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©

2001 Analytical Imaging and Geophysics, LLC.All rights reserved. ACORN is an AIG product developed by ImSpec LLC with MODTRAN licensed technology

ACORN Version 3.12January, 2001 Edition

Copyright © ImSpec, LLC.All Rights Reserved

ACORN Tutorial

Stand Alone Version

ACORN Tutorial

3

Tutorial:

ACORN Modes 1 and 2 Atmospheric Correction (Stand-Alone)

The following topics are covered in this tutorial:

Radiance Data Preview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Review or Prepare ACORN Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Running ACORN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Create and/or Edit ACORN Control File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Standard ACORN Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Mode 1-Specific Parameters and Execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Mode 2-Specific Parameters and Execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Atmospheric Correction Quality Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4

Tutorial: ACORN Modes 1 and 2 Atmospheric Correction (Stand-Alone)

Files Used in this Tutorial

ACORN Tutorial

Overview of This Tutorial

This tutorial is designed to introduce you to basic atmospheric correction of hyperspectral data using the Atmospheric COrrection Now (ACORN) software. Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data from Cuprite, Nevada, are used to investigate the details of atmospheric correction of hyperspectral data. We will examine AVIRIS Radiance Data, correct these data to apparent reflectance using several ACORN options, and evaluate the atmospheric correction performance.

Note

This tutorial describes only ACORN Mode 1 and Mode 2 applications. See the

ACORN User’s Guide

for additional information about ACORN modes, required parameters, file formats, and

general information about atmospheric correction.


The files listed below are required to run this exercise. Generated files listed below are created when the tutorial is run.

Required Files

Generated Files

cup0rdn Cuprite 1997 Radiance Data

cup0rdn.hdr ENVI Header for Above

cup97av.wvl ACORN Wavelength File

cup1.gain ACORN Gain File

cup1.off ACORN Offset File

cup1.in ACORN Control File (Baseline Mode 1)

cup1v1.in ACORN Control File (Visibility On)

cup1a1.in ACORN Control File (Type 1 Suppression)

cup1a2.in ACORN Control File (Type 1&2 Suppression)

cup1a3.in ACORN Control File (Type 1,2,3 Suppression)

cup2.in ACORN Control File (Mode 2)

cup2.meas Known Cuprite Measured Reflectance Spectrum

cup2.ext Cuprite Image Spectrum corresponding to Spectrum Above

Cup1rfl Cuprite ACORN Reflectance (No artifact suppression)

cup1rfl.hdr ENVI Header for Above

cup1a1.ref Cuprite ACORN Reflectance (Type 1artifact suppression)


5

ACORN Tutorial


cup1a1.hdr ENVI Header for Above

cup1a2.ref Cuprite ACORN Reflectance (Type 1 and Type 2 artifact suppression)

cup1a2.hdr ENVI Header for Above

cup1a3.ref Cuprite ACORN Reflectance (Type 1, Type 2, and Type 3 artifact suppression)

Cup1a3.hdr ENVI Header for Above

Cup1v1.ref Cuprite ACORN Reflectance (Types 1,2,3 artifact suppression, plus visibility estimate)

cup1v1.hdr ENVI Header for Above

cup1eff Cuprite ACORN Reflectance (with EFFORT correction)

cup1eff.hdr ENVI Header for Above

cup2rfl Cuprite ACORN Reflectance (with Mode 2 Single Spectrum Enhancement)

cup2rfl.hdr ENVI Header for Above

6


Radiance Data Preview

ACORN Tutorial

ACORN Overview

The Atmospheric COrrection Now (ACORN) software 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.

The hyperspectral data must be spectrally and radiometrically calibrated 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 Guide

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.

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.

The Atmospheric CORection Now (ACORN) software has been developed to offer a range of atmospheric correction capabilities. These capabilities include:

Mode 1. Radiative transfer atmospheric correction of calibrated hyperspectral data

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 6. Single spectrum enhancement of a multispectral atmospheric correction

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


Start ENVI by double clicking on the ENVI icon or alternatively start another image processing package to display the Cuprite AVIRIS radiance data and extract radiance spectra as described. The instructions below assume that you are running ENVI 3.4.


7

ACORN Tutorial


Display Radiance Image(s)

Display the Cuprite AVIRIS radiance data. These data are scaled to integer data utilizing a gain and offset.

