km visibility used. 10 km is excessively hazy and causes

negative reflectance results in the visible portion of the spectrum.

Figure 4-13. Atmospheric correction with ACORN visibility estimation used.

50

Chapter 5 Mode 1.5. Radiative transfer atmospheric correction of calibrated hyperspectral data with water vapor and liquid water spectral fitting The following topics are covered in this chapter:

Description…………………………….…………………………………………………………… 51

Input Image Data………………...…………………………………………………………….……51

Starting ACORN……………………………………………………………...………………...….. 53

Reviewing and Editing the Control File…………………………………….…………………...… 54

Running ACORN Mode 1.5…...……………………………..…….…………………………....… 58

Reflectance Results…...……………………………..………………………………………...…… 58

Water Vapor Image…...……………………………..…………………………………...………… 59

Liquid Water Image…...……………………………………………………..………..…………… 60

Additional Output Files…...…………………………………………...……………..…….……… 61

Other Tutorial Examples…...………………………………………………..………..……….…… 61

51

Description ACORN mode 1.5 is an advanced mode that uses radiative transfer calculations and the measured, calibrated

hyperspectral data to deduce a subset of the atmospheric effects present in the hyperspectral data set. This mode

includes spectral fitting to simultaneously determine the water vapor and liquid water in vegetation (or water

saturated soil) on the surface. This gives an improved estimation of water vapor for the atmospheric correction

and provides a surface liquid water parameter data set that may be used to describe liquid water related properties

of vegetation. The derived atmospheric properties are used in conjunction with the other constraint parameters to

correct for the effects of the atmosphere in the hyperspectral data set. With an input of calibrated spectral radiance

data, ACORN produces an output of apparent surface reflectance.


and estimation.

As always, the quality of the atmospheric correction is closely tied to the quality of the calibration of the image

data. At present, perfect calibration and perfect knowledge of the atmosphere are not achievable. Some artifacts

will be present in every atmospheric correction. The strength of the artifacts will be related to the quality of the

calibration, the knowledge of the atmosphere, and the ability to model the atmosphere. Several options are offered

in ACORN to help suppress artifacts in the atmospheric correction result.

Input Image Data

For this example the AVIRIS data set acquired over Jasper Ridge, California is used. Begin by examining the

Jasper Ridge calibrated radiance data set provided with the ACORN software.

1. Start ENVI software

2. Open the Jasper Ridge data provide with ACORN. The default installation location for this data set is

c:\Program Files\ACORN6\Data\Jasper\jsp0rdn.



Load Band button. The resulting displayed image is shown in Figure 5-1. Figure 5-2 shows a set of extracted

spectra the Jasper Ridge data. Table 5-1 gives the site, and location of these extracted spectra.

52

Figure 5-1. Jasper Ridge AVIRIS radiance image.

Figure 5-2 shows a set of extracted spectra the Jasper Ridge data. Table 5-1 gives the site, image label, and

location of these extracted spectra.











Data>ASCII.



53



Site Sample Line





Felt Lake 398 355

These image radiance data contain the effects of the solar illumination, two-way transmittance and scattering of


radiance values fall in a range of efficiently stored integers. ACORN mode 1.5 is designed to use input and

derived parameters with spectral fitting for water vapor and surface liquid water to assess and correct for these

atmospheric effects.

Starting ACORN

In this example ACORN Mode 1.5 will be used to atmospherically correct the Jasper Ridge calibrated radiance

data set.


The main ACORN program interface will appear as shown in Figure 5-3.

54

Figure 5-3. ACORN 5 Control Panel




1. From the ACORN main interface, select mode 1.5. The mode 1.5 control file editor will appear as shown in

Figure 5-4.

2. For this example, select the Jasper Ridge mode 1.5 control file. The default location for this file is

C:\Program Files\ACORN6\Data\Jasper\jsp1_5a3.in.

Use the open button to find this file.

3. Examine the ACORN control file parameter entry panel shown in Figure 5-4. These are the files and

parameters necessary to perform the atmospheric correction. Each parameter and file is described below for this

Jasper Ridge ACORN mode 1.5 example.

55

Figure 5-4. ACORN Mode 1.5 control file parameter entry window.

Input Image: This is the name of the input radiance image file. The default location is


Note: The input image data must be stored as 16 bit integer data for use by ACORN.

Output Reflectance Image: This is the name of the output image to be generated by ACORN. The default

location is

C:\Program Files\ACORN6\Data\Jasper\jsp1_5a3rfl.

Image Spectral Calibration: This is the spectral calibration file for the input image. The default location is C:\Program Files\ACORN6\Data\Jasper\avr97.wvl

This is a two column file. The first column is the center position of each spectral band in the image in

nanometers. The second column is the full-width-at-half-maximum (FWHM) of the appropriate Gaussian

function to describe the spectral response function of the band in nanometers. Table 4-2 shows a subset of this

file.

56

Table 4-2. Jasper Ridge spectral calibration file.

369.85 9.61

379.69 9.58

389.53 9.55

… …

2486.99 10.07

2496.90 10.05

2506.81 10.03

Image Gain: This is the gain file that converts the image file integer values to radiance in units of

(W/m^2/um/sr). The default location is C:\Program Files\ACORN6\Data\Jasper\avr97.gain

This is a one column file that is multiplied by the image integer values to convert the integers to the correct

radiance units. Table 4-3 shows a subset of this file.


0.2

0.2

0.2

…

0.1

0.1

0.1

Image Offset: This is the offset file that corrects for any offset in the image file integer values in the conversion

to radiance in units of (W/m^2/um/sr). The default location is

C:\Program Files\ACORN6\Data\Jasper\avr97.off.

This is a one column file that is added to the radiance values. Table 4-4 shows a subset of this file. For this data

set the offset values are zero.

Table 4-4. Contents of the Jasper Ridge offset file.

0.0

0.0

0.0

…

0.0

0.0

0.0




Image Integer Format: This parameter specifies whether the image data are stored as big endian (host, intel) or

little endian (network or IEEE). The Jasper Ridge integer format is big endian.

57


band-interleave-by-pixel (BIP). The Jasper Ridge data set is BIP file format.

Image Latitude: These are the central degrees, minute, seconds of the image data set. Each parameter may be

integer or decimal. If decimal degrees are used the minutes and second field should be zero. North latitude is

positive. The central latitude of the Jasper Ridge data set is 37.18.

Image Longitude: These are the central degrees, minute, seconds of the image data set. Each parameter may be

integer or decimal. If decimal degrees are used the minutes and second field should be zero. The central

longitude of the Jasper Ridge data set is -122.3. East longitude is positive.

Image Acquisition Time: This is the date and time of image acquisition. Date is input as day, month, year

format. This example data set was acquired on the 3rd

of April, 1997. The nominal time of acquisition in hours,

minutes, and seconds. The time must be Greenwich Mean Time (GMT). Each parameter may be integer or

decimal. The example Jasper Ridge data set was acquired at 19:57:23 GMT.

Average Surface Elevation: This is the average elevation of the surface in the input image in meters. The

approximate average elevation of the Jasper Ridge example data set is 200 m.

Acquisition Altitude: This is the altitude of the instrument that acquired the image data in kilometers. The

altitude of acquisition of this example Jasper Ridge data set is 20 km.




Derive Water Vapor: This option selects the type of water vapor derivation for ACORN Mode 1.5.

i) 940 water vapor band


iii) 940 and 1140 water vapor band

For this example the option to use both the 940 and 1140 nm water vapor bands is selected.

Path Radiance in Spectral Fit: This option includes fine tuning of the path radiance parameter in the spectral

fitting of water vapor and liquid water. This may improve the quality of the water vapor and liquid water result.

For this example this option is selected.

Baseline Visibility: This parameter controls the visibility of the atmospheric model used in the atmospheric

correction. A visibility of 60 km is entered for this example.

Estimated Visibility: If selected ACORN will attempt to estimate the visibility from the data themselves. This

option is not selected for this example.


i) Artifact suppression type 1 attempts to address spectral calibration mismatches between the

image data set and the radiative transfer code model of the atmosphere.

ii) Artifact suppression type 2 attempts to address radiometric calibration mismatches between

the image data set and the radiative transfer code model of the atmosphere.


For this example all artifact suppression options are selected.

58

Running ACORN Mode 1.5

To execute ACORN mode 1.5, click Save + Run on the ACORN control file editor. A processing status box will


program will report the time required for atmospheric correction. Click Okay to return to the control file editor.

Reflectance Results

The primary result of ACORN Mode 1.5 is the atmospherically corrected apparent reflectance image.

1. With ENVI open the Jasper Ridge reflectance image. The default location is c:\program files\ACORN6\Data\Jasper\jsp1_5a3rfl



Figure 5-6. These spectra are stored as integer of reflectance multiplied by 10000. This preserves the precision of

the measurement and allows the data to be stored efficiently.


atmosphere have been compensated. The reflectance spectra are smooth over vegetation in the 980 and 1200 nm

region where liquid water absorption in vegetation occurs.

