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ENVI Orthorectification Module Version 4.7December, 2009Copyright © ITT Visual Information SolutionsAll Rights Reserved

ENVI Orthorectification Module User’s Guide

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AcknowledgmentsENVI® and IDL® are registered trademarks of ITT Corporation, registered in the United States Patent and Trademark Office. ION™, ION Script™, ION Java™, and ENVI Zoom™ are trademarks of ITT Visual Information Solutions.ESRI®, ArcGIS®, ArcView®, and ArcInfo® are registered trademarks of ESRI.Portions of this work are Copyright © 2009 ESRI. All rights reserved.PowerPoint® and Windows® are registered trademarks of Microsoft Corporation in the United States and/or other countries.Macintosh® is a registered trademark of is a trademark of Apple Inc., registered in the U.S. and other countries.UNIX® is a registered trademark of The Open Group.Adobe Illustrator® and Adobe PDF® Print Engine are either registered trademarks or trademarks of Adobe Systems Incorporated in the United States and/or other countries.Numerical Recipes™ is a trademark of Numerical Recipes Software. Numerical Recipes routines are used by permission.GRG2™ is a trademark of Windward Technologies, Inc. The GRG2 software for nonlinear optimization is used by permission.NCSA Hierarchical Data Format (HDF) Software Library and Utilities. Copyright © 1988-2001, The Board of Trustees of the University of Illinois. All rights reserved.NCSA HDF5 (Hierarchical Data Format 5) Software Library and Utilities. Copyright © 1998-2002, by the Board of Trustees of the University of Illinois. All rights reserved.CDF Library. Copyright © 2002, National Space Science Data Center, NASA/Goddard Space Flight Center.NetCDF Library. Copyright © 1993-1999, University Corporation for Atmospheric Research/Unidata.HDF EOS Library. Copyright © 1996, Hughes and Applied Research Corporation.SMACC. Copyright © 2000-2004, Spectral Sciences, Inc. and ITT Visual Information Solutions. All rights reserved.This software is based in part on the work of the Independent JPEG Group.Portions of this software are copyrighted by DataDirect Technologies, © 1991-2003.BandMax®. Copyright © 2003, The Galileo Group Inc.Portions of this computer program are copyright © 1995-2008 Celartem, Inc., doing business as LizardTech. All rights reserved. MrSID is protected by U.S. Patent No. 5,710,835. Foreign Patents Pending.Portions of this software were developed using Unisearch’s Kakadu software, for which ITT has a commercial license. Kakadu Software. Copyright © 2001. The University of New South Wales, UNSW, Sydney NSW 2052, Australia, and Unisearch Ltd, Australia.This product includes software developed by the Apache Software Foundation (www.apache.org/).MODTRAN is licensed from the United States of America under U.S. Patent No. 5,315,513 and U.S. Patent No. 5,884,226.QUAC and FLAASH are licensed from Spectral Sciences, Inc. under U.S. Patent No. 6,909,815 and U.S. Patent No. 7,046,859 B2.Portions of this software are copyrighted by Merge Technologies Incorporated.Support Vector Machine (SVM) is based on the LIBSVM library written by Chih-Chung Chang and Chih-Jen Lin (www.csie.ntu.edu.tw/~cjlin/libsvm), adapted by ITT Visual Information Solutions for remote sensing image supervised classification purposes.IDL Wavelet Toolkit Copyright © 2002, Christopher Torrence.IMSL is a trademark of Visual Numerics, Inc. Copyright © 1970-2006 by Visual Numerics, Inc. All Rights Reserved.Other trademarks and registered trademarks are the property of the respective trademark holders.

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Contents
Chapter 1 Introduction ....................................................................................................... 5Introduction to the ENVI Orthorectification Module ..................................................................... 6
Supported Image Formats ........................................................................................................ 6The ENVI Orthorectification Module Workflow .................................................................. 10Installing and Starting the ENVI Orthorectification Module ................................................. 11

Chapter 2 The ENVI Orthorectification Layout Manager ............................................... 13Working with the ENVI Orthorectification Layout Manager ...................................................... 14

Location Bar ........................................................................................................................... 15Layout View Area .................................................................................................................. 15Identification Area ................................................................................................................. 16Display Controls .................................................................................................................... 17

Chapter 3 Orthorectifying Images ................................................................................... 19The ENVI Orthorectification Wizard ........................................................................................... 20

Common Wizard Tools .......................................................................................................... 20Saving and Restoring Projects ............................................................................................... 20Selecting Bands ...................................................................................................................... 21Viewing the Project Summary ............................................................................................... 21

Selecting Input Images and DEMs ............................................................................................... 22Starting the ENVI Orthorectification Module ....................................................................... 23

Collecting and Editing Ground Control Points ............................................................................. 25

ENVI Orthorectification Module User’s Guide 3

4

GCP File Format .................................................................................................................... 26Restoring GCP Files ............................................................................................................... 27Adding GCPs ......................................................................................................................... 27Editing GCPs .......................................................................................................................... 29Turning GCPs On and Off ..................................................................................................... 29GCP Coordinate Projection .................................................................................................... 30Saving GCP Files ................................................................................................................... 30Evaluating Residual Errors in GCPs ...................................................................................... 30

Collecting and Editing Tie Points ................................................................................................. 33Tie Point File Format ............................................................................................................. 35Restoring Tie Point Files ........................................................................................................ 35Manually Adding Tie Points Between Two Images .............................................................. 36Automatically Generating Tie Points ..................................................................................... 37Editing Tie Points ................................................................................................................... 38Saving Tie Point Files ............................................................................................................ 39

Reordering Images and Defining Cutlines ................................................................................... 40Setting the Image Order ......................................................................................................... 40Drawing Cutlines ................................................................................................................... 41

Selecting Output Parameters ........................................................................................................ 45Setting the Output Projection ................................................................................................. 46

Chapter 4 Programming ................................................................................................... 49

Chapter 5 Tips and Tricks for the ENVI Orthorectification Module .............................. 55Tips and Tricks ............................................................................................................................. 56

Appendix A Glossary ........................................................................................................... 59

ENVI Orthorectification Module User’s Guide Contents

Chapter 1

Introduction

This chapter covers the following topics:

Introduction to the ENVI Orthorectification Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6


6 Chapter 1: Introduction

Introduction to the ENVI Orthorectification Module

An orthorectified image (or orthophoto) is one where each pixel represents a true ground location and all geometric, terrain, and sensor distortions have been removed to within a specified accuracy. Orthorectification transforms the central perspective of an aerial photograph or satellite-derived image to an orthogonal view of the ground, which removes the effects of sensor tilt and terrain relief. Scale is constant throughout the orthophoto, regardless of elevation, thus providing accurate measurements of distance and direction. Geospatial professionals can easily combine orthophotos with other spatial data in a geographic information system (GIS) for city planning, resource management, and other related fields.

ENVI's existing orthorectification tools (accessed through Map → Orthorectification on the ENVI main menu bar) allow you to orthorectify images using rational polynomial coefficients (RPCs), elevation and geoid information, and optional ground control points (GCPs). RPCs attempt to model the sensor geometry, using a polynomial approximation to model light rays from a ground location to the focal plane of the sensor. However, RPCs and elevation information do not provide enough details to build a rigorous model representing the path of light rays from a ground object to the sensor.