1. Select

File > Open Data File

from the ENVI Main menu to open the file.

2. Navigate to the

Cuprite

subdirectory (usually installed by default in

c:\program files\ACORN\Data\Cuprite

).

3. Select the file

cup0rdn

in the Enter Data Filenames dialog and click “Open”.

The Available Bands List dialog automatically appears on your screen when the file is opened with a list of the bands and wavelengths.

4. Scroll down the band list using the scroll bar on the right edge of the dialog, click on Band 31, and then click “Load Band” to display the image.

Extract Radiance Spectra

ENVI’s “Z” profile capabilities provide integrated spectral analysis.

1. From the displayed image, select

Tools > Profiles > Z Profile (Spectrum)

in the Main Image Display menu bar to start a spectral profile.

The spectrum for the current cursor location will be plotted in a plot window. A vertical line on the plot is used to mark the wavelength position of the currently displayed band.

Figure 1: The ENVI Available Bands List with the Cuprite AVIRIS radiance bands

8



ACORN Tutorial

2. Move the cursor position in the Main or Zoom window by clicking the left mouse button in the window and the spectrum for the position under the cursor will be extracted and plotted for the new location.

3. Browse the spectral profile by clicking and holding the left mouse button outside the Zoom indicator box and dragging the box across the image.

The spectrum will be updated as the Zoom indicator box moves. Note that the spectra you are viewing are radiance—not reflectance spectra, as you are currently working with Cuprite radiance data. Also note how similar the radiance spectra appear. The overall shape of the spectra is caused by the typical combined solar/atmospheric response.

Extracting Spectra

To extract selected image radiance spectra for specific targets in the AVIRIS radiance data:

1. From the Main Display window menu bar, select

Tools > Pixel Locator

.

2. Position the zoom window over Stonewall Playa, centered around the pixel at sample 590, line 570 by entering these pixel coordinates in the Pixel Locator and clicking “Apply”.

3. Extract the radiance spectrum for this location using the Z-Profile function and save in a new plot window.

A. Select

Options > New Window > Blank

to open a new plot window to contain saved image spectra.

B. Display the labels in your new plot window by clicking the right mouse button anywhere in the spectral profile to display the spectrum name (plot label) to the right of the plot.

C. Click and hold the left mouse button on the first character of the spectrum name, drag the name to the new plot window, and release the mouse button.

Figure 2: Cuprite Radiance Image, band 31 (0.67 micrometers) (left), and a radiance spectrum (right)


9

ACORN Tutorial


D. Select a new spectrum from the image by moving the current pixel location in either the Main Display or Zoom window and repeat the drag-and-drop process to build a collection of spectra in the new plot window.

E. Once you have several plots in the plot window, select

Options > Stack Data

in the plot window. The spectra will be offset vertically to allow interpretation.

F. To change the color and line style of the different spectra, select

Edit > Data Parameters

in the new plot window. Each spectrum is listed by name/location in the Data Parameters dialog. Select a line and change its name and color, alter other properties as desired. When completed, click “Cancel” to close the dialog.

4. Now extract radiance spectra for the following locations listed in Table 1 and load into the same plot window for comparison.

Compare AVIRIS Radiance Spectra and Spectral Library Spectra

Now compare apparent reflectance spectra from the image to selected library reflectance spectra.

1. Select

Spectral > Spectral Libraries > Spectral Library Viewer

from the ENVI Main Menu.

2. When the Spectral Library Input File dialog appears, click “Open Spectral Library” and select

JPL1.SLI

from the list then click “OK”.

3. Click on the file name and click “OK” to open the Spectral Library Viewer dialog. Select

Options > Edit (x, y) Scale Factors

in the dialog and enter a value of 10,000 for the “Y Data Multiplier.”

4. Plot the following spectra in the Spectral Library Viewer window by clicking on the appropriate mineral name in the list of spectra:

ALUNITE SO-4A

BUDDINGTONITE FELDS TS-11A

CALCITE C-3D

KAOLINITE WELL ORDERED PS-1A

Location Name Sample Line

Stonewall Playa 606 293

Varnished Tuff 465 291

Silica Cap 546 247

Opalite Zone w/Alunite 554 272

Strongly Argillized Zone w/Kaolinite

526 319

Buddingtonite Zone 472 235

Calcite 295 348

Table 1: AVIRIS pixel locations for materials of interest at Cuprite, Nevada.