Figure 5-5. Atmospherically corrected Jasper Ridge image following use of ACORN mode 1.5.

59

Figure 5-6. Extracted spectra from Jasper Ridge data set after mode 1.5 atmospheric correction.

Water Vapor Image

In addition to the atmospherically corrected reflectance image at water vapor image is generated from the spectral

fitting algorithm. Figure 5-7 shows the water vapor image for this tutorial example. Water vapor is reported in

units of precipitable microns (um).

1. With ENVI open the Jasper Ridge water image. The default location is c:\program files\ACORN6\Data\Jasper\jsp1_5a3rfl.wtrv

2. Use the ENVI cursor value location capability to examine the water vapor values. From the ENVI image

window select Function>Interactive Analysis> Cusor Location/Value. Move the cursor to various parts of the

image to see the change in water vapor. Water vapor varies from 5000 um to 7000 precipitable um (5 to 7

precipitable mm) across the image. This is a dry atmosphere data set. The image is largely devoid of surface

structure as expected for water vapor. Some surface leakage occurs over dark targets such as the open water areas

of lakes.

60

Figure 5-7. Water vapor image from ACORN atmospheric correction mode 1.5.

Liquid Water Image

A second result of the water vapor and liquid water spectral fitting algorithm used in ACORN is a surface liquid

water image. Figure 5-8 shows the liquid water image for this tutorial example. Liquid water is reported in units

of expressed path absorption in microns (um). This parameter may be used to derive vegetation water, species-

type, and phonological state properties.

1. With ENVI open the Jasper Ridge water image. The default location is c:\program files\ACORN6\Data\Jasper\jsp1_5a3rfl.wtrl



image to see the change in expressed liquid water. Liquid water varies from 0 to 4000 um expressed path

absorption in um (0 to 4 mm) over the image. Paved surfaces, soils, and dry grass give low to zero values.

Healthy vegetation gives a range of values depending on species, water status, and phonological state. Some of

the highest liquid water values are found in the redwood forest in the upper left corner of the data set.

61

Figure 5-8. Suface liquid water image from ACORN atmospheric correction mode 1.5.



.in This is the control file that provides the files and parameters that are used by ACORN. The file jsp1_5a3.in

was created in the default location for this tutorial example.

.eco In the same file location of the jsp1_5a3.in file will be a jsp1_5a3.in.eco. This file is an echo of the input

file as it is being interpreted by the ACORN software. If problems are detected interpreting the control file these

will be indicated in this file.

.diag1 In the same directory as the output reflectance file will be a jsp1_5a3rfl.diag1. The diag1 file confirms

the input parameters used in the atmospheric correction and indicates problems encountered during the run.

.diag2 In the same directory as the output reflectance the jsp1_5a3rfl.diag2 file will be created. This file contains


Other Tutorial Examples Additional tutorial examples are provided with ACORN for the Cuprite Nevada AVIRIS and Hymap data. The

default location for these files is c:\Program Files\ACORN6\Data\Cuprite\cup1_5.in

and c:\Program Files\ACORN6\Data\Cupriteh\cuph1_5.in


mode 1.5.

62

Chapter 6 Mode 1.5pb. Mode 1.5 Radiative transfer atmospheric correction of calibrated hyperspectral data with water vapor and liquid water spectral fitting for pushbroom sensors The following topics are covered in this chapter:

Description…………………………….………………………………………………...…….…… 63

Input Image Data………………...…………………………………………………………….……64

Starting ACORN……………………………………………………………...…………...……….. 66

Reviewing and Editing the Control File…………………………………….……………..…….… 66

Running ACORN Mode 1.5pb…...…………………..…….………………………………...….… 70

Reflectance Results…...……………………………..……………………………………..….…… 70

Water Vapor Image…...……………………………..…………………………………......….…… 72

Liquid Water Image…...……………………………………………………..………..…...….…… 73

Additional Output Files…...…………………………………………...……………..……….…… 74

63

Description ACORN mode 1.5pb is an advanced mode that uses radiative transfer calculations and the measured, calibrated

hyperspectral data to deduce a subset of the atmospheric effects present in the hyperspectral data set. This mode

is designed to account for the cross-track spectral calibration variation of pushbroom sensors. Mode 1.5pb

includes spectral fitting to simultaneously determine the water vapor and liquid water in vegetation (or water

saturated soil) on the surface. This gives an improved estimation of water vapor for the atmospheric correction

and provides a surface liquid water parameter data set that may be used to describe liquid water related properties

of vegetation. The derived atmospheric properties are used in conjunction with the other constraint parameters to

correct for the effects of the atmosphere in the hyperspectral data set. With an input of calibrated spectral radiance

data, ACORN produces an output of apparent surface reflectance.


and estimation.

As always, the quality of the atmospheric correction is closely tied to the quality of the calibration of the image

data. At present, perfect calibration and perfect knowledge of the atmosphere are not achievable. Some artifacts

will be present in every atmospheric correction. The strength of the artifacts will be related to the quality of the

calibration, the knowledge of the atmosphere, and the ability to model the atmosphere. Several options are offered

in ACORN to help suppress artifacts in the atmospheric correction result.

ACORN mode 1.5pb is a new mode introduced in ACORN version 5 that allows use of a spectral calibration file

that account for the cross track spectral variation in pushbroom imaging spectrometers such as Hyperion. Figure

6-1 shows the cross-track variation in spectral calibration for channel 40 of Hyperion. Across the 256 spatial

elements of Hyperion the spectral calibration varies by ~3 nm. This is a 30% variation with respect to the 10 nm

full-width-at-half-maximum of the Hyperion instrument. Accounting for this cross-track spectral variation is

essential for accurate atmospheric correction of spectra measured by pushbroom imaging spectrometers.

Note: Because considerable file reformatting is required to utilize the full cross-track spectral calibration

the Mode requires longer computation time that the non pushbroom mode.

749

750

751

752

753

754

0 32 64 96 128 160 192 224 256

Cross-Track Sample (#)

Wav

elen

gth

(n

m)

Channel 40

Figure 6-1. Cross-track spectral calibration variation of the Hyperion pushbroom imaging spectrometer.

64

Input Image Data

For this example of ACORN mode 1pb a Hyperion data set acquired south of Jasper Ridge that includes the

Stanford University campus in California is used. Begin by examining the Stanford calibrated radiance data set

provided with the ACORN software.


2. Open the Stanford Hyperion data provided with ACORN. The default installed location is

c:\program files\ACORN6\Data\stanford\stnhyp_rdn.



Load Band button. The resulting displayed image is shown in Figure 6-2. Figure 6-3 shows a set of extracted

spectra from the Stanford Hyperion data. Table 6-1 gives the site name, and location of these extracted spectra.

Figure 6-2. Stanford radiance image bands near 650, 550, 450 nm as red, green and blue.

65











Data>ASCII.




Site Sample Line

Golf Course 58 272

Red roof 36 94

Dry Grass 39 195

Felt Lake 13 227

Figure 6-3. Extracted spectra from Stanford Hyperion radiance image.



radiance values fall in a range of efficiently stored integers. ACORN mode 1.5pb is designed to use input and

derived parameters with spectral fitting for water vapor and surface liquid water to assess and correct for these

atmospheric effects.

66

Starting ACORN

In this example ACORN Mode 1.5pb will be used to atmospherically correct the Stanford calibrated radiance data

set.


The main ACORN program interface will appear as shown in Figure 6-4.

Figure 6-4. ACORN 5 Control Panel




1. From the ACORN main interface, select mode 1.5pb. The mode 1.5pb control file editor will appear as shown

in Figure 6-5.

67

2. For this example, using the Open button, select the Stanford mode 1.5pb control file. The default location for

this file is

C:\Program Files\ACORN6\Data\Stanford\stnhyp15pb_a111.in.



Stanford Hyperion ACORN mode 1.5pb example.

Figure 6-5. ACORN Mode 1.5pb control file parameter entry window.

Input Image: This is the name of the input radiance image file. The default location is

C:\Program Files\ACORN6\Data\Stanford\stnhyp_rdn.

Output Reflectance Image: This is the name of the output image to be generated by ACORN. The default

location is

C:\Program Files\ACORN6\Data\Stanford\stnhyp15pb_rfl.

68

Cross Track Wavelength Calibration: This is the spectral calibration file for the input image. The format is

ASCII. The default location is C:\Program Files\ACORN6\Data\Stanford\hyp_wvlpb.txt

This is a two dimensional file. Each row corresponds to the cross-track sample (1 to 256 for Hyperion). Each

column is the wavelength position for the corresponding spectral channel (1 to 242 for Hyperion). Table 6-2

shows a subset of this file.

Table 6-2. Stanford cross-track wavelength spectral calibration file.

355.37 365.56 375.75 . . . 2557.01 2567.01 2577.11

355.4 365.59 375.77 . . . 2557.01 2567.01 2577.11

355.43 365.61 375.8 . . . 2557.02 2567.02 2577.12

. . . . . .