The ENVI Orthorectification Module allows you to build highly accurate orthorectified images by rigorously modeling the object-to-image transformation. The details of this transformation are mostly transparent to the user, which means you can create orthorectified images without defining any detailed model parameters.

With the ENVI Orthorectification Module, you can easily see the spatial coverage of images, DEMs, GCPs, and tie points, along with error magnitudes for each GCP. You can adjust your GCPs and tie points to improve the root mean square error (RMSE) for the orthorectified output. ENVI uses the latest innovative research methods to perform mosaicking and color balancing in the output product.

The ENVI Orthorectification Module is a joint collaboration between ITT Visual Information Solutions and Spacemetric AB of Stockholm, Sweden. Spacemetric designed the underlying block-adjustment model, which provides a precision orthorectification solution for various sensors. For more technical information on Spacemetric’s orthorectification models, see their website at http://www.spacemetric.se/technology.

Supported Image FormatsFollowing is a list of image formats that have been successfully tested with the ENVI Orthorectification Module. The metadata listed next to each image format or sensor must

Introduction to the ENVI Orthorectification Module ENVI Orthorectification Module User’s Guide

Chapter 1: Introduction 7

accompany the imagery, and the imagery must be in its native format, filename, and directory structure, exactly as delivered by the data provider.

Satellite/Sensor

Image Format

Required Metadata Product Notes

ASTER HDF *_metadata.txt

(Generated by ENVI)

Level-1B, EOS v2.8

Landsat TM and ETM+

FAST

IKONOS TIFFNITF

metadata.txt Ortho Kit

GeoEye-1 TIFF .pvl, .eph, .att BASIC Math Model Kit PAN

OrbView-3 TIFF .pvl, .eph, .att BASIC

QuickBird TIFFNITF

.xml, .rpb,

.imd, .att,

.eph, .geo, .til

Product Levels LV1B and LV2A

WorldView-1 TIFFNITF

.xml, .rpb,

.imd, .att,

.eph, .geo, .til

Product Levels LV1B and LV2A

SPOT TIFF metadata.dim (DIMAP version=’1.1’ only)

SPOTSCENE_2ASPOTSCENE_1BSPOTSCENE_1A

XML version attribute must be version=1.1

FORMOSAT-2 TIFF .dim (DIMAP version=’1.1’ only)

F2_Level_1A XML version attribute must be version=1.1

Table 1-1: Supported data formats

ENVI Orthorectification Module User’s Guide Introduction to the ENVI Orthorectification Module


KOMPSAT-2 TIFF .txt, .rpc, .eph L1G Opening these files through the ENVI Ortho Module results in a black/white image display. To open the files as multi-color images, select File > Open Exernal File > KOMPSAT-2 from the ENVI main menu bar.

CARTOSAT-1 TIFF product_met .txt

PRODUCT_RPC .TXT

DECRYPTED_PRODUCT_RPC.TXT

ORTHO Kit To open CARTOSAT-1 data with RPC positioning, select File > Open External File > CARTOSAT-1 > Open with RPC Positioning from the ENVI main menu bar.

EROS-A .1A .pass 1A Open .1A files with File > Open Exernal File > EROS > Level 1A from the ENVI main menu bar.

RADARSAT-1 CEOS dat_01. 001

lea_01.001

nul_vdf.001

tra_01.001

vdf_dat.001

SGF, SGX

ERS CEOS dat_01. 001

lea_01.001

nul_vdf.001

tra_01.001

vdf_dat.001

BERS2-PRI-6.1 Open ERS files with File > Open Exernal File > Radar > ERS from the ENVI main menu bar.

Satellite/Sensor

Image Format



Introduction to the ENVI Orthorectification Module ENVI Orthorectification Module User’s Guide


Aerial Camera Data

The ENVI Orthorectification Module provides limited support for digital aerial image files from the UltraCam (Level-3 processing only) and Applanix DSS frame-central cameras. When opening aerial camera data from the ENVI Orthorectification Wizard, select Aerial in the Select Sensor Type dialog (see “Starting the ENVI Orthorectification Module” on page 23).

You can add support for other frame cameras by creating an exterior/interior orientation description of your frame camera in the form of an XML file. Contact ITT Visual Information Solutions Technical Support for more information.

For UltraCam data, you must separate RGB images, color-infrared (CIR) images, and panchromatic images, in different directories. All must be at the same level as the Lvl03 directory. Following is a suggested directory structure:

Lvl03/

Lv103_pan/put panchromatic data here

Lv103_rgb/put RGB images here

Lv103_cir/put CIR images here

UltraCam (aerial camera)

TIFF Level-3 Map coordinates are not shown in the ENVI display group.

Applanix DSS (aerial camera)

TIFF Map coordinates are not shown in the ENVI display group.

Satellite/Sensor

Image Format



ENVI Orthorectification Module User’s Guide Introduction to the ENVI Orthorectification Module


The ENVI Orthorectification Module WorkflowFigure 1-1 illustrates a typical orthorectification workflow. The only required steps in the ENVI Orthorectification Module workflow are (1) select input images and DEMs, (2) build an adjustment model, (3) select output parameters, and (4) rectify the input image(s) to produce the output image. However, you will achieve the most accurate results by incorporating optional ground control points (GCPs) and tie points into the model, while iteratively reviewing the overall model error and editing points as needed to reduce the error. You can also optionally create and/or add cutlines and set some basic parameters for the final output.

The shaded boxes in Figure 1-1 represent workflow steps that have an associated Wizard dialog to guide you through the process. See “The ENVI Orthorectification Wizard” on page 20 for more information.

Figure 1-1: The ENVI Orthorectification Module workflow

The ENVI Orthorectification Module Workflow ENVI Orthorectification Module User’s Guide


Installing and Starting the ENVI Orthorectification ModuleThe ENVI Orthorectification Module requires a separate installer and license from ENVI. You must have ENVI 4.7 SP1 installed. The ENVI Orthorectification Module is supported on the following platforms (both 32-bit and 64-bit): Windows 7, Vista, and XP SP2.

When you purchase a license for the ENVI Orthorectification Module, you will have access to an installer for your specific platform. If you do not have an ENVI Orthorectification Module license, contact ITT Visual Information Solutions or your ENVI distributor.

For information about entering a new ENVI license, refer to the Installation and Licensing Guide, which is available on the ITT Visual Information Solutions website, on the ENVI installation DVD, or on the ENVI Resource DVD that shipped with your software.

From the ENVI main menu bar, select Map → Rigorous Orthorectification to start the ENVI Orthorectification Module. The ENVI Orthorectification Layout Manager and ENVI Orthorectification Wizard appear. To begin orthorectifying images now, see “Selecting Input Images and DEMs” on page 22. Or, you can learn more about the interface first by going to “Working with the ENVI Orthorectification Layout Manager” on page 14 and “The ENVI Orthorectification Wizard” on page 20.

NoteThe recommended minimum screen resolution for using the ENVI Orthorectification Wizard is 1152 x 864.