10


Review or Prepare ACORN Files

ACORN Tutorial

5. It is helpful to change the X-Axis scale by selecting

Edit > Plot Parameters

and entering the values 2.0 and 2.5 for the range. This will allow direct visual comparison of radiance and spectral library reflectance (though the Y-axes will not have the same units).

6. Click “Cancel” to close the Plot Parameters dialog.

7. Visually compare and contrast the corresponding AVIRIS radiance spectra with the laboratory measurements for alunite, buddingtonite, calcite, and kaolinite.

Note how difficult it is to visually identify the minerals by comparing features in the radiance spectra to absorption features shown in the laboratory spectra.

Note the effect of the superimposed convex-upward solar-atmospheric signature in the AVIRIS radiance data on visual identification.


ACORN requires supporting files in specific formats. For ACORN Mode 1, these include a calibrated radiance image file, a spectral calibration file (wavelength and FWHM), a gain file, and an offset file. These files must be prepared in advance of the ACORN run and can either be obtained from the hyperspectral instrument manufacturer, built from scratch, or modified from existing ACORN files. For the purposes of this exercise, these files are provided, however, you should examine these using a text editor for familiarity. Additional information about the required files is summarized below and also in the ACORN User’s Guide.

Calibrated Radiance File

ACORN only works with image files that are stored as 16 bit integers in either BIP or BIL format. The format of the integers may be big endian (NETWORK) or little endian (HOST). The integer format little endian or big endian must be known and specified by the user. Data written by PCs are often little endian integer format. The image interleave must be Band Interleaved by Pixel (BIP) or Band Interleaved by Line (BIL) and it must be specified by the user. If the input radiance image has an ENVI header, ACORN copies the ENVI header information to an output ENVI header to allow easy use of the reflectance corrected output file in ENVI.

Spectral Calibration File

The ACORN spectral calibration file must be an ASCII file with two columns. The first column is the wavelength position of each band in units of nanometers. The second column is the full-width-half-maximum (FWHM) of the Gaussian function that describes the spectral response of each band.

1. Open the file

cup1.wvl

in the

Cuprite

directory and examine the spectral calibration for the

cup0rdn

AVIRIS image data.

Table 2: Portion of an example spectral calibration file for ACORN Mode 1. The first column is wavelength center position of each band in nanometers

and the second column is the FWHM of the Gaussian spectral response of each band.

369.85 9.61 . . .

379.69 9.58

389.53 9.55

399.37 9.53


11

ACORN Tutorial


Gain File

The ACORN gain file is an ASCII file with one column with a value for each image band. The values in the gain file are the real numbers that convert the image integer values to radiance in units Watts/meter

2

/micron/steradian (W/m

2

/µm/sr). This file can usually be constructed from information provided with the radiometrically calibrated image data set. Typically, some form of gain file is provide with the image data set that can be modified to give the correct conversion of image integer values to radiance in units of (W/m

2

/µm/sr). Table 3 shows a portion of a gain file for ACORN Mode 1.

2. Open the file

cup1.gain

in the

Cuprite

directory using a text editor and examine the spectral calibration for the

cup0rdn

AVIRIS image data.

409.21 9.50

. .

. .

. .

2467.16 10.11

2477.08 10.09

2486.99 10.07

2496.90 10.05

2506.81 10.03 . . .

Table 3: This is a portion of a gain file for ACORN Mode 1. The values in this file convert the image integer values to radiance in units of (W/m2/µm/sr).

0.2

0.2

0.2

0.2

0.2

.

.

.

0.1

0.1

0.1

0.1

0.1

12


Running ACORN

ACORN Tutorial

Offset File

The ACORN offset file is an ASCII file with one column with a value for each image band. The values in this offset file are the real numbers that are added to the image radiance values after the gain file has been applied. The units of the offset file are (W/m2/

µ

m/sr). For most data sets the offset file values will be 0.0. Table 4 shows a portion of a gain file for ACORN Mode 1.

3. Open the file

cup1.off

in the

Cuprite

directory using a text editor and examine the offsets for the

cup0rdn

AVIRIS image data.