. . . . . .

. . . . . .

352.83 363.02 373.21 . . . 2556.6 2566.6 2576.7

352.79 362.98 373.17 . . . 2556.59 2566.59 2576.69

352.74 362.93 373.12 . . . 2556.58 2566.58 2576.68

Cross Track FWHM Calibration: This is the spectral calibration file for the input image. The format is ASCII.

The default location is

C:\Program Files\ACORN6\Data\Stanford\hyp_fwhmpb.txt.

This is a two dimensional file. Each row corresponds to the cross-track sample. Each column is the full-width-at-

half-maximum for the corresponding spectral channel. Table 6-3 shows a subset of this file.

Table 6-3. Stanford cross-track FWHM spectral calibration file.

11.23 11.23 11.23 . . . 10.44 10.44 10.44

11.23 11.23 11.23 . . . 10.44 10.44 10.44

11.23 11.23 11.23 . . . 10.44 10.44 10.44

. . . . . .

. . . . . .

. . . . . .

11.25 11.25 11.25 . . . 10.19 10.19 10.19

11.25 11.25 11.25 . . . 10.19 10.19 10.19

11.25 11.25 11.25 . . . 10.19 10.19 10.19



C:\Program Files\ACORN6\Data\Stanford\hyp_gain.txt.

This is a one column file that is use by ACORN to convert the image file integers to the correct radiance units.

Table 6-4 shows a subset of this file.

Table 6-4. Contents of the Stanford gain file.

0.025

0.025

0.025

…

69

0.0125

0.0125

0.0125


in units of (W/m^2/um/sr). For this example, the default location is

C:\Program Files\ACORN6\Data\Stanford\hyp_offset.txt This is a one column file that is

added to the radiance values. Table 6-5 shows a subset of this file. For most data sets the radiance offset is zero.

Table 6-5. Contents of the Stanford offset file.

0.0

0.0

0.0

…

0.0

0.0

0.0


offset parameter is the number of bytes used to skip any embedded image headers. The dimensions of the

Stanford data set are 242 bands, 400 lines, and 256 samples with 0 byte offset.

Image Integer Format: This parameter specifies whether the image data are stored as big endian (host, intel) or

little endian (network or IEEE). The Stanford integer format is big endian.


band-interleave-by-pixel (BIP). The Stanford data set is BIL file format.



approximate latitude of the Stanford data set is 37.47.



of the Stanford data set is -122.13. East longitude is positive.


acquired on the 31st of July, 2002.


Greenwich Mean Time (GMT). Each parameter may be integer or decimal. The example Stanford Hyperion data

set was acquired at 18:35:00 GMT.

Image Elevation: This is the average elevation of the surface in the input image. The nominal elevation of the

Stanford example data set is 200 m.


The altitude of acquisition of this example Stanford data set is 700 km.




70

Derive Water Vapor: This option selects the type of water vapor derivation for ACORN Mode 1pb. ACORN


i) 940 water vapor band


iii) 940 and 1140 water vapor band

For this example the option to use the 1140 nm water vapor bands is selected. The signal to noise ratio of

Hyperion is poor in the 940 nm spectral region.

Path Radiance in Spectral Fit: This option includes fine tuning of the path radiance parameter in the spectral

fitting of water vapor and liquid water. This may improve the quality of the water vapor and liquid water result.

For this example this option is selected.

Baseline Visibility: This parameter controls the visibility of the atmospheric model used in the atmospheric

correction. A visibility of 60 km is entered for this example.

Estimated Visibility: If selected ACORN will attempt to estimate the visibility from the data themselves. This

option is not selected for this example.







For this example all three types of artifact suppression options are selected.

4. Click Save As to save the ACORN mode 1.5pb parameter file. The control file may be saved as the original

name or a new file name.

Running ACORN Mode 1.5pb

To execute ACORN mode 1.5pb, click Save + Run on the ACORN control file editor. A processing status box

will appear to indicate the progress of the atmospheric correction. When the atmospheric correction is complete

the program will report the time required for atmospheric correction. Click Okay to return to the control file

editor.

Reflectance Results

The primary result of ACORN Mode 1.5pb is the atmospherically corrected apparent reflectance image.

1. With ENVI open the Stanford Hyperion reflectance image. The default location is c:\program files\ACORN6\Data\Stanford\stnhyp_a111_rfl


71


Figure 6-7. These spectra are stored as integer of reflectance multiplied by 10000. This preserves the precision of

the measurement and allows the data to be stored efficiently.


atmosphere have been compensated. The reflectance spectra are smooth over vegetation in the 980 and 1200 nm

region where liquid water absorption in vegetation occurs.

Figure 6-6. Atmospherically corrected Stanford image following use of ACORN mode 1.5pb.

72

Figure 6-7. Extracted spectra from Stanford data set after mode 1.5pb atmospheric correction.

Water Vapor Image

In addition to the atmospherically corrected reflectance image at water vapor image is generated from the spectral

fitting algorithm. Figure 6-8 shows the water vapor image for this tutorial example. Water vapor is reported in

units of precipitable microns (um).

1. With ENVI open the Stanford Hyperion water vapor image. The default location is c:\program files\ACORN6\Data\Stanford\stnhyp15_a111_rfl.wtrv



image to see the change in water vapor. Water vapor varies from 15000 um to 30000 precipitable um (15 to 30

precipitable mm) across the image. This is a fairly wet atmosphere data set. The image is largely devoid of

surface structure as expected for water vapor. Some surface leakage occurs over dark targets such as the open

water areas of lakes.

Note: vertical striping results from calibration and detector response variation associated with this sensor.

73

Figure 6-8. Water vapor image from ACORN atmospheric correction mode 1.5pb.

Liquid Water Image

A second result of the water vapor and liquid water spectral fitting algorithm used in ACORN is a surface liquid

water image. Figure 6-9 shows the liquid water image for this tutorial example. Liquid water is reported in units

of expressed path absorption in microns (um). This parameter may be used to derive vegetation water, species-

type, and phonological state properties.

1. With ENVI open thr Stanford Hyperion liquid water image. The default location is c:\program files\ACORN6\Data\Stanford\stnhyp_a111_rfl_wtrl



image to see the change in expressed liquid water. Liquid water varies from 0 to 4000 um and higher expressed

path absorption in um (0 to 4 mm) over the image. Paved surfaces, soils, and dry grass give low to zero values.

Healthy vegetation gives a range of values depending on species, water status, and phonological state. Some of

the highest liquid water values are found in the redwood forest in the upper left corner of the data set.

Note: vertical striping results from calibration and detector response variation associated with this sensor.

74

Figure 6-9. Suface liquid water image from ACORN atmospheric correction mode 1.5pb.



.in This is the control file that provides the files and parameters that are used by ACORN. The file

stnhyp1pb_a111.in was created in the default location for this tutorial example.

.eco In the same file location of the stnhyp1pb_a111.in file will be a stnhyp1pb_a111.in.eco. This file is an echo

of the input file as it is being interpreted by the ACORN software. If problems are detected interpreting the

control file these will be indicated in this file.

.diag1 In the same directory as the output reflectance file will be a stnhyp_a111_rfl.diag1. The diag1 file

confirms the input parameters used in the atmospheric correction and indicates problems encountered during the

run.

.diag2 In the same directory as the output reflectance the stnhyp_a111_rfl.diag2 file will be created. This file

contains selected spectral outputs that may be helpful to understand problems with the atmospheric correction.

75

Chapter 7 Mode 2. Single spectrum enhancement of a hyperspectral atmospheric correction. The following topics are covered in this chapter:

Description…………………………….……………………………………………….……...…… 76

Input Image Data………………...………………………………………………………….....……76

Starting ACORN……………………………………………………………...……………...…….. 78

Reviewing and Editing the Control File……………………………………….………….……..… 79

Running ACORN Mode 2…………………………..…….……………………………………..… 81

ACORN Mode 2 Results…...………………………………………………………....…………… 81

Additional Output Files…...……………………………………………………….....……….…… 83

Other Tutorial Examples…...…………………………………………………….…..…….……… 83

76

Description

For single spectrum enhancement of a hyperspectral atmospheric correction, ACORN uses a spectrum extracted

from an atmospherically corrected hyperspectral data set and an accurately known spectrum for the same target.

With these two spectra, the full atmospherically corrected hyperspectral data set is corrected to the accuracy of the

known spectrum. ACORN accurately and automatically convolves the known spectrum to the spectral

characteristics of the hyperspectral data set for this atmospheric correction enhancement. This is a powerful

method to compensate for residual errors in the hyperspectral atmospheric correction due to data calibration as

well as atmospheric modeling.

Input Image Data

For this example the AVIRIS data set acquired over Cuprite, Nevada is used. Begin by using ACORN Mode 1 to

atmospherically correct the Cuprite data set. Do this by following the ACORN Mode 1 tutorial, but use the input

control file for Cuprite. The default installation location for the Cuprite control file is

c:\Program Files\ACORN6\Data\Cuprite\cup1.in.