ENVI Orthorectification Module User’s Guide Installing and Starting the ENVI Orthorectification Module


Installing and Starting the ENVI Orthorectification Module ENVI Orthorectification Module User’s Guide

Chapter 2

The ENVI Orthorectification Layout Manager


Working with the ENVI Orthorectification Layout Manager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14


14 Chapter 2: The ENVI Orthorectification Layout Manager

Working with the ENVI Orthorectification Layout Manager

The ENVI Orthorectification Layout Manager (hereafter called the “Layout Manager”) provides a visual and tabular view of the current state of the orthorectification process. You can quickly see the spatial coverage of all of the components used to create an orthorectified image, including the input images, DEMs, ground control points (GCPs), tie points, cutlines, and output area. The Layout Manager also displays the root mean square error (RMSE) of the model.

This dialog is for viewing only. It cannot be used to edit GCP or tie point locations or to interactively move the vectors representing images, DEMs, cutlines, output area, or residuals.

As you edit GCPs and tie points in the ENVI Orthorectification Wizard, the Layout Manager automatically updates to show the locational changes. The orthorectification model is recomputed anytime you edit GCPs or tie points. You can quickly determine if your GCPs have broad spatial coverage across the input images and if you have enough tie points to cover image overlap regions. You can view all of these components at the same time, or you can select to view specific items of interest.

For more information on using the Layout Manager, select Help → Help on Layout Manager from the ENVI Orthorectification Wizard menu bar (for any step in the workflow). Select Help → Mouse Button Descriptions to display a list of mouse button controls when working in the Layout Manager.

Working with the ENVI Orthorectification Layout Manager ENVI Orthorectification Module User’s Guide

Chapter 2: The ENVI Orthorectification Layout Manager 15

The dialog consists of three parts, as shown in Figure 2-1: Location Bar, Layout View Area, Identification Area, and Display Controls. Each of these is described in the following sections.

Location BarThe Location Bar indicates the cursor position in meters for a UTM WGS-84 map projection.

Layout View AreaThe Layout View Area shows the spatial coverage (in map space) of the input images, DEMs, GCPs, tie points, cutlines, output area, and residuals. When the project consists of multiple

Figure 2-1: ENVI Orthorectification Layout Manager

Location

DisplayControls

Identification

Bar

Identification Area

LayoutView Area

ENVI Orthorectification Module User’s Guide Working with the ENVI Orthorectification Layout Manager


images, the image boundaries are shown as white lines and filled with different colors to visually show the image stack order.

Layout View Area Controls

Following are controls for working in the Layout View Area.

Identifying Images and DEMs

Moving the cursor over the Layout View Area shows the map coordinates in the Location Bar and lists the images (in stack order) and DEMs in the Identification Area.

Selecting the Active Image

• Shift-click to select the active image. (The top image in the image stack under the cursor will become active in the ENVI display group.) The active image is highlighted with a red border.

• Ctrl-click to select a second active image in the tie points step (see “Collecting and Editing Tie Points” on page 33). The secondary active image is highlighted with a yellow border.

Zooming

• Click once inside the Layout View Area to set the zoom center, then use the middle mouse wheel to zoom in and out of the display.

• Right-click and select Zoom In or Zoom Out to zoom incrementally.

• Right-click and select Reset to revert back to the original view.

Changing the Image Stack Order

You typically select the active image using Shift-click; however, if an image is behind all other images with no exposed area visible in the Layout View Area, you cannot select it using Shift-click. To expose the image, use the Layout View Area controls to move the upper images down the stack. The image stack order affects the final mosaic output. Parts of images that are underneath other images will not be seen in the final mosaic output (see “Reordering Images and Defining Cutlines” on page 40).

1. In the Layout View Area, Shift-click inside an image boundary.

2. Right-click and select one of the following options:

• Move Image Down: Moves the active image down one position in the stack.

• Move Image Up: Moves the active image up one position in the stack.

• Move to Bottom: Moves the active image to the bottom of the stack.

• Move to Top: Moves the active image to the top of the stack.

Identification AreaThe Identification Area lists the images and DEMs under the cursor location. Images are listed in their stack order; the first listed image is at the top of the stack. Use the Layout View


Chapter 2: The ENVI Orthorectification Layout Manager 17

Area and Identification Area to identify images, to verify the image stack order, and to ensure that DEMs cover a spatial extent greater than the image area.

Display ControlsUse the check boxes in the Display Controls area to select which components of the orthorectification process to view in the Layout View Area:

• Image: Bounding boxes for all input images (white)

• DEM: Bounding boxes for all input DEMs (red). By default, the Layout View Area is set to the total image output area. If a DEM is significantly larger than this area, then the DEM boundary is only visible if you zoom out. DEMs are not automatically subsetted to the total image output area. If you use a global DEM, the Layout View Area will not display its full spatial extent. A global DEM is shown with a bounding box that is twice the size of the total image output area.

• GCP: Symbols representing locations of GCPs (yellow)

• GCP Labels: Labels identifying GCPs (red)

• Tie Point: Symbols representing locations of tie points (magenta)

• Tie Point Labels: Labels identifying tie points (purple)

• Cutline: Polygons representing cutlines (blue)

• Output Area: Polygon representing output area for the orthorectified output (yellow). The default is the collective boundary of all input images.

• Residual Vectors: Vectors representing the magnitude and direction of errors in the image plane at each GCP location (cyan). See “Evaluating Residual Errors in GCPs” on page 30.

• Residual Image: A contour view of the overall residual error as a function of GCPs. Red areas represent the highest error values, and blue-to-white values represent the lowest values. The Residual Image option is not a recommended choice for viewing the error magnitudes of GCPs that share the same map location or are very close to one another. Use the Residual Vectors option and the Orthorectification Ground Control Points List instead. See “Evaluating Residual Errors in GCPs” on page 30.

The RMSE field shows the root mean square error (RMSE) for the overall model.

To the right of the RMSE field is an area that lists the filenames of the input image and DEM underlying the cursor location in the Layout View Area.

Figure 2-2: Identification Area

ENVI Orthorectification Module User’s Guide Working with the ENVI Orthorectification Layout Manager


The Residual Exaggeration slider controls the relative magnitude of the residual vectors when you select the Residual Vectors check box. The multiplication factor ranges from 1 to 10,000.


Chapter 3

Orthorectifying Images


The ENVI Orthorectification Wizard . . . . . . . . 20Selecting Input Images and DEMs . . . . . . . . . . 22Collecting and Editing Ground Control Points . 25

Collecting and Editing Tie Points . . . . . . . . . . . 33Reordering Images and Defining Cutlines . . . . . 40Selecting Output Parameters . . . . . . . . . . . . . . . 45


20 Chapter 3: Orthorectifying Images

The ENVI Orthorectification WizardThe ENVI Orthorectification Wizard dialogs have two control buttons at the bottom: Previous and Next, which allow you to maneuver backward and forward, respectively, through the steps of the Wizard.

• Next continues to the next step in the Wizard, but it is only enabled when the required information has been provided in the current step.

• Previous allows you to go back to a previous step in the workflow. Note, however, that you will lose all data in the current step.