Running ACORN

Figure 3 shows the ACORN Control Panel dialog after starting a registered version of ACORN. The top portion of this window provides option buttons to create a “New” control file, “Open” an existing control file, “Edit” the control file, or “Save” a control file. The processing mode, output reflectance file, and available disk space are shown in the bottom portion of the dialog when a control file has been designated and saved.

1. Start ACORN by double clicking on the ACORN icon or finding ACORN under the

Start > Programs

option on your Windows Menu Bar.

The ACORN Control Panel dialog will appear.

Table 4: This is a portion of an offset file for ACORN mode 1. The values in this file are added to the image radiance values (W/m2/µ

m

/sr). The units of the offset file values are (W/m2/µ

m

/sr).

0.0

0.0

0.0

0.0

0.0

.

.

.

0.0

0.1

0.1

0.1


13

ACORN Tutorial

Create and/or Edit ACORN Control File

2. Examine the dialog and the different available options.

Create and/or Edit ACORN Control File

An ACORN Control File is required for each ACORN mode to be run. It can be created in a text editor or created and edited using the control file editing capabilities built into ACORN.

Note

If the “New” option is selected in the ACORN Control Panel dialog, ACORN will prompt for the control file type based upon the ACORN mode. This allows easy entry of control file parameters. If a control file already exists, however, it is easier to select the OPEN option to open and allow editing

of existing information rather than entering information from scratch.

1. Click on the “OPEN” button in the ACORN Control Panel dialog to allow selection of the ACORN Control File.

2. Navigate to the

Cuprite



) and select the file

cup1.in

and click “Open.”

3. Click on the “Edit” button in the ACORN Control Panel to start the Edit Control File dialog.

Figure 3: Initial ACORN Control Panel dialog

14


Standard ACORN Parameters

ACORN Tutorial

• The user can interactively change values in this dialog when running with their own hyperspectral data. Simply change the desired values then click “OK.” Be sure to click on the “Save “button in the ACORN Control Panel dialog to save the changed parameters.

Note

You shouldn’t have to change any values for the purposes of this exercise. You may, however, have to change the path for all of the input files to correspond to your ACORN install directory if it is different from the default


. Also check to be sure that the integer format of your input image is correct in the Control File or ACORN will not execute

properly.

Standard ACORN Parameters

All ACORN modes utilize common required image parameters. These include the input and output image filenames; and the image descriptors integer format (byte order), image format (interleave), number of samples, number of lines, number of bands, and offset (header offset). These are shown at the top of the Edit Control File dialog (Figure 4).

1. Review the standard ACORN parameters at the top of the ACORN Edit Control File dialog.

Figure 4: The ACORN Edit Control File dialog


15

ACORN Tutorial

Mode 1-Specific Parameters and Execution


Each ACORN mode also has specific parameters that only pertain to the selected mode. For Mode 1 these include the Image Spectral Calibration File (wavelengths and FWHM); the Gain File; the Offset File; and hyperspectral dataset-specific parameters such as the Image Latitude and Longitude, Image Acquisition Date and Time, and the Image Acquisition Altitude and Mean Elevation of the site. Other required parameters include the spectral regions to use for water vapor determination (None, 940nm, 1140nm, or Both), a fixed water vapor to use if not calculated from the data, and a fixed atmospheric visibility to use if not calculated from the data. The final four parameters determine how ACORN executes. One option is provided to calculate atmospheric visibility and three artifact suppression modes are offered.

2. Review the ACORN Mode 1-specific parameters in the ACORN Edit Control File dialog.

Note

This exercise uses both the 940 and 1140nm bands to estimate water vapor. Normally we would only use the 1140 band, for atmospheric correction over geologic targets, because most typical geologic targets contain minerals with Fe+3 (Iron Oxide), which has a significant absorption feature near 940nm. Inclusion of the 940nm band for estimation of water vapor in terrains with iron oxide may result in overestimation of the column water vapor and removal of key Fe+3 absorption features caused by geology. It is our opinion, however, that including the 940nm band for this particular

scene produces better results than calculations made without using this band.

Execute Baseline ACORN (No Artifact Suppression)

The starting point for the ACORN radiative transfer atmospheric correction is the calibrated radiance spectra of the input hyperspectral image. The provided ACORN Control File

cup1.in

is set up to run ACORN without modifications for demonstration purposes. Optionally run ACORN with the visibility calculation enabled by checking the appropriate box or utilizing the ACORN Control File

cup1v1.in

.