Once the Mode 1 atmospheric correction is complete examine the Cuprite reflectance image


2. Using ENVI, open the Cuprite, Nevada mode 1 reflectance data calculated with ACORN. The default location

for this data set is

c:\Program Files\ACORN6\Data\Cuprite\cup1rfl.

3. Select bands 200, 130, 30 to display as red, green, blue. This is done with in the Available Bands List window

by clicking the RGB Color option and then scrolling and clicking on band 200, 130 and 30 in that order. Then

click the Load Band button. The resulting displayed image is shown in Figure 7-1. Figure 7-2 shows a set of

extracted spectra the Cuprite atmospherically corrected data set. Table 7-1 gives the site, image label, and

location of these extracted spectra.

77

Figure 7-1. Cuprite, Nevada ACORN calculated reflectance image.











Data>ASCII.



78

Figure 7-2. Extracted spectra from Cuprite ACORN Mode 1 atmospherically corrected image.


Site Sample Line

Stonewall Playa 606 293

Opalite Zone w/Alunite 554 272

Strongly Argillized Zone w/Kaolinite 526 319

Buddingtonite Zone 472 235

Calcite 295 348

These extracted spectra from the Cuprite ACORN Mode 1 atmospherically corrected image have some residual

spectral variability across the spectral range. This is due to imperfections in the data calibration as well as the

atmospheric modeling used by ACORN. ACORN Mode 2 is designed to suppress this systematic spectral

variability.

Starting ACORN

In this example ACORN Mode 2 will be used to enhance the atmospheric correction for the Cuprite ACORN

Mode 1 reflectance data set.


The ACORN6 users interface will appear as shown in Figure 7-3.

79

Figure 7-3. ACORN Control Panel

Reviewing and Editing the Control File Note: Except for the examples provided with ACORN you must obtain the appropriate calibration files and


1. From the ACORN Control Panel, select Mode 2. The mode 2 control file editor will be displayed.

2. For this example, use Open to choose the Cuprite Mode 2 tutorial control file. The default location for this

control file is C:\Program Files\ACORN6\Data\Cuprite\cup2.in


parameters necessary to perform the single spectrum enhancement tutorial example. Each parameter and file is

described below for this tutorial example.

80


Input Image File Name: This is the name of the input radiance image file. The default location is

C:\Program Files\ACORN6\Data\Cuprite\cup1rfl.

Note: You must have created cup1rfl by running ACORN Mode 1 on the Cuprite radiance data set.


this tutorial, the default location is


Image Target Reflectance Spectrum File: This is a single column file with the extracted spectrum from

cup1rfl that corresponds to the location where an accurate surface reflectance spectrum has been acquired. Table

7-3 shows a portion of this file. Negative and noisy values have been set to 0.000.

Table 7-3. Cuprite Image Target Reflectance Spectrum File.

0.000

0.000

516.721

1399.558

1863.000

…

3529.698

5338.349

5243.674

Image Spectral Calibration file: This is the spectral calibration file for the input image. The default location is C:\Program Files\ACORN6\Data\Cuprite\avr97.wvl




file.

81

Table 7-3. Cuprite image spectral calibration file.

369.85 9.61 . . .

379.69 9.58

389.53 9.55

… …

2486.99 10.07

2496.90 10.05

2506.81 10.03

Measured Target Reflectance Spectrum File : This is the measured reflectance for the same target as the

Image Target Reflectance Spectrum File. The default location of the file for this tutorial is C:\Program Files\ACORN6\Data\Cuprite\Cup2.meas

This is a two column file. The first column is wavelength in nm and the second column is reflectance. The

reflectance is multiplied by 10000 to have the same scaling as the Image Target Reflectance Spectrum File. Table

7-4 shows a portion of this file.

Table 7-4. Contents of the Measured Target Reflectance Spectrum File.

350 1636.862

351 1638.537

352 1642.535

… …

2503 4079.836

2504 4081.076

2505 4082.106


offset parameter is the number of bytes used to skip any embedded image headers. The dimensions of the Cuprite

data set are 224 bands, 512 lines, and 614 samples with 0 byte offset.


endian (network or IEEE). The Cuprite integer format is big endian.


band-interleave-by-pixel (BIP). The Cuprite data set is BIP file format.


To execute ACORN mode 2, click Save + Run on the ACORN control panel. A processing status box will appear

to indicate the progress of the atmospheric correction. When the atmospheric correction is complete the program

will report the time required for the atmospheric correction. Click Okay to return to the control file editor.


The principal result of ACORN Mode 2 is the single spectrum enhanced reflectance image.

1. With ENVI open the Cuprite single spectrum enhanced image. The default location for the tutorial is c:\program files\ACORN6\Data\Cuprite\cup2rfl.

82


2. Extract the single spectrum enhanced spectra from the sites in table 7-1. The extracted spectra are shown in

Figure 7-6.

3. Compare the extracted single spectrum enhanced spectra with the input reflectance spectra in Table 7-1. The

systematic spectral roughness has been removed. Spectral artifacts remain only in the 1400 and 1900 nm region

where the signal is extremely low due to the absorption of atmospheric water vapor.

Figure 7-5. Single Spectrum Enhance image following use of ACORN Mode 2.

83

Figure 7-6. Extracted spectra from Cuprite data set after Mode 2 Single Spectrum Enhancement.



.in This is the control file that provides the files and parameters that are used by ACORN. The file cup2.in was


.eco In the same file location of the cup2.in file will be a cup2.in.eco. This file is an echo of the input file as it is



.diag1 In the same directory as the output reflectance file will be a cup2rfl.diag1. The diag1 file confirms the


.diag2 In the same directory as the output reflectance the cup2rfl.diag2 file will be created. This file contains


Other Tutorial Examples Additional tutorial examples are provided with ACORN for the Jasper Ridge AVIRIS and Cuprite Hymap data.

The default location for these files is c:\Program Files\ACORN6\Data\Jasper\jsp2.in

and c:\Program Files\ACORN6\Data\Cupriteh\cuph2.in


mode 2.

84

Chapter 8 Mode 3. Atmospheric correction using the empirical line method for hyperspectral data. The following topics are covered in this chapter:

Description…………………………….…………………………………….………………….….. 85

Input Image Data………………...…………………………………….……………………………85

Starting ACORN………………………………………………………………...………………..... 87

Reviewing and Editing the Control File………………………………………….……….……..… 88

Running ACORN Mode 3 …………………………..……………………………………......…… 91

ACORN Mode 3 Results…...………………………………………………………...…...….…… 91

Additional Output Files…...……………………………….………………..………..……….…… 92

Other Tutorial Examples…...………………………………………………..………..……….…… 93

85

Description

ACORN uses a dark and bright extracted target spectrum from a hyperspectral data set and corresponding

measured ground spectra for these same targets. These four input spectra are used to implement an empirical line

atmospheric correction. ACORN accurately and automatically convolves the known measured spectra to the

spectral characteristics of the hyperspectral data set for this mode of atmospheric correction. This mode is less

dependent on the accuracy of the radiometric calibration of the hyperspectral data and can even be run on the raw

image data from the image provider.

To implement ACORN empirical line correction a measured dark and bright target reflectance spectrum is

required from within the region of the hyperspectral data set. Typically there must be at least 0.10 reflectance

difference between the dark and bright spectra. Figure 8-1 shows an example measured dark and bright target

reflectance spectra from the Cuprite, NV area. The measured spectra must have equal or better spectral resolution

and sampling than the hyperspectral data set. The spectral range must be equal or greater as well. Spectra from the

corresponding areas of the hyperspectral data set must be extracted using image processing software. Figure 8-1

also shows the extracted dark and bright target image radiance spectra. With these spectra and the spectral

calibration parameters of the hyperspectral data set ACORN performs an empirical line atmospheric correction.

0

2000

4000

6000

8000

10000

12000

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Ref

lect

ance

/1000

0.

Dark Target

Bright Target

0

2000

4000

6000

8000

10000

12000

14000

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Imag

e S

cale

d R

adia

nce

Dark Target

Bright Target

Figure 8-1: Left: dark and bright target measured spectra from the Cuprite area. Right: spectra for the

dark and bright targets extracted from the Cuprite hyperspectral radiance data

Input Image Data

For this example the AVIRIS data set acquired over Cuprite, Nevada is used. Begin by examining the scaled

radiance spectra of the Cuprite, NV data set.

1. Start ENVI software.

2. Open the Cuprite, Nevada data provide with ACORN. The default installation location for this data set is

c:\Program Files\ACORN6\Data\Cuprite\cup0rdn.

3. Select bands 200, 130, 30 to display as red, green, blue. This is done with in the Available Bands List window

by clicking the RGB Color option and then scrolling and clicking on band 200, 130 and 30 in that order. Then

click the Load Band button. The resulting displayed image is shown in Figure 8-2. Figure 8-3 shows a set of

extracted radiance spectra the Cuprite. Table 8-1 gives the site and location of these extracted spectra. Extract

these spectra for later comparison with empirical line result.