Selecting Help → Help on current step from the Wizard menu bar provides information describing the current Wizard step.

Common Wizard ToolsFollowing are some tools that are common to the GCP, tie point, and/or cutline steps.

Saving and Restoring ProjectsAt any point throughout the orthorectification workflow, you can save your project and restore it later if needed. Saving a project saves the complete state of the project including GCP and tie point on/off status and colors, cutline image coordinates and colors, and various parameter settings available in the workflow. The on/off status for GCPs and tie points is only preserved when restoring a project to the relavent GCP or tie point step.

When restoring a project, the images and DEMs must be in the same directory as when you saved the project.

Follow these steps to save your project:

1. From the ENVI Orthorectification Wizard menu bar, select File → Save Project. The Select Project File dialog appears. You can save projects during Step 2 or later in the Wizard.

Button Description

Set the color for on/off GCPs, tie points, and cutlines.

Open a GCP or tie point file.

Save a GCP or tie point file.

If a multiband image is intially displayed as a grayscale image in the display group, you can click the MB button in the ENVI Orthorectification Wizard to display a multispectral image of Bands 3, 2, and 1 as red, green, and blue, respectively. Click the MB button again to display a grayscale image. This button is only available for images with three or more bands. See “Selecting Bands” on page 21.

Table 3-1:

The ENVI Orthorectification Wizard ENVI Orthorectification Module User’s Guide

Chapter 3: Orthorectifying Images 21

2. Select an output location and filename, and click Save. ENVI saves the project state in XML format. A file named *_envidata.xml contains any cutline polygons you created (in original image space), and the state of the Layout Manager, including GCPs, tie points, cutline color, auto tie point parameters, and GCP/tie point on/off status. Do not modify this XML file or else the project will not restore correctly.

Follow these steps to restore a previously saved project. You can only restore a project during Step 1 of the Wizard.

1. From the ENVI Orthorectification Wizard menu bar, select File → Restore Project. The Select Project File dialog appears.

2. Select a project file (in XML format), and click Open. ENVI automatically opens all input images, DEMs, GCPs, and tie points.

Selecting BandsFrom the ENVI Orthorectification Wizard menu bar, select Options → Band Selection. The Bands dialog appears. Use this dialog to change the default multi-band and single-band display options. Band 1 is the default setting for single-band images. To change this setting, enter a band number in the Single Band Number field. Bands 3, 2, and 1 are used by default for red, green, and blue, respectively, in multi-band images. To change this setting, enter band numbers in the Red, Green, and Blue fields.

Using this method ensures that band settings persist throughout the orthorectification workflow. You can also change bands through ENVI’s Available Bands List, but the band settings will not persist throughout the orthorectification workflow.

Viewing the Project SummaryFrom the ENVI Orthorectification Wizard menu bar, select Options → View Project Summary. The Project Summary dialog appears. Use the Project Summary to review characteristics of the images, DEMs, GCPs, tie points, and cutlines currently used in the project.

ENVI Orthorectification Module User’s Guide The ENVI Orthorectification Wizard


Selecting Input Images and DEMsFollowing are the steps for selecting input images for the ENVI Orthorectification Module.

Input images must be in their native format, filenames, and directory structure, exactly as they are delivered by the data provider. For example, you cannot spatially subset a QuickBird GeoTIFF image, rename the subset, save it as a TIFF image, and read it into the ENVI Orthorectification Module. The imagery must include all associated metadata and ephemeris data. See “Introduction to the ENVI Orthorectification Module” on page 6 for a list of supported image formats and accompanying metadata.

You should copy data to a directory with write access because the ENVI Orthorectification Module creates temporary files during the orthorectification process and writes them to the same directory as your data files.

It is recommended that you copy input images and DEMs to a local drive rather than access them through a network.

Selecting Input Images and DEMs ENVI Orthorectification Module User’s Guide


Starting the ENVI Orthorectification Module1. Select Map → Rigorous Orthorectification from the ENVI main menu bar. The

ENVI Orthorectification Wizard opens with the Input Data Selection panel (Step 1 of 5, Figure 3-1).

2. Click the button next to the Input Images section. The Select Input Files dialog appears.

3. Click the Open drop-down button and select New File. A file selection dialog appears.

4. Select an input image. See “Introduction to the ENVI Orthorectification Module” on page 6 for a list of supported image formats.

NoteFor SPOT DIMAP files, select the .dim file. Open KOMPSAT-2 and EROS Level-1A files by selecting File → Open External

Figure 3-1: Input Data Selection panel (Step 1 of 5)

ENVI Orthorectification Module User’s Guide Selecting Input Images and DEMs


File → sensor type from the ENVI main menu bar, versus opening them from the ENVI Orthorectification Wizard.

5. Click Open. The Select Sensor Type dialog appears.

6. Select the sensor type for your input image. Click OK.

7. Repeat Steps 2-6 to open more images.

8. The Input Images section of the ENVI Orthorectification Wizard lists the full paths and filenames of the file(s) you selected. To remove a file from the Input Images list, select the file and click . The file will also be removed from the Available Bands List.

9. Click next to the Input DEMs section to select one or more DEM files for input. The DEMs should cover a contiguous map region about 10% or greater than the total image area (to account for relief displacement), and elevations should be expressed in meters above sea level.

10. Click Next to continue. The Layout View Area of the Layout Manager shows the boundaries and names of each input image and DEM. A new display group also appears of the first image that you selected for input.

Selecting Input Images and DEMs ENVI Orthorectification Module User’s Guide


Collecting and Editing Ground Control PointsWhen you click Next in Step 1, the ENVI Orthorectification Wizard proceeds to the GCP Selection panel (Step 2 of 5, Figure 3-2). In this step, you associate image pixels to points on the ground whose locations are known through a horizontal coordinate system and vertical datum. These points are called ground control points (GCPs).

NoteBe sure that your GCPs are of a quality that is consistently better than the initial accuracy of the sensor model; this varies with data type and vendor. For example, mid-latitude QuickBird scenes of a relatively flat area may provide an accurate orthorectification without the use of GCPs. GCP elevations should be expressed in meters above mean sea level (orthometric height), not as height above the ellipsoid.

ENVI Orthorectification Module User’s Guide Collecting and Editing Ground Control Points


The controls in this step of the Wizard allow you to optionally restore GCP files, to add new GCPs, and to edit existing GCPs. You may choose to bypass this step by clicking Next; the orthorectified solution will be computed without GCPs in this case.

GCP File FormatThe ENVI Orthorectification Module uses the Rigorous Orthorectification GCP file format, which is an ASCII file that contains image file IDs, map X/Y coordinates, elevation in meters, and image X/Y coordinates. Fields are tab-delimited. You can either create a Rigorous Orthorectification GCP file from scratch using your map and elevation data (see example below), or you can enter map/image coordinates and elevations directly in the Wizard and save it to the Rigorous Orthorectification GCP file format.