3. To run ACORN in its baseline mode without artifact suppression, click “OK” or “Cancel” on the ACORN Edit Control File dialog to return to the ACORN Control Panel dialog. If you have changed any of the parameters, be sure to click “Save” in the dialog to save the changes.

4. Click “Run” in the ACORN Control Panel dialog to start ACORN execution. A status bar will appear and a completion message will be posted when ACORN completes the run. The file

cup1rfl

(ACORN Apparent Reflectance with Type 1 Artifact Suppression) is created.

Figure 5 shows four target spectra from the Cuprite hyperspectral data set and the atmospherically corrected spectra for the target radiance spectra. To a large degree, the effects of the atmosphere and solar source have been suppressed. The residual artifacts result from an imperfect spectral and radiometric calibration of the data as well as an imperfect radiative transfer model of the atmosphere.

16


Display Baseline Apparent Reflectance Image(s) and Spectra

ACORN Tutorial


Display the Cuprite AVIRIS apparent reflectance data calculated utilizing ACORN Mode 1 atmospheric correction. Apparent reflectance are scaled from 0 – 10000 (0 – 100% reflectance).

1. Select

File > Open Data Fil

e from the ENVI Main menu to open the file.

2. Navigate to the

Cuprite



).

3. Select the file

cup1rfl

in the Enter Data Filenames dialog and click “Open”.

The Available Bands List dialog automatically appears on your screen when the file is opened with a list of the bands and wavelengths.

4. Scroll down the band list using the scroll bar on the right edge of the dialog, click on Band 31, and then click “Load Band” to display the radiance image.

5. Select

Tools > Profiles > Z Profile (Spectrum)

in the Main Image Display menu bar to start a spectral profile. The spectrum for the current cursor location will be plotted in a plot window.

Figure 5: Left, AVIRIS calibrated measured radiance spectra for targets from a hyperspectral data set acquired over Cuprite, NV, USA. Right, ACORN baseline-corrected spectra for key cuprite

geologic materials


17

ACORN Tutorial


6. Now extract apparent reflectance spectra for the locations listed in Table 1 and load into a plot window for comparison. Compare to the radiance spectra extracted from the

cup0rdn

file. (This can be accomplished by opening both the radiance file and the apparent reflectance file, then selecting

File > Link > Link Displays

, turning off dynamic overlay, and starting a spectral profile from each image display for comparison).

7. Compare apparent reflectance spectra from the image to the selected library reflectance spectra previously used with the radiance data.

Artifact Suppression Overview

To help address the residual artifacts in the atmospherically corrected data, ACORN has three types of artifact suppression. Type 1 attempts to assess and correct for any mismatch in the spectral calibration of the hyperspectral data set and the spectral radiative transfer calculations. Artifact suppression type 1 suppresses the artifacts near the strong atmospheric absorption features at 760, 940, 1150, 2000 nm. There are still some small artifacts across the spectral range. Artifact suppression type 2 attempts to suppress these. The resulting spectra are considerably smoother with a number of additional artifacts suppressed. The portions of the spectrum across the 1400 and 1900 nm water vapor bands give very noisy reflectance results. These noisy values result from the low signals in these portions of the spectrum. ACORN artifact suppression type 3 assesses the signal levels of the calibrated radiance and suppresses the lowest signal portions where the atmospheric correction is excessively noisy. Resulting spectra are close to the quality that would be measured on the ground. ACORN offers complete flexibility in applying the artifact suppression types. They may be applied in any combination. If it appears the artifact suppression is not improving the quality of the atmospheric correction, all three types of artifact suppression may be turned off.

Figure 6: Cuprite Mode 1 apparent reflectance image, band 31 (0.67 micrometers) (left), and apparent reflectance spectrum (right). Compare to radiance spectrum in Figure 2

18 Tutorial: ACORN Modes 1 and 2 Atmospheric Correction (Stand-Alone)

Display Baseline Apparent Reflectance Image(s) and Spectra ACORN Tutorial

Execute ACORN Utilizing Type 1 Artifact Suppression

1. Click on the “Open” button in the ACORN Control Panel dialog to allow selection of the ACORN Control File.

2. Navigate to the Cuprite subdirectory (usually installed by default in c:\program files\ACORN\Data\Cuprite) and select the file cup1a1.in and click “Open.”