86

Figure 8-2. Cuprite AVIRIS radiance image.











Data>ASCII.



87

Figure 8-3. Extracted spectra from Cuprite radiance image.


Site Sample Line

Stonewall Playa 606 293

Opalite Zone w/Alunite 554 272

Strongly Argillized Zone w/Kaolinite 526 319

Buddingtonite Zone 472 235

Calcite 295 348



radiance values fall in a range of efficiently stored integers. ACORN mode 3 is designed to use the empirical line

method to correct these radiance spectra to reflectance.

Starting ACORN

In this example ACORN Mode 3 will be used to perform an empirical line atmospheric correction on the Cuprite

radiance data set.

1. Start ACORN by selecting Start>Programs>ACORN6>ACORN6

The ACORN software interface will appear as shown in Figure 8-4.

88





1. From the ACORN software interface select mode 3. The Mode 3 Control file editor will open.

2. For this example, use the Open button to open the Cuprite Mode 3 tutorial control file. The default installation

location for this control file is C:\Program Files\ACORN6\Data\Cuprite\cup3.in


parameters necessary to perform the empirical line atmospheric correction. Each parameter and file is described

below for this tutorial example.

89



C:\Program Files\ACORN6\Data\Cuprite\cup0rdn.




Image Spectral Calibration file: This is the spectral calibration file for the input image. The default location is C:\Program Files\ACORN6\Data\Cuprite\avr97.wvl




file.


369.85 9.61 . . .

379.69 9.58

389.53 9.55

… …

2486.99 10.07

2496.90 10.05

2506.81 10.03

Dark Target Image Spectrum File: This is a single column file with the extracted dark target spectrum from

the input image. This spectrum corresponds to the location where an accurate surface reflectance spectrum has

been acquired. The default location of the file for this tutorial is C:\Program Files\ACORN6\Data\Cuprite\Cup3d.ext.

90

Table 8-3 shows a portion of this file.

Table 8-3. Dark Target Image Spectrum File.

864.64

1147.03

1587.77

…

10.35

15.77

11.48

Dark Target Reflectance File : This is the measured reflectance for the dark target. The default location of the

file for this tutorial is C:\Program Files\ACORN6\Data\Cuprite\Cup3d.meas


reflectance is multiplied by 10000 to product a scaling between 0 and 10000 in the output file. Table 8-4 shows a

portion of this file.

Table 8-4. Dark Target Reflectance File.

350 1636.86

351 1638.53

352 1642.53

… …

2503 4079.83

2504 4081.07

2505 4082.10

Bright Target Image Spectrum File: This is a single column file with the extracted dark target spectrum from

the input image. This spectrum corresponds to the location where an accurate surface reflectance spectrum has

been acquired. The default location of the file for this tutorial is C:\Program Files\ACORN6\Data\Cuprite\Cup3b.ext.

Table 8-5 shows a portion of this file.

Table 8-5. Bright Target Image Spectrum File.

1524.44

2022.41

2694.53

…

31.85

45.26

31.55

Bright Target Reflectance File : This is the measured reflectance for the bright target. The default location of

the file for this tutorial is C:\Program Files\ACORN6\Data\Cuprite\Cup3b.meas

91


reflectance is multiplied by 10000 to product a scaling between 0 and 10000 in the output file. Table 8-6 shows a

portion of this file.

Table 8-6. Bright Target Reflectance File.

351 462.23

351 469.62

352 473.59

… …

2503 1174.56

2504 1173.76

2505 1173.2


offset parameter is the number of bytes used to skip any embedded image headers. The dimensions of the Cuprite

data set are 224 bands, 512 lines, and 614 samples with 0 byte offset.


endian (network or IEEE). The Cuprite integer format is big endian.


band-interleave-by-pixel (BIP). The Cuprite data set is BIP file format.


To execute ACORN mode 3, click Save + run on the ACORN control panel. A processing status box will appear

to indicate the progress of the atmospheric correction. When the empirical atmospheric correction is complete the

program will report the time elapsed. Click Okay to return to the control file editor.


The principal result of ACORN Mode 3 is the empirical line reflectance image.

1. With ENVI open the Cuprite empirical line reflectance image. The default location for the tutorial is c:\program files\ACORN6\Data\Cuprite\cup3rfl


2. Extract the empirical line corrected spectra from the sites in table 8-1. The extracted spectra are shown in

Figure 8-7.

3. Compare the extracted empirical line corrected spectra with the input image radiance spectra in Figure 8-3.

The radiance data have been corrected to apparent surface reflectance. Some residual artifacts remain near the

water vapor bands because the empirical line correction assumes a homogenous atmosphere over the entire data

set.

92

Figure 8-6. Empirical line atmospheric correction image following use of ACORN Mode 3.

Figure 8-7. Extracted spectra from Cuprite data set after Mode 3 empirical line atmospheric correction.



.in This is the control file that provides the files and parameters that are used by ACORN. The file cup3.in was


93

.eco In the same file location of the cup3.in file will be a cup3.in.eco. This file is an echo of the input file as it is



.diag1 In the same directory as the output reflectance file will be a cup3rfl.diag1. The diag1 file confirms the


.diag2 In the same directory as the output reflectance the cup3rfl.diag2 file will be created. This file contains



The default location for these files is

c:\Program Files\ACORN6\Data\Jasper\jsp3.in.

and

c:\Program Files\ACORN6\Data\Cupriteh\cuph3.in. You may run these examples and examine

the input and output files to gain additional experience with ACORN mode 3.

94

Chapter 9

Mode 4. Convolution of hyperspectral data to multispectral data. The following topics are covered in this chapter:

Description…………………………….…………………………………………………...….…… 95

Input Image Data………………...…………………………………………………………….……95

Starting ACORN……………………………………………………...…………………...……….. 97

Reviewing and Editing the Control File……………………………….……………………...…… 98

Running ACORN Mode 4 …………………………..………………………………….…….…… 101

ACORN Mode 4 Results…...…………………………………………………..……..…………… 101

Additional Output Files…...…………………………………………………...……..……….…… 102

Other Tutorial Examples…...……………………………………………..…………..……….…… 102

95

Description

ACORN uses the designated multispectral spectral response functions to convolve a spectrally and

radiometrically calibrated hyperspectral data set to the specified multispectral data set. This convolution handles

the case of overlapping or irregularly spaced sampling in the hyperspectral data set. ACORN mode 4 requires

input of a calibrated hyperspectral data set. For the convolution, the spectral response functions of the

multispectral data set to be generated are required. Figure 9-1 shows the spectral response functions for a six band

multispectral sensor in the solar reflected spectrum. Any multispectral data set may be generated that is within

the spectral range of the hyperspectral data set. Multispectral band response functions for existing, planned, or

imagined multispectral sensors may be used.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

No

rmal

ized

Res

po

nse Response

Figure 9-1: Multispectral spectral response functions for six bands in the solar reflected spectrum from 400 to

2500 nm.

Input Image Data For this example of ACORN mode 1 the AVIRIS data set acquired over Jasper Ridge, California is used. Begin

by examining the Jasper Ridge calibrated radiance data set provided with the ACORN software.


2. With ENVI, open the Jasper Ridge data provide with ACORN. The default installed location is

c:\program files\ACORN6\Data\Jasper\jsp0rdn.



Load Band button. This image is shown in Figure 9-2. Figure 9-3 shows a set of extracted spectra from the

Jasper Ridge data. Table 9-1 gives the site, and location of these extracted spectra.

96

Figure 9-2. Jasper Ridge radiance image.











Data>ASCII.



97



Site Sample Line





Felt Lake 398 355

Starting ACORN

In this example ACORN Mode 4 will be used to perform spectral convolution on the Jasper radiance data set.


The ACORN Control Panel will appear as shown in Figure 9-4. ACORN operates based on control files that

provide the parameters and files for the ACORN algorithm.

98





1. From the ACORN Control Panel, select Mode 4. The Mode 4 control file editor will open.

2. For this example, use Open to open the Jasper Ridge Mode 4 tutorial control file. The default installation

location for this control file is C:\Program Files\ACORN6\Data\Jasper\jsp4.in


parameters necessary to perform the hyperspectral to multispectral convolution. Each parameter and file is

described below for this tutorial example.

99






C:\Program Files\ACORN6\Data\Jasper\jps4tm.

Image Spectral Calibration file: This is the spectral calibration file for the input image. The default location is

C:\Program Files\ACORN6\Data\Jasper\avr97.wvl.




file.


369.85 9.61 . . .

379.69 9.58

389.53 9.55

… …

2486.99 10.07

2496.90 10.05

2506.81 10.03

Hyperspectral Gain File: This is the gain file that converts the image file integer values to radiance in units of

(W/m^2/um/sr). For this tutorial example, the default installation location is C:\Program

Files\ACORN6\Data\Jasper\avr97.gain. This is a one column file that is multiplied by the image

integer values to convert the integers to the correct radiance units. Table 9-3 shows a subset of this file. This gain

factor is required to account changing gains across the spectrum before the convolution is applied.