A Rigorous Orthorectification GCP file requires a .pts file extension and begins with the words ENVI Rigorous Orthorectification GCP File. The first several rows that begin with semicolons are header rows. Following is an example:

; ENVI Rigorous Orthorectification GCP File ; FileName0=image1.TIF ; FileName1=image2.TIF

Figure 3-2: GCP Selection panel (Step 2 of 5)

Collecting and Editing Ground Control Points ENVI Orthorectification Module User’s Guide


; projection info = {UTM, 12, North, WGS-84, units=Meters} ; ImageFile#, Map (x,y,elev), Image (x,y) ; 1 408620.653090 3698080.965069 314.977000 4366.000000 447.000000 1 406652.054304 3698011.485774 311.083000 3569.000000 479.000000 1 398163.369353 3694523.413367 309.603000 245.000000 1861.000000

The header rows that say FileNamex refer to the input image filenames, where x is an index number used to reference the filename throughout the rest of the GCP file. In the example above, a line that begins with “1” indicates GCP data for the file image2.TIF, as indicated in the header. If you ever change the mapped drive, you must remember to update the GCP file headers.

If you are creating your own GCP file in a text editor or other application, you do not need to include a full path to the image filenames (in the lines that begin with FileNamex) as long as your filenames are unique. In cases where the filenames may be the same for all images (for example, CARTOSAT-1 and SPOT data), you need to include the full path to each image file in the FileNamex lines.

NoteFiles in ENVI GCP format will not work with the ENVI Orthorectification Wizard; they must be in the Rigorous Orthorectification GCP file format. The ENVI GCP file format is described in the topic “ENVI ASCII Files” in ENVI Help.

Restoring GCP FilesFollow these steps to restore an existing GCP file:

1. Click the Load a new GCP file button . The Select GCP File dialog appears.

2. Select an ASCII file with GCPs, and click Open. The GCP locations are displayed in the Layout View Area of the Layout Manager (if you enabled the GCP check box) and in the display group. You have two options to view the list of GCPs:

• Click Show List in the ENVI Orthorectification Wizard. The Orthorectification Ground Control Points List appears. See “Editing GCPs” on page 29 for instructions on working with this dialog.

3. Evaluate the RMSE for the entire model and the error residuals for individual GCPs (as described in “Evaluating Residual Errors in GCPs” on page 30), and proceed with adding new GCPs or editing GCPs as needed.

Adding GCPsFollow these steps to add new GCPs to an existing set, or to create a new set of GCPs.

1. A display group shows the image selected as the Active Image in the GCP Selection panel. To change the image that is displayed, select a different image from the Active Image list or Shift-click an image boundary in the Layout Manager to make that image active. The active image is shown with a red border in the Layout Manager, and ENVI will load the appropriate display group. (See “Changing the Image Stack Order” on page 16.)



2. In the Image window of the display group, position the red Zoom box over the area where you want to add a new GCP.

3. In the Zoom window of the display group, click on a specific pixel to center the Zoom window over that pixel or portion of that pixel.

The Image X/Y fields in the ENVI Orthorectification Wizard show the image coordinates of the selected pixel underlying the crosshairs in the Zoom window. Subpixel coordinates are supplied to provide higher accuracy in selecting GCPs. Whole-number pixel coordinates correspond to the upper-left corner of the pixel. The x,y values increase to the right and bottom of the pixel, respectively.

4. If your image is not already georeferenced, you can enter known map coordinates for the GCP location into the E/N or Lat/Lon fields of the ENVI Orthorectification Wizard. See “Selecting Map Projection Types” in the ENVI User’s Guide for instructions on changing the projection.

• To enter map coordinates, select a map projection and datum. Click the toggle button next to the map projection name to switch between E/N and Lat/Lon fields.

• Use negative (-) longitude for the western hemisphere and negative (-) latitude for the southern hemisphere.

• To change the latitude and longitude values to decimal degrees, click DDEG. To use degrees, minutes, and seconds, click DMS.

5. If your image is georeferenced, a recommended method is to use ENVI’s Pixel Locator dialog to enter coordinates into the ENVI Orthorectification Wizard. From the Display group menu bar, select Tools → Pixel Locator. The Pixel Locator dialog appears. The Sample and Line values reflect the pixel location where the Zoom window is centered.

6. Set the correct map projection and datum (if needed) in the Pixel Locator dialog.

7. Click Export in the Pixel Locator dialog. The map coordinates are transferred to the corresponding fields in the ENVI Orthorectification Wizard. (You do not need to perform this step with the Image X/Y fields.)

8. Enter the elevation in meters above mean sea level for the point described by the Lat/Lon (or E/N) values, into the Elev field of the ENVI Orthorectification Wizard.

9. Click Add Point in the ENVI Orthorectification Wizard to add the point to the GCP list.

When you add a GCP, a marker is placed in the Layout View Area of the Layout Manager and in the display group. The marker consists of an ID next to a + symbol and indicates the selected pixel (or subpixel location). The center of the marker (located under the + symbol) indicates the GCP location.

10. Add more GCPs as needed, using the same procedures.

When you enter at least three GCPs, the GCP list shows the residual errors for each GCP, along with the RMSE for the entire model. While ENVI accepts as few as one GCP, it is desirable to have four or more GCPs evenly distributed around each image. See “Evaluating Residual Errors in GCPs” on page 30.



Editing GCPsYou can edit the location of a GCP by changing the map and/or image coordinates in the ENVI Orthorectification Wizard.

To edit the location of a GCP in the ENVI Orthorectification Wizard, perform the following steps:

1. Click Show List. The Orthorectification Ground Control Points List appears.

2. Click a row number to select a given GCP, or click in a table cell corresponding to a GCP and click Goto. The display group centers over that GCP location, and the GCP is highlighted in the Layout Manager. If the image that corresponds to the GCP is not already displayed, ENVI loads the appropriate image into the display group.

3. Edit the E/N (or Lat/Lon), Image X/Y, and Elev values in the Orthorectification Ground Control Points List. You can also use the Pixel Locator dialog to determine map coordinates, as described in “Adding GCPs” on page 27.

4. Click Update in the Orthorectification Ground Control Points List. The changes are reflected in the GCP list, display group, and the Layout Manager.

Removing GCPs

To remove any GCP from the table in the ENVI Orthorectification Wizard, select the row for that GCP and click Delete. To remove all GCPs, select Options → Clear All Points from the Orthorectification Ground Control Points List menu bar.

Turning GCPs On and Off1. In the Orthorectification Ground Control Points List, click on a row corresponding to a

GCP you want to turn off.

2. Click On/Off. The color of the GCP marker in the display group changes to green (by default) to indicate that it is off. You can change the default colors by clicking the Set GCP Color button . The Point Color dialog appears. Right-click on a color box and select the desired color.

GCPs that you turn off are moved to the end of the Orthorectification Ground Control Points List if you have sorted the GCPs by error magnitude (see “Evaluating Residual Errors in GCPs” on page 30). Their ID values have a minus symbol to indicate they are turned off, and their Magnitude (m) values are 0. These GCPs are not used in the bundle adjustment model as long as they are off. The RMSE calculation is based only on GCPs that are turned on.

When you click Next to proceed to the Tie Points panel, GCPs that were previously off are turned back on and used in the model. The purpose of the On/Off functionality is so that you can experiment with GCPs and decide whether or not to permanently delete any.