3. Click on the “Edit” button in the ACORN Control Panel to start the Edit Control File dialog and review both the standard ACORN parameters and the ACORN Mode 1-specific parameters in the ACORN Edit Control File dialog. Note the check mark on Artifact Suppression (Type 1).

4. Click “OK” or “Cancel” on the ACORN Edit Control File dialog to return to the ACORN Control Panel dialog. If you have changed any of the parameters, be sure to click “Save” in the dialog to save the changes.

5. Click “Run” in the ACORN Control Panel dialog to start ACORN execution. A status bar will appear and a completion message will be posted when ACORN completes the run. The file cup1a1.ref (ACORN Apparent Reflectance with Type 1 Artifact Suppression) is created.

Display ACORN Type 1 Artifact Suppression Apparent Reflectance Image(s) and Spectra

Display the Cuprite AVIRIS apparent reflectance data calculated utilizing ACORN Mode 1 atmospheric correction. Apparent reflectance are scaled from 0 – 10000 (0 – 100% reflectance).

1. Select File > Open Data File from the ENVI Main menu to open the file.

2. Navigate to the Cuprite subdirectory and select the file cup1a1.ref in the Enter Data Filenames dialog and click “Open.”


4. Select Tools > Profiles > Z Profile (Spectrum) in the Main Image Display menu bar to start a spectral profile. The spectrum for the current cursor location will be plotted in a plot window.

5. Extract apparent reflectance spectra for the locations listed in Table 1 and load into a plot window for comparison. Compare to the radiance spectra and apparent reflectance spectra extracted from the cup0rdn and cup1rfl files.

Tutorial: ACORN Modes 1 and 2 Atmospheric Correction (Stand-Alone) 19

ACORN Tutorial Display Baseline Apparent Reflectance Image(s) and Spectra


Execute ACORN Utilizing Type 1 and 2 Artifact Suppression


2. Navigate to the Cuprite subdirectory and select the file cup1a2.in and click “Open.”

3. Click on the “Edit” button in the ACORN Control Panel to start the Edit Control File dialog and review the both the standard ACORN parameters and the ACORN Mode 1-specific parameters in the ACORN Edit Control File dialog. Note the check mark on Artifact Suppression (Type 1 and Type 2).


5. Click “Run” in the ACORN Control Panel dialog to start ACORN execution. A status bar will appear and a completion message will be posted when ACORN completes the run. The file cup1a2.ref (ACORN Apparent Reflectance with Type 1 and Type 2 Artifact Suppression) is created.

Figure 7: Cuprite Mode 1 with Type 1 artifact suppression apparent reflectance image, band 31 (0.67 micrometers) (left), and apparent reflectance spectrum (right). Compare to Figure 2 and Figure 6


Display Baseline Apparent Reflectance Image(s) and Spectra ACORN Tutorial

Display ACORN Type 1 and 2 Artifact Suppression Apparent Reflectance Image(s) and Spectra

Display the Cuprite AVIRIS apparent reflectance data calculated utilizing ACORN Mode 1 atmospheric correction with Type 1 and Type 2 artifact suppression. Apparent reflectance are scaled from 0 – 10000 (0 – 100% reflectance).


2. Navigate to the Cuprite subdirectory and select the file cup1a2.ref in the Enter Data Filenames dialog and click “Open”.



5. Extract apparent reflectance spectra for the locations listed in Table 1 and load into a plot window for comparison. Compare to the radiance spectra and apparent reflectance spectra extracted from the various Mode 1 apparent reflectance corrections.


Figure 8: Cuprite Mode 1 with Type 1 and Type 2 artifact suppression apparent reflectance image, band 31 (0.67 micrometers) (left), and apparent reflectance spectrum (right). Compare to

Figure 2, Figure 6, and Figure 7


ACORN Tutorial Display Baseline Apparent Reflectance Image(s) and Spectra

Execute ACORN Utilizing Types 1, 2, and 3 Artifact Suppression


2. Navigate to the Cuprite subdirectory and select the file cup1a3.in and click “Open.”

3. Click on the “Edit” button in the ACORN Control Panel to start the Edit Control File dialog and review both the standard ACORN parameters and the ACORN Mode 1-specific parameters in the ACORN Edit Control File dialog. Note the check mark on Artifact Suppression (Type 1, Type 2, and Type 3).