100


0.2

0.2

0.2

…

0.1

0.1

0.1

Mulispectral Spectral Response File: This is a two column ASCII file containing the multispectral spectral

response functions. The first column is the wavelength in nanometers and the second column is the response value

for the multispectral band. The spectral responses for each band are contained in this one file with the end of each

band indicated by a negative value in the second column. The default location of the file for this tutorial is C:\Program Files\ACORN6\Data\Jasper\tm.rsp.

Table 9-4 shows a portion of the spectral response file.

Table 9-4. Mulispectral Spectral Response File.

428 0.0000

430 0.0021

432 0.0087

… …

504 1.0000

506 0.9609

508 0.914

… …

552 0.0003

554 0.0001

556 -0.0001

502 0.0000

504 0.0006

506 0.0057

Multispectral Gain File: This is the gain file that converts the convolved image radiance values to integer

values for the output file. For this tutorial example, the default installation location is C:\Program Files\ACORN6\Data\Jasper\Jsp4tm.gain

Table 9-5 shows a subset of this file. This gain factor is required to preserve the precision of the convolution in

the output integer data.

Table 9-5. Multispectral Gain File.

100

100

100

100

100

100

101









To execute ACORN Mode 4, click Save + Run on the ACORN control panel. A processing status box will

appear to indicate the progress of the atmospheric correction. When the convolution is complete the program will

report the elapsed time. Click Okay to return to the control file editor.


The principal result of ACORN Mode 4 is the convolved image.

1. With ENVI open the Jasper Ridge convolved image. The default location for the tutorial is c:\program

files\ACORN6\Data\Jasper\jsp4tm. Figure 9-6 shows the convolved image with bands 3, 4, 2 displayed

as Red, Green, Blue respectively.

2. Extract the convolved image spectra from the sites in table 9-1. These spectra are shown in Figure 9-7.

3. Compare the extracted convolved spectra with the input spectra in Figure 9-3. The Hyperspectral data set has

been converted to multispectral.

Figure 9-6. Convolved image following use of ACORN Mode 4.

102

Figure 9-7. Extracted spectra from Jasper Ridge data set after Mode 4 convolution








.diag1 In the same directory as the output file will be a jsp4tm.diag1. The diag1 file confirms the input

parameters used in the atmospheric correction and indicates problems encountered during the run.

.diag2 In the same directory as the output the jsp4tm.diag2 file will be created. This file contains selected

spectral outputs that may be helpful to understand problems with the convolution.


The default location for these files is c:\Program Files\ACORN6\Data\Cuprite\cup4.in


103


mode 4.

104

Chapter 10 Mode 5. Radiative transfer atmospheric correction of calibrated multispectral data. The following topics are covered in this chapter:

Description…………………………….……………………………………………...……….…… 105

Input Image Data………………...…………………………………………………………….……105

Starting ACORN……………………………………………………………………..….…………. 107

Reviewing and Editing the Control File…………………………………….……………..….…… 108

Running ACORN Mode 5…...………………………………………………………….…….…… 111

ACORN Mode 5 Results…...………………..…….…………………………………...………..… 111

Additional Output files…...……………………………………………………………..…….…… 113

Other Tutorial Examples…………………………………………………………………………… 113

105

Description

ACORN uses radiative transfer calculations to atmospherically correct multispectral data. With an input of

calibrated multispectral radiance data and baseline atmospheric parameters, ACORN produces an output of

apparent surface reflectance. An accurate spectral response functions file is required for this mode. The

multispectral data must be spectrally and radiometrically calibrated.

Figure 10-1 shows four radiance spectra from the example Jasper Ridge multispectral data set and the

atmospherically corrected spectra after ACORN mode 5.

0

20

40

60

80

100

120

140

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Rad

ian

ce (

W/m

^2

/um

/sr)

Target 1

Target 2

Target 3

Target 4

0

2000

4000

6000

8000

10000

12000

400 700 1000 1300 1600 1900 2200 2500

Wavelength (nm)

Ref

lect

ance

/10

00

0.

Target 1

Target 2

Target 3

Target 4

Figure 10-1: Left: radiance spectra for targets in the example Jasper Ridge multispectral data set. Right ACORN Mode 5 results showing the apparent reflectance spectra for the same targets

Input Image Data

For this example of ACORN Mode 5 the AVIRIS data set acquired over Jasper Ridge, California convolved to the

Landsat Thematic Mapper multispectral bands is used. This data set must be generated by following the Mode 4

example in the tutorial for the Jasper Ridge data.

Once the data have been convolved, begin by examining the Jasper Ridge convolved multispectral calibrated

radiance data.


2. With ENVI, open the Jasper Ridge convolved to Landsat TM data. The default location is

c:\program files\ACORN6\Data\Jasper\jsp5tm.





106

Figure 10-2. Jasper Ridge Multispectral convolved radiance image.











Data>ASCII.



107

Figure 10-3. Extracted spectra from Jasper Ridge convolved mulitpsectral radiance image.


Site Sample Line





Felt Lake 398 355



radiance values fall in a range of efficiently stored integers. ACORN Mode 5 is designed to correct for these

illumination and atmospheric effects in Multispectral data.

Starting ACORN

In this section ACORN Mode 5 will be used to atmospherically correct the Jasper Ridge calibrated multispectral

radiance data set.


The ACORN software user interface will appear as shown in Figure 10-4.

108





1. Select mode 5 to open the mode 5 control file editor.

2. Select Open to open the Jasper Ridge mode 5 control file. The default location for this file is

C:\Program Files\ACORN6\Data\Jasper\jsp5.in.

3. Examine the ACORN Mode 5 control file parameter entry panel shown in Figure 10-5. These are the files and

parameters necessary to perform a Mode 5 atmospheric correction. Each parameter and file is described for the

Jasper Ridge example.

109

Figure 10-5. ACORN 4 Mode 5 control file parameter entry window.

Input Image File Name: This is the name of the input radiance image file. For this example, the default location

is

C:\Program Files\ACORN6\Data\Jasper\jsp4tm.

Output Reflectance Image File Name: This is the name of the output image to be generated by ACORN Mode

5. For this example, the default location is

C:\Program Files\ACORN6\Data\Jasper\jsp5tmrfl.

Image Spectral Response File: This is a two column ASCII file containing the multispectral spectral response

functions. The first column is the wavelength in nanometers and the second column is the response value for the

multispectral band. The spectral responses for each band are contained in this one file with the end of each band

indicated by a negative value in the second column. The default location of the file for this tutorial is C:\Program Files\ACORN6\Data\Jasper\tm.rsp



428 0.0000

430 0.0021

432 0.0087

110

434 0.0154

436 0.0221

… …

500 0.9791

502 0.9883

504 1.0000

506 0.9609

508 0.914

… …

552 0.0003

554 0.0001

556 -0.0001

502 0.0000

504 0.0006

506 0.0057

Gain File: This is the gain file that converts the image integer values to radiance (W/m^2/um/sr). For this

tutorial example, the default installation location is

C:\Program Files\ACORN6\Data\Jasper\Jsp5tm.gain.



0.01

0.01

0.01

0.01

0.01

0.01

Offset File: This is the offset file that corrects for any offset in the image file integer values in the conversion to

radiance in units of (W/m^2/um/sr). For this example, the default location is C:\Program Files\ACORN6\Data\Jasper\jsp5tm.off

This is a one column file that is added to the radiance values. Table 10-4 shows a subset of this file.

Table 10-4. Contents of the Jasper Ridge multispectral offset file.

0.0

0.0

0.0

0.0

0.0

0.0



Ridge data set are 6 bands, 512 lines, and 614 sample with 0 byte offset.



111





approximate latitude of the Jasper Ridge data set is 37.18.



of the Jasper Ridge data set is -122.3. East longitude is positive.

Image Date: This is the date of image acquisition in day, month, and year format. This example data set was

acquired on the 3rd

of April, 1997.

Image Time: This is the average time of acquisition in hours, minutes, and seconds. The time must be



Image Elevation: This is the average elevation of the surface in the input image in meters. The approximate

elevation of the Jasper Ridge example data set is 200 m.

Image Acquisition Altitude: This is the altitude of the instrument that acquired the image data. The altitude of

acquisition of this example Jasper Ridge data set is 20 km.




Fixed Water Vapor: A water vapor amount must be specified in units of precipitable mm of water vapor. For

this example a value of 15 mm is used.






will report the elapsed time. Click Okay to return to the control file editor.


The primary result of ACORN Mode 5 is the atmospherically corrected image.

1. With ENVI open the Jasper Ridge reflectance image. The location specified in the tutorial control file is

c:\program files\ACORN6\Data\Jasper\jsp5tmrfl.


112



precision of the measurement and allows the data to be stored efficiently.


atmosphere have been compensated.

Figure 10-6. Atmospherically corrected Jasper Ridge image following implementation of ACORN mode 5.

Figure 10-7. Extracted spectra from Jasper Ridge data set after mode 1 atmospheric correction.