GCP Coordinate ProjectionIf you change the map projection in the Orthorectification Wizard, close the Orthorectification Ground Control Points List if it is currently open. Then re-open it to see the updated coordinate values.

Saving GCP FilesTo save edits to an existing GCP file, or to save new GCPs to a new file:

1. Click the Save current GCP list button in the ENVI Orthorectification Wizard.

2. The Select GCP File dialog appears.

3. Specify an output location and filename, then click Save.

GCP files are saved in the current projection.

Evaluating Residual Errors in GCPsFor the best orthorectification results, try to minimize the RMSE for the entire model by refining the positions of the pixels with the largest errors or by removing them. The RMSE is calculated when three or more GCPs are available. You can reduce errors by adding more GCPs. If you only have a few GCPs, place them near the image corners or widely scatter them throughout the image.

To see which GCPs have the highest error magnitudes, within the GCP list:

1. Click Show List in the ENVI Orthorectification Wizard. The Orthorectification Ground Control Points List appears.

2. Scroll to the right until you see the Magnitude (m) column. From the Orthorectification Ground Control Points List menu bar, select Options → Order Points by Error. The GCPs are reordered so that those with the highest errors appear at the top of the list.

3. Click on a row number for a given GCP to center the Layout View Area (in the Layout Manager) over that GCP.

4. You can examine the impact of the model RMSE by turning GCPs on and off.

You can also enable the Residual Vectors option in the Layout Manager and increase the Residual Exaggeration slider to view the error magnitudes at each GCP location, as shown



in Figure 3-3. You should have at least three GCPs before enabling the Residual Vectors option.

Enable the Residual Image option in the Layout Manager to see a contour view of the residual error magnitudes across the entire model. Red areas represent the highest errors, and

Figure 3-3: Error magnitudes and directions at each GCP location, indicated by arrows



blue-to-white areas represent the lowest errors. This area is restricted to the minimum bounding rectangle of the GCPs.

NoteThe accuracy of this plot is highly dependent on the distribution of GCPs. True errors may not follow a trend, but this contour plot provides an alternative to the residual vector display. Also, the Residual Image option is not a recommended choice for viewing the error magnitudes of GCPs that share the same map location or are very close to one another. Use the Residual Vectors option and Orthorectification Ground Control Points List instead.

Use the Residual Image option along with the Residual Vectors option, Residual Exaggeration slider, and RMSE field to evaluate which GCPs have the highest residual values. You can then correct or delete GCPs as needed to decrease the overall model error. The Residual Image option does not become available until you have selected at least three GCPs.

Figure 3-4: Displaying the residual error magnitudes across the entire model, by enabling the Residual Image option in the Layout Manager



Collecting and Editing Tie PointsTie points are pairs of pixels corresponding to the same locations where images overlap. They are an integral part of the bundle adjustment and account for most of the relative accuracy in the orthorectified product.

NoteBe sure that your tie points are of a quality that is consistently better than the initial accuracy of the sensor model; this varies with data type and vendor. For example, mid-latitude QuickBird scenes of a relatively flat area may provide an accurate orthorectification without the use of tie points.

When you click Next in Step 2, the ENVI Orthorectification Wizard proceeds to the optional Tie Point Selection panel (Step 3 of 5, Figure 3-5) if you selected more than one image for input. Two display groups appear.

ENVI Orthorectification Module User’s Guide Collecting and Editing Tie Points


The controls in this step of the wizard allow you to optionally restore tie point files, to add new tie points, and to edit existing tie points. You may choose to bypass this step by clicking Next. The orthorectified solution will be computed without tie points in this case.

Figure 3-5: Tie Point Selection panel (Step 3 of 5)

Collecting and Editing Tie Points ENVI Orthorectification Module User’s Guide


Tie Point File FormatThe ENVI Orthorectification Module supports tie point files in the following format:

; ENVI Rigorous Orthorectification TiePoint File; FileName0=image1.TIF; FileName1=image2.TIF; Image1File#, Image2File#, Image1(ImageX, ImageY), Image2(ImageX, ImageY); 0 1 195.250000 370.250000 6562.750000 398.250000

The first five lines in this example contain header information that lists the image filenames and the column labels.

The header rows that say FileNamex refer to the input image filenames, where x is an index number used to reference the filename throughout the rest of the tie points file. In the example above, “0” refers to image1.TIF and “1” refers to image2.TIF, as indicated in the header.

195.250000 and 370.250000 are the ImageX and ImageY values for image1.TIF.

6562.750000 and 398.250000 are the ImageX and ImageY values for image2.TIF.

If you are creating your own tie point file in a text editor or other application, you do not need to include a full path to the image filenames (in the lines that begin with FileNamex) as long as your filenames are unique. In cases where the filenames may be the same for all images (for example, CARTOSAT-1 and SPOT data), you need to include the full path to each image file in the FileNamex lines.

When you restore a tie point file, ENVI appends five new columns to the table displayed in the Orthorectification Tie Points List: Correlation, RMSE, DX, DY, and Magnitude.

Correlation and RMSE are image-to-image error metrics that are used to estimate the quality of a tie point match. These metrics are calculated for image chips that are the same size as the moving window size and that are centered on the estimated tie point image coordinates. (See Step 4 in “Automatically Generating Tie Points” on page 37 for a description of moving window size and image chips).

DX, DY, and Magnitude show the difference (in meters) in model-predicted map coordinates for the two tie-point image coordinates.

The image error metrics and image-to-ground difference metrics can be used to determine potentially poor quality tie points.

Restoring Tie Point FilesFollow these steps to restore an existing tie point file:

1. Click the Load Tiepoints button . The Select Tiepoints File dialog appears.

2. Select a tie points file, and click Open. The tie points are listed in the Orthorectification Tie Points List (click Show List in the ENVI Orthorectification Wizard), and their locations are displayed in the Layout View Area of the Layout Manager (if you enabled the Tie Point check box).



Manually Adding Tie Points Between Two ImagesFollow these steps to add new tie points to an existing set, or to create a new set of tie points between two images.

1. From the Image 1 drop-down list, select a filename for the first image from the list of open files. Or, Shift-click an image boundary in the Layout Manager to select it as Image 1. The first display group updates to show the image you selected. Image 1 is shown with a red border in the Layout Manager.

2. From the Image 2 drop-down list, select a filename for the second image from the list of open files. Or, Ctrl-click an image boundary in the Layout Manager to select it as Image 2. The second display group updates to show the image you selected. Image 2 is shown with a yellow border in the Layout Manager.

See “Changing the Image Stack Order” on page 16.

3. In the Image window of the display group corresponding to the first image, position the red Zoom box over the area where you want to add a new tie point.

4. In the Zoom window of this same display group, left-click on a specific pixel to center the Zoom window over that pixel or portion of that pixel.

The X/Y fields next to the Image 1 field in the table at the bottom of the ENVI Orthorectification Wizard show the image coordinates of the selected pixel underlying the crosshairs in the Zoom window. Subpixel coordinates are supplied to provide higher accuracy in selecting tie points. Whole-number pixel coordinates correspond to the upper-left corner of the pixel. The x,y values increase to the right and bottom of the pixel, respectively.