5. Click “Run” in the ACORN Control Panel dialog to start ACORN execution. A status bar will appear and a completion message will be posted when ACORN completes the run. The file cup1a3.ref (ACORN Apparent Reflectance with Type 1, 2, and 3 Artifact Suppression) is created.

Display ACORN Type 1, 2, and 3 Artifact Suppression Apparent Reflectance Image(s) and Spectra

Display the Cuprite AVIRIS apparent reflectance data calculated utilizing ACORN Mode 1 atmospheric correction with Type 1, 2, and 3 artifact suppression. Apparent reflectance are scaled from 0 – 10000 (0 – 100% reflectance).


2. Navigate to the Cuprite subdirectory and select the file cup1a3.ref in the Enter Data Filenames dialog and click “Open.”



5. Extract apparent reflectance spectra for the locations listed in Table 1 and load into a plot window for comparison. Compare to the radiance spectra and apparent reflectance spectra extracted from the various Mode 1 apparent reflectance corrections.



Mode 2-Specific Parameters and Execution ACORN Tutorial


ACORN mode 2 supplements the radiative transfer atmospheric correction with a single spectrum enhancement. Single spectrum enhancement begins with calculation of ACORN Mode 1 apparent reflectance as described and demonstrated previously. To implement ACORN single spectrum enhancement, a known measured reflectance spectrum from an area in the atmospherically corrected data set is required (Measured Target Reflectance). The known measured spectrum must have equal to or better spectral resolution than the hyperspectral data and the spectral range of the measured spectrum must span that of the hyperspectral data set. A corresponding ACORN apparent reflectance spectrum (Image Target Reflectance) is extracted from the hyperspectral data set using image processing software such as ENVI. With these two spectra and the spectral calibration parameters of the hyperspectral data set, ACORN calculates a gain factor for each spectral channel to match the image and known spectra. The ACORN single spectrum enhancement suppresses the majority of artifacts present in typical atmospherically corrected apparent reflectance spectra.

In addition to the parameters required for Mode 1, the following additional parameters are required for Mode 2.

Image Target Reflectance Spectrum File

This file has one column containing the average values extracted for a known target from the reflectance image generated in ACORN Mode 1. There is one value for each band in the image. The units are typically reflectance *10000. Table 5 shows a portion of an image reflectance spectrum file. Open the file cup2.ext in the Cuprite directory using a text editor and examine the format.

Figure 9: Cuprite Mode 1 with Type 1, 2, and 3 artifact suppression apparent reflectance image, band 31 (0.67 micrometers) (left), and apparent reflectance spectrum (right). Compare to Figure 2,

Figure 6, Figure 7, and Figure 8


ACORN Tutorial Mode 2-Specific Parameters and Execution

Measured Target Reflectance Spectrum File

This is a two-column file. The first column is the wavelength in nanometers for a ground reflectance spectrum. The second column is the reflectance value for each wavelength position in the first column. The reflectance values must be at the same scale as the Image Reflectance Spectrum File values. The second column will usually be reflectance*10000. The spectral sampling of this file must be equal-to or higher than the target image reflectance spectrum and must cover the entire spectral range. Table 6 shows a portion of a ground reflectance spectrum file. Open the file cup2.ext in the Cuprite directory using a text editor and examine the format.

Table 5: Portion of an image reflectance spectrum file for ACORN mode 2. The values in this file are the ACORN mode 1 generated apparent surface

reflectance*10000.

10.83

29.38

820.34

1748.72

.

.

2211.55

4142.69

4475.00

6505.76

6785.41

Table 6: Portion of a ground reflectance spectrum file for ACORN Mode 2. The first column is wavelength center position of each band in nanometers. The

second column is the reflectance value for each wavelength position in the first column.

400.00 1991.69

401.00 2002.42

402.00 2013.18

403.00 2023.99

. . .

. .

2502.00 4146.05

2503.00 4142.71

2504.00 4139.88

2505.00 4137.72


Mode 2-Specific Parameters and Execution ACORN Tutorial

Execute ACORN Mode 2 Single Spectrum Enhancement


2. Navigate to the Cuprite subdirectory and select the file cup2.in and click “Open.”

3. Click on the “Edit” button in the ACORN Control Panel to start the Edit Control File dialog and review both the standard ACORN parameters and the ACORN Mode 2-specific parameters in the ACORN Edit Control File dialog.