113








.diag1 In the same directory as the output file will be a jsp5tmrfl.diag1. The diag1 file confirms the input


.diag2 In the same directory as the output the jsp5tmrfl.diag2 file will be created. This file contains selected


Other Tutorial Examples Additional tutorial examples are provided with ACORN for the Cuprite AVIRIS and Cuprite Hymap data. The

default location for these files is c:\Program Files\ACORN6\Data\Cuprite\cup5.in



mode 5.

114

Chapter 11 Mode 5.5. Radiative transfer atmospheric correction of calibrated multispectral data with water vapor image. The following topics are covered in this chapter:

Description…………………………….…………………………………………………...….…… 115

Input Image Data………………...…………………………………………………………….……115

Starting ACORN……………………………………………………………………….…..………. 118

Reviewing and Editing the Control File…………………………………….………………...…… 119

Running ACORN Mode 5.5...………………………………………………………..……….…… 122

ACORN Mode 5.5 Results…...……………..…….……………………………………………..… 122

Additional Output files…...…………………………………………………………..……….…… 124

Other Tutorial Examples…………………………………………………………………………… 124

115

Description

ACORN uses radiative transfer calculations to atmospherically correct multispectral data. With an input of

calibrated multispectral radiance data and baseline atmospheric parameters, ACORN produces an output of

apparent surface reflectance. An accurate spectral response functions file is required for this mode. The

multispectral data must be spectrally and radiometrically calibrated. For this mode an image of atmospheric water

vapor is used to correct for water vapor on a pixel by pixel basis. The water vapor image must be registered and

sampled to match the multispectral data set and be of the same integer format.

Input Image Data

For this example of ACORN Mode 5.5 the AVIRIS data set acquired over Jasper Ridge, California convolved to

the Landsat Thematic Mapper multispectral bands is used. This data set must be generated by following the

Mode 4 example in the ACORN tutorial.

Once the multispectral data have been generated, begin by examining the Jasper Ridge convolved multispectral

calibrated radiance data.

1. Start the ENVI software on your computer

2. Open the Jasper Ridge data generated with ACORN. The default location is






116












Data>ASCII.



117

Figure 11-2. Extracted spectra from Jasper Ridge convolved mulitpsectral radiance image.


Site Sample Line





Felt Lake 398 355



radiance values fall in a range of efficiently stored integers. ACORN Mode 5.5 is designed to correct for these

illumination and atmospheric effects in Multispectral data.

ACORN6 Mode 5.5 requires input of a water vapor image in units of precipitable microns that matches the

multispectral data set. Precipitable microns refer to the thickness of the liquid water layer if the atmosphere was

condensed. Figure 11-3 shows the water vapor image for the Jasper Ridge data set. Water vapor varies from

4700 to 8000 precipitable microns in the image. It is up to the user to acquire or generate the water vapor image.

118

Figure 11-3. Water vapor image corresponding to the Jasper Ridge data set.

Starting ACORN In this section ACORN Mode 5.5 will be used to atmospherically correct the Jasper Ridge calibrated multispectral

radiance data set.



119

Figure 11-4. ACORN software user interface


1. Select mode 5.5. This opens the mode 5.5 control file editor.

2. Select the Jasper Ridge mode 5.5 control file. The default location for this file is C:\Program

Files\ACORN6\Data\Jasper\jsp5_5.in. This control file appears in the Mode 5.5 control file editor

box.

3. Examine the ACORN Mode 5.5 control file parameter entry panel shown in Figure 11-5. These are the files

and parameters necessary to perform a Mode 5.5 atmospheric correction. Each parameter and file is described for

the Jasper Ridge example.

120

Figure 11-5. ACORN 4 Mode 5.5 control file parameter entry window.

Input Image File Name: This is the name of the input radiance image file. For this example, the default location

is

C:\Program Files\ACORN6\Data\Jasper\jsp4tm.

Output Reflectance Image File Name: This is the name of the output image to be generated by ACORN Mode

5.5. For this example, the default location is

C:\Program Files\ACORN6\Data\Jasper\jsp5_5tmrfl.






121


428 0.0000

430 0.0021

432 0.0087

434 0.0154

436 0.0221

… …

500 0.9791

502 0.9883

504 1.0000

506 0.9609

508 0.914

… …

552 0.0003

554 0.0001

556 -0.0001

502 0.0000

504 0.0006

506 0.0057

Gain File: This is the gain file that converts the image integer values to radiance (W/m^2/um/sr). For this

tutorial example, the default installation location is C:\Program Files\ACORN6\Data\Jasper\Jsp5tm.gain



0.01

0.01

0.01

0.01

0.01

0.01

Offset File: This is the offset file that corrects for any offset in the image file integer values in the conversion to

radiance in units of (W/m^2/um/sr). For this example, the default location is

C:\Program Files\ACORN6\Data\Jasper\jsp5tm.off.


Table 11-4. Contents of the Jasper Ridge multispectral offset file.

0.0

0.0

0.0

0.0

0.0

0.0

Water Vapor Image: This is the name of the independently provide water vapor image to be used by ACORN

Mode 5.5. For this example, the default location is

C:\Program Files\ACORN6\Data\Jasper\ jsp1_5rfla2_wtrv.

122



Ridge data set are 6 bands, 512 lines, and 614 sample with 0 byte offset.





Image Elevation: This is the average elevation of the surface in the input image in meters. The approximate

elevation of the Jasper Ridge example data set is 200 m.

Image Acquisition Altitude: This is the altitude of the instrument that acquired the image data. The altitude of

acquisition of this example Jasper Ridge data set is 20 km.








approximate latitude of the Jasper Ridge data set is 37.18.



of the Jasper Ridge data set is -122.3. East longitude is positive.

Image Date: This is the date of image acquisition in day, month, and year format. This example data set was

acquired on the 3rd

of April, 1997.

Image Time: This is the average time of acquisition in hours, minutes, and seconds. The time must be



Running ACORN Mode 5.5

To execute ACORN mode 5.5, click Save + Run on the ACORN control panel. A processing status box will


program will stop and indicated the elapsed time. Click Okay to return to the control file editor.

ACORN Mode 5.5 Results

The primary result of ACORN Mode 5.5 is the atmospherically corrected image.

1. Using ENVI, open the Jasper Ridge reflectance image. The location specified in the tutorial control file is c:\program files\ACORN6\Data\Jasper\jsp5tmrfl

123




precision of the measurement and allows the data to be stored efficiently.

3. Compare the extracted reflectance values with the input radiance spectra. The effects of the solar source and

atmosphere have been compensated. These reflectance values are more accurate because the appropriate water

vapor has been used to correct the measured radiance at each point in the image.

Figure 11-6. Atmospherically corrected Jasper Ridge image following implementation of ACORN mode 5.5.

Figure11-7. Extracted spectra from Jasper Ridge data set after mode 5.5 atmospheric correction.

124



.in This is the control file that provides the files and parameters that are used by ACORN. The file jsp5_5.in was


.eco In the same file location of the jsp5_5.in file will be a jsp5_5.in.eco. This file is an echo of the input file as

it is being interpreted by the ACORN software. If problems are detected interpreting the control file these will be


.diag1 In the same directory as the output file will be a jsp5_5tmrfl.diag1. The diag1 file confirms the input


.diag2 In the same directory as the output the jsp5_5tmrfl.diag2 file will be created. This file contains selected



default location for these files is c:\Program Files\ACORN6\Data\Cuprite\cup5_5.in


mode 5.5.

125

Chapter 12 Mode 6. Single spectrum enhancement of a multispectral atmospheric correction. The following topics are covered in this chapter:

Description…………………………….………………………………………….………..….…… 126

Input Image Data………………...…………………………………………………………….……126

Starting ACORN………………………………………………………...………………...……….. 127

Reviewing and Editing the Control File……………………………….…………………..….…… 128

Running ACORN Mode 6…………………………..……………………………………..….…… 131

ACORN Mode 6 Results…...…………………….……………………………...…..………..…… 131

Additional Output Files…...……………………………………………...…………..……….…… 132

Other Tutorial Examples…...……………………………………………..…………..……….…… 133

126

Description

ACORN Mode 6 uses a set of bands extracted from an atmospherically corrected multispectral data set and an

accurately known spectrum for the same target. With the band values and spectrum, the full atmospherically

corrected multispectral data set is then corrected to the accuracy of the known spectrum. ACORN accurately and

automatically convolves the known spectrum to the spectral characteristics of the multispectral data set for this

atmospheric correction enhancement.

Input Image Data

For this example the AVIRIS data set acquired over Jasper Ridge that has been convolved to Landsat Thematic

Mapper bands and atmospherically corrected is used. The convolved and atmospherically corrected data set are

created by using ACORN modes 4 and 5 as described in proceeding chapters of this tutorial.

Once the input data set have been generated, begin by examining the Jasper Ridge convolved multispectral

atmospherically corrected data.