5. Identify the same point in the second image, and repeat Steps 3-4 to define the pixel coordinates for that point in the display group corresponding to the second image. The X/Y fields next to the Image 2 field in the ENVI Orthorectification Wizard show the image coordinates of the selected pixel.

NoteYou can also enter values directly into the Image 1 and Image 2 X/Y fields if you have already determined their image coordinates.

6. Click Add Point in the ENVI Orthorectification Wizard.

7. Click Show List to display the Orthorectification Tie Points List. The new tie point is added to this table.

When you add a tie point, a marker is placed in the Layout View Area of the Layout Manager, which consists of an ID next to an X symbol.The marker indicates the selected pixel (or subpixel location). The center of the marker (located under the X symbol) indicates the tie point location.

8. Add more tie points as needed, using the same procedures.

NoteThe RMSE value will update when you add tie points.



Automatically Generating Tie PointsImages must have rational polynomial coefficients (RPCs) for automatic tie point generation to work. If your images do not have RPCs, you will need to manually add tie points.

The ENVI Orthorectification Module uses an area-based image matching method, which compares the gray scale values of patches of two or more images and tries to find conjugate image locations based on similarity in those gray scale value patterns. Tie points are generated on the single-band image that you set in Options → Band Selection.

Follow these steps to automatically generate tie points between overlapping images.

1. To generate tie points for two images, select an image filename from the Image 1 and Image 2 lists. The two images must be different. Enable the All Images option, and click Auto Generate Points.

NoteA warning message will display if you select the same image from the Image 1 and Image 2 drop-down lists.

2. To generate tie points for all images where there is at least a 50% overlap in one dimension (for example, along an image edge), enable the All Images and Majority Overlap options, and click Auto Generate Points.

3. To generate tie points for all images, enable the All Images option only and click Auto Generate Points.

4. An Auto Tie Points dialog shows the progress of generating tie points for each pair of overlapping images, and the tie points are displayed in the Layout View Area of the Layout Manager and in the ENVI display groups that show the active images.

5. Tie points are generated based on a default set of parameters. To change these parameter values, select Options → Auto Tie Point Parameters from the ENVI Orthorectification Wizard menu bar. The Generate Tie Points dialog appears. The following parameters are used:

• Number of Tie Points: Specify the number of tie points to generate. This can be as few as 1, but the recommended value is 6 (default setting).

• Search Window Size: Specify the search window size, in square pixels. The search window is a defined subset of the image, within which the smaller moving window scans to find a topographic feature match for a tie point placement. The search window size can be any integer greater than or equal to 21, but it must be larger than the Moving Window Size. The default is 101. This value depends upon the accuracy of RPC information of the images, and it also depends on the roughness of terrain.

• Moving Window Size: Specify the moving window size, in pixels. The moving window scans methodically in the image subset area defined by the Search Window Size, looking for matches to a topographic feature. The moving window size must be an odd integer. The smallest allowable value is 5. The default is 31. Using a larger value results in a more reliable tie point placement, but takes longer processing time; conversely, a smaller value takes less processing time, but the tie



points are less reliable. Determining a good moving window size largely depends upon the image resolution and terrain type. Some general guidelines follow:

A. For a 10 meters or higher resolution image, use a range of 9–15.

B. For a 5–10 meter resolution image, use a range of 11–21.

C. For a 1–5 meter resolution image, use a range of 15–41.

D. For a 1 meter or less resolution image, use a range of 21–81 or higher.

E. Because most buildings in very high-resolution images in an urban area appear similar, specify a larger value to take the surrounding area into account for more robust matching results. To reduce the computational time for image matching, ENVI automatically uses a hierarchical matching method when the moving window size is greater than or equal to 19.

• Minimum Correlation: Specify a floating-point value for the minimum correlation coefficient. Any matches with a correlation coefficient smaller than this value are discarded. The default is 0.55.

• Chip Size: Specify an integer value for one side of a square image chip size for finding the interest points in the base image. Larger values require more computational time; however, the obtained tie points may be more useful. The default is 128, meaning a 128 x 128 image chip size.

Editing Tie PointsYou can edit the location of a tie point by changing the image coordinates in the ENVI Orthorectification Wizard, or by interactively selecting new locations in the Zoom window.

To manually edit the location of a tie point, perform the following steps:

1. Click Show List to display the Orthorectification Tie Points List.

2. Click a row number to select a given tie point. Both display groups center over the tie point location.

3. Enter updated Image 1 and Image 2 X/Y values in the ENVI Orthorectification Wizard.

4. Click Update in the Orthorectification Tie Points List. The updated tie point location is reflected in this table, in the display groups, and in the Layout Manager.

To interactively move a tie point, perform the following steps:

1. Select the row for that tie point in the Orthorectification Tie Points List, or click in a table cell within that row and click Goto. Both display groups center over the tie point, and the tie point is highlighted in the Layout Manager. If both images have consistent georeferencing (usually the case for adjacent along-track images in the same pass), tie points in the Layout Manager will be closely spaced and will appear as a single point. Otherwise, they will be shown as two distinct points.

2. In each display group, center the crosshairs in the Zoom window over a new pixel location for the tie point.

3. In the Orthorectification Tie Points List, click Update. The changes are reflected in the table, the display groups, and the Layout Manager.

To see which tie points have the highest error magnitudes:



1. Scroll to the right until you see the Magnitude (m) column. From the Orthorectification Tie Points List menu bar, select Options → Order Points by Error.

Removing Tie Points

To remove a tie point, select the row for that tie point in the Orthorectification Tie Points List, and click Delete. The RMSE value will update when you delete any tie points.

Turning Tie Points On and Off

Each time you turn a tie point on or off, the model is recomputed and the RMSE updates. If there are no GCPs, then turning on or off a tie point has no effect on the RMSE, but it does affect the model.

1. In the Orthorectification Tie Points List, click on a row corresponding to a tie point you want to turn off.

2. Click On/Off. The color of the tie point marker in the display group changes to green (by default) to indicate that it is off. You can change the default colors by clicking the Set TiePoint Color button . The Point Color dialog appears. Right-click on a color box and select the desired color

When you click Next to proceed to the Image Order and Cutline panel, tie points that were previously off are turned back on and used in the model. Tie points that you deleted will remain deleted. The purpose of the On/Off functionality is so that you can experiment with tie points and decide whether or not to permanently delete any.

NoteRestoring a project during Step 3 (tie point selection) means that all GCPs will be turned on but some tie points may still be turned off, depending on how you set them before saving the project. See “Saving and Restoring Projects” on page 20.

Saving Tie Point FilesTo save edits to an existing tie points file, or to save tie points to a new file:

1. Click the Save Tiepoints button in the ENVI Orthorectification Wizard.

2. The Select TiePoint File dialog appears.

3. Specify an output location and filename, then click Save.



Reordering Images and Defining CutlinesWhen you click Next in Step 3, the ENVI Orthorectification Wizard proceeds to the Image Order and Cutline Selection panel (Step 4 of 5, Figure 3-6). In this step, you optionally define which areas between two or more overlapping images that you want to appear in the final output. With each input image, you perform two steps: (1) define the hierarchy of the image relative to the others, and (2) define an optional cutline for the image.