5. Click “Run” in the ACORN Control Panel dialog to start ACORN execution. A status bar will appear and a completion message will be posted when ACORN completes the run. The file cup2rfl (ACORN Apparent Reflectance Mode 2 Single Spectrum Enhancements) is created.

Display ACORN Mode 2 Apparent Reflectance Image(s) and Spectra

Display the Cuprite AVIRIS apparent reflectance data calculated utilizing ACORN Mode 2 atmospheric correction (Figure 10). Apparent reflectance are scaled from 0 – 10000 (0 – 100% reflectance).


2. Navigate to the Cuprite subdirectory and select the file cup2rfl in the Enter Data Filenames dialog and click “Open”.



5. Extract apparent reflectance spectra for the locations listed in Table 1 and load into a plot window for comparison. Compare to the radiance spectra and apparent reflectance spectra extracted from the cup0rdn and cup1rfl files.


ACORN Tutorial Mode 2-Specific Parameters and Execution


The figures below show an example for the Cuprite dataset.

Figure 10: Cuprite Mode 2 single spectrum enhancement apparent reflectance image, band 31 (0.67 micrometers) (left), and apparent reflectance spectrum (right). Compare to previous figures.

Figure 11: Result of applying baseline ACORN Mode 1 radiative transfer atmospheric correction. Left, the measured spectrum of the known target in the cuprite hyperspectral data set. right, the acorn-

corrected data without artifact suppression. A number of artifacts are evident and related to the spectral and radiometric calibration of the data set and to the accuracy of the

radiative transfer model

Bright Target

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Atmospheric Correction Quality Assessment ACORN Tutorial

Atmospheric Correction Quality Assessment

ACORN provides several diagnostic files to help assess atmospheric correction quality. These include the “Echo” (.eco) file and two other diagnostic files (.diag1 and .diag1).

The ACORN echo file simply echoes the ACORN Control File, listing the parameters in the file. If an error occurs during execution, it is documented in this file. The filename of the echo file for a specific run takes the form of filename.in.eco. For example, for the ACORN Mode 1 run in this tutorial utilizing the ACORN Control file cup1.in, the echo file has the name cup1.in.eco. Examine the .eco files for the ACORN runs performed during this tutorial to gain an understanding of their format and content.

The first ACORN diagnostic file contains additional information about the actual parameters used in the ACORN calculations. This includes files successfully opened and created, the solar zenith, and the water vapor and visibility models used. The filename of the 1st ACORN diagnostic file for a specific run takes the form of reflectancefilename.diag1. For example, for the ACORN Mode 1 run in this tutorial utilizing the ACORN Control File cup1.in and creating the output apparent reflectance file cup1refl, the 1st diagnostic file has the name cup1refl.diag1. Examine the .diag1 files for the ACORN runs performed during this tutorial to gain an understanding of their format and content.

The diag2 file contains sample calculations from the ACORN run, including the wavelengths and FWHM used, input image values, gain, offset, calculated radiance values, calculated apparent reflectance values, path radiance and reflected radiance. The filename of the second ACORN diagnostic file for a specific run takes the form of reflectancefilename.diag2. For example, for the ACORN Mode 1 run in this tutorial utilizing the ACORN Control File cup1.in and creating the output apparent reflectance file cup1refl, the second diagnostic file has the name cup1refl.diag2. Examine the diag2 files for the ACORN runs performed during this tutorial to gain an understanding of their format and content.

Finally, the best way to assess the quality of the ACORN atmospheric corrections is to compare spectra for known targets to the model-based ACORN-calculated spectra. Examine the spectra from

Figure 12: The Result of ACORN Mode 2, single spectrum enhancement for the Cuprite hyperspectral data set. Left, ACORN baseline-corrected spectra for key Cuprite geologic materials. Right, ACORN

Mode 2-corrected data showing suppression of artifacts present in the Mode 1 radiative transfer atmospheric correction

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ACORN Tutorial Atmospheric Correction Quality Assessment

each of the different corrections performed during this tutorial and compare to each other and the spectral library spectra for the sites specified in Table 1. When you have completed these comparisons, exit the ACORN Control Panel by clicking on the “Exit” button and exit ENVI or your alternate image processing and analysis system.


Atmospheric Correction Quality Assessment ACORN Tutorial

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