2. Open the Jasper Ridge data. The default location is

c:\program files\ACORN6\Data\Jasper\jsp5tmrfl.





Figure 12-1. Atmospherically corrected Jasper Ridge image following implementation of ACORN mode 4 and

Mode 5.

L

127











Data>ASCII.



Figure 12-2. Extracted spectra from Jasper Ridge data set after Mode 4 convolution to multispectral data and

Mode 5 atmospheric correction.


Site Sample Line





Felt Lake 398 355

Starting ACORN

In this example ACORN Mode 6 will be used to enhance the atmospheric correction for the Jasper Ridge Mode 4

and Mode 5 generated reflectance data set.


128


Figure 12-3. ACORN software user interface

Reviewing and Editing the Control File Note: Except for the examples provided with ACORN you must obtain the appropriate calibration files and


1. From the ACORN software user interface, select mode 6.

2. For this example, use Open to open the Japser Ridge Mode 6 example control file. The default installation

location for this control file is C:\Program Files\ACORN6\Data\Jasper\jsp6.in

129


parameters necessary to perform the single spectrum enhancement to the atmospheric correction. Each parameter

and file is described below for this tutorial example.



C:\Program Files\ACORN6\Data\jasper\jsp5tmrfl.



C:\Program Files\ACORN6\Data\Jasper\jps6tmrfl.






130


428 0.0000

430 0.0021

432 0.0087

434 0.0154

436 0.0221

… …

500 0.9791

502 0.9883

504 1.0000

506 0.9609

508 0.914

… …

552 0.0003

554 0.0001

556 -0.0001

502 0.0000

504 0.0006

506 0.0057

Image Target Spectrum File: This is a single column file with the extracted spectrum from jsp5tmrfl that

corresponds to the location where a accurate surface reflectance spectrum has been acquired. The default location

of the file for this tutorial is C:\Program Files\ACORN6\Data\Jasperr\jsp6tm.ext

Table 12-3 shows a portion of this file. The units of this file are reflectance * 10000

Table 12-3. Jasper Image Target Reflectance File.

777.59

1171.09

1498.30

2196.30

3567.10

3194.80

Measured Target Reflectance Spectrum File : This is the measured reflectance for the same target as the

Image Target Reflectance Spectrum File. The default location of the file for this tutorial is C:\Program Files\ACORN6\Data\Jasper\jps6tm.meas


reflectance is multiplied by 10000 to have the same scaling as the Image Target Reflectance Spectrum File. Table

12-4 shows a portion of this file.

Table 12-4. Contents of the Measured Target Reflectance Spectrum File.

350 250.02

351 251.14

352 253.81

… …

2498 2573.75

2499 2574.65

2500 2575.773

131










to indicate the progress of the single spectrum enhancement. When the processing is complete the program will

return to the ACORN Control Panel.


The principal result of ACORN Mode 6 is the single spectrum enhanced reflectance image.

1. With ENVI open the Jasper Ridge single spectrum enhanced image. The default location for the tutorial is c:\program files\ACORN6\Data\Jasper\jsp6tmrfl



Figure 12-6.

3. Compare the extracted single spectrum enhanced spectra with the input reflectance spectra in Figure 6-2. The

radiative transfer code based reflectance has been enhanced to the accuracy of the surface reflectance spectrum of

the target area.

132

Figure 12-5. Multispectral Single Spectrum Enhanced image following use of ACORN Mode 6.

Figure 12-6. Extracted spectra from Jasper Ridge data set after Mode 6 Single Spectrum Enhancement.





133












mode 6.

134

Chapter 13 Mode 7. Atmospheric correction by the empirical line method for multispectral data. The following topics are covered in this chapter:

Description…………………………….……………………………………………………..…….. 135

Input Image Data………………...…………………………………………………………….……135

Starting ACORN……………………………………………………...………………………...….. 137

Reviewing and Editing the Control File…………………………….…………………………...… 137

Running ACORN Mode 7…………………………..………………………………...…...….…… 141

ACORN Mode 7 Results…...……………………………………………..…………..…….…...… 141

Additional Output Files…...…………………………………………...……………..…….……… 142

Other Tutorial Examples…...………………………………………..………………..…….……… 142

135

Description

ACORN uses a dark and bright extracted target spectrum from a multispectral data set and corresponding

measured ground spectra for these targets. These input pair of band values and pair of known spectra are used to

implement an empirical line atmospheric correction. ACORN accurately and automatically convolves the known

spectra to the spectral characteristics of the multispectral data set for this mode of atmospheric correction. This

mode is less dependent on the accuracy of the radiometric calibration of the multispectral data. In some cases raw

integer values can be used.

Input Image Data

For this example the AVIRIS data set acquired over Jasper Ridge that has been convolved to Landsat thematic

mapper bands is used. This convolution is achieved by using ACORN mode 4 as described in an earlier chapter

of this tutorial.

Once the input file has been generated, begin by examining the Jasper Ridge convolved multispectral data.


2. Open the Jasper Ridge data. The default location is









L

136









Data>ASCII.



Figure 13-2. Extracted spectra from Jasper Ridge convolved multipsectral radiance image.


Site Sample Line





Felt Lake 398 355



radiance values fall in a range of efficiently stored integers. ACORN mode 7 is designed implement an empirical

line atmospheric correction.

137

Starting ACORN

In this example ACORN Mode 7 will be used to implement an empirical line correction on the Jasper Ridge data

set that has been convolved to the Landsat Thematic Mapper multispectral bands.



Figure 13-3. ACORN software user interface.


1. From the ACORN Control Panel, select Mode 7. This will start the mode 7 control file editor.

2. For this example, use Open to open the Japser Ridge Mode 7 example control file. The default installation

location for this control file is

138

C:\Program Files\ACORN6\Data\Jasper\jsp7.in



tutorial example.



C:\Program Files\ACORN6\Data\jasper\jsp4tm.



C:\Program Files\ACORN6\Data\Jasper\jps7tmrfl.






139


428 0.0000

430 0.0021

432 0.0087

434 0.0154

436 0.0221

… …

500 0.9791

502 0.9883

504 1.0000

506 0.9609

508 0.914

… …

552 0.0003

554 0.0001

556 -0.0001

502 0.0000

504 0.0006

506 0.0057

Dark Target Image Value File: This is a single column file with the extracted values from the image jsp4tm

that corresponds to the location where a accurate surface reflectance spectrum of a dark target has been acquired.

The default location of the file for this tutorial is C:\Program Files\ACORN6\Data\Jasperr\jsp7tmd.ext

Table 13-3 shows a portion of this file. The units are the image value units.

Table 13-3. Jasper Image Dark Target File.

6782.2

5066.2

3874.5

3076.0

507.2

154.9

Dark Target Reflectance Spectrum File : This is the measure reflectance for the dark target. The default

location of the file for this tutorial is C:\Program Files\ACORN6\Data\Jasper/jps7tmd.meas


reflectance is multiplied by 10000 so that the output file will have this scale.

140

Table 13-4. Contents of the Dark Target Reflectance Spectrum File.

350 400.03

351 402.22

352 407.38

… …

2498 686.61

2499 685.99

2500 685.38

Bright Target Image Value File: This is a single column file with the extracted values from the image jsp4tm

that corresponds to the location where a accurate surface reflectance spectrum of a bright target has been acquired.

The default location of the file for this tutorial is C:\Program Files\ACORN6\Data\Jasperr\jsp7tmd.ext

Table 13-5 shows a portion of this file. The units are the image value units.

Table 13-5. Jasper Image Bright Target File.

23062.2

22706.1

20291.2

14731.5

2494.4

757.9

Bright Target Reflectance Spectrum File : This is the measure reflectance for the bright target. The default

location of the file for this tutorial is C:\Program Files\ACORN6\Data\Jasper/jps7tmb.meas


reflectance is multiplied by 10000 so that the output file will have this scale. Table 13-6 shows a portion of this

file

Table 13-6. Contents of the Bright Target Reflectance Spectrum File.

350 1920.492

351 1928.312

352 1946.853

… …

2498 3249.936

2499 3246.004

2500 3242.096








141




will stop and indicate the elapsed time. Click Okay to return to the control file editor.


The principal result of ACORN Mode 7 is the empirical line reflectance image.

1. With ENVI open the Jasper Ridge empirical line reflectance image. The default location for the tutorial is c:\program files\ACORN6\Data\Jasper\jsp7tmrfl

Figure 13-5 shows the reflectance image with bands 3, 4,2 displayed as Red, Green, Blue respectively.


Figure 13-6.

3. Compare the extracted single spectrum enhanced spectra with the input radiance spectra in Figure 13-2. The

effects of the illumination and the atmosphere have been suppressed.

Figure 13-5. Multispectral empirical line image following use of ACORN Mode 7.

142

Figure 13-6. Extracted spectra from Jasper Ridge data set after Mode 7 empirical line.
















mode 7.

acorn6 tut 080101 - 國立臺灣大學homepage.ntu.edu.tw/~choying/acorn_tut.pdf · 2017-02-16 ·...

Documents