You may choose to bypass this step by clicking Next. If you do not specify a cutline for a given image, ENVI uses the image frame by default. The image frame is the area of usable data within an image, excluding any background pixels (Figure 3-10).

You can only define one cutline per image, but they can be complex inclusion polygons encompassing detailed paths across the entire image.

Setting the Image OrderUse the Image order list to define how multiple images are ordered in the final output. This list is populated with the image filenames you added to the workflow earlier (see “Selecting

Figure 3-6: Image Order & Cutline Selection panel (Step 4 of 5)

Reordering Images and Defining Cutlines ENVI Orthorectification Module User’s Guide


Input Images and DEMs” on page 22). The image at the top of the list will be ordered first, followed by the second image, and so forth. Image ordering only pertains to areas of overlap between two or more images. Figure 3-7 shows an example:

Follow these steps to reorder images:

1. Click Reorder. The Reorder Input Images dialog appears.

2. Click-and-drag image filenames in the preferred order.

3. Click OK. The Image Order list updates to show the new hierarchy.

You can also set the image order in the Layout Manager, as described in “Layout View Area Controls” on page 16.

Drawing CutlinesCutlines are polygons that are used in combination with image ordering to define areas that you want to keep in the final output. They are typically used to hide seams in the final mosaic or to exclude poor quality regions and instead use higher quality regions that are lower in the image stack. A cutline is an inclusion polygon, similar to a cookie cutter applied to one or more images. Cutlines are often defined by drawing a line along a geographic feature such as

Figure 3-7: Example output from three ordered images with no cutlines. Image 1 is on top, and Image 3 is on the bottom.

ENVI Orthorectification Module User’s Guide Reordering Images and Defining Cutlines


a river or street, and snapping to the nearest image edges or corners to define a polygon. The following example shows a cutline for a single image.

Following is an example of applying a cutline to the first-order (top-most) image when you have multiple images:

Figure 3-8: Cutline on a single image

Figure 3-9: Example output when defining a cutline



If you do not specify a cutline for a given image, ENVI uses the image frame by default. The image frame is the area of usable data within an image, excluding any background pixels (Figure 3-10).

Follow these steps to define cutlines:

1. From the Active Image list, select an image that the cutline will apply to. Or, Shift-click on an image boundary in the Layout Manager to select an active image; it is shown with a red border. ENVI loads the corresponding display group.

2. Click the button. A new cutline name is added to the Cutlines list.

3. In the Layout Manager, enable the Cutline option to view cutlines.

In the Scroll window of the ENVI display group, draw a cutline using Alt-click to define the vertices. Use Ctrl-click to snap to the nearest image edge. Use Shift-click to snap to the nearest corner. Click the middle mouse button (if available) to delete the last vertex. Right-click to close the polygon. The cutline is shown in the Layout Manager. You can only add one cutline per image. Following is an example:

A suggested method is to draw the cutline in the Scroll window of the display group because you can see the entire image. However, if you need to more precisely draw a

Figure 3-10: Example of image frame

Figure 3-11: Example sequence for drawing a cutline

ENVI Orthorectification Module User’s Guide Reordering Images and Defining Cutlines


line along a geographic feature, you can follow these steps: a. Click in the Scroll window to center the Image window over a specific location. b. Add vertices (Alt-click) for your cutline in the Image window. When you reach the edge of the Image window and need to continue the cutline, switch back to the Scroll window and click once in a new location. The Image window will center over that location, and you can continue adding vertices for the cutline. c. Use Ctrl-click and Shift-click in the Scroll window to snap to the nearest edge and corner, respectively. Right-click in the Scroll window to to close the cutline polygon.

4. To remove a cutline, select the cutline name from the Cutlines list and click the button.

5. To change the default color of cutlines as they appear in the display group, click the Set cutline color button . The Point Color dialog appears. Left-click to cycle through various colors, or right-click to select a specific color.

6. If the active image is intially displayed as a grayscale image in the display group, you can click the MB button in the ENVI Orthorectification Wizard to display a multispectral image of Bands 3, 2, and 1. Click the MB button again to display a grayscale image. See “Selecting Bands” on page 21.

7. Restoring a project during Step 4 (cutline selection) means that all GCPs and tie points will be turned on by default. See “Saving and Restoring Projects” on page 20, “Turning GCPs On and Off” on page 29, and “Turning Tie Points On and Off” on page 39 for more information.

8. When you are done drawing cutlines, click Next in the ENVI Orthorectification Wizard.



Selecting Output ParametersWhen you click Next in Step 4, the ENVI Orthorectification Wizard proceeds to the Output Parameters panel (Step 5 of 5, Figure 3-12)..

Follow these steps to select an output filename, format, pixel size, and projection for the orthorectified product, and to preview the result before creating an output file.

1. In the Enter Output Filename field, select an output location and rootname for the output. ENVI will create a GeoTIFF file or band-interleaved-by-line (BIL) raster file in ENVI format, along with an output summary text file that lists the data used in the project.

2. From the File Format drop-down list, select GeoTIFF or ENVI for the output image.

3. Select the Auto Color Adjust option to perform automatic color and contrast adjustment in the output mosaic. Spacemetric developed the underlying method, which uses a least-squares adjustment to ensure radiometric consistency among the scenes that comprise the mosaic. Results will vary when running the same workflow more than once because the color adjustment is based on a randomized statistical calculation.

Figure 3-12: Output Parameters panel (Step 5 of 5)

ENVI Orthorectification Module User’s Guide Selecting Output Parameters


4. Enter an output pizel size in the Pixel Size (m) field, and press Enter. The projection information in the Output Parameters panel updates to show your changes. Pixels in orthorectified output are square, so this value is used for both the x and y dimensions of the pixel. By default, the pixel size is set to the smallest pixel size of the input images. The pixel size is always based on the initial UTM projection used in the workflow. A larger pixel size will result in a lower resolution image with faster processing time, while the default pizel size will result in the highest resolution image with slower processing time.

5. Set the output projection as described in “Setting the Output Projection” on page 46.

6. The Preview button allows you to create a reduced-resolution version of the orthorectified product so that you can look for potential problems before creating the full-resolution product. When you click Preview, ENVI performs orthorectification, creates the reduced-resolution mosaic, adds the mosaic to the Available Bands List, and automatically loads it into a new display group. The reduced-resolution mosaic is georeferenced to UTM WGS-84 and shows you the results of your cutline selection, auto color adjustment (if you previously selected the Auto Color Adjust option), and overall appearance of the final product. You can also use the reduced-resolution mosaic to check geolocation accuracy of the orthorectified product before creating a full-resolution version.

7. If you are satisfied with the preview results, you can choose to display the output by selecting Display Result on Finish. When processing is complete (after you click Finish), the ENVI Orthorectification Module closes and the orthorectified output image loads into a new display group.

8. Click Finish to create the orthorectified product. The ENVI Orthorectification Module closes.

Setting the Output ProjectionThe Post Processing Reprojection section of the Output Parameters step allows you to change the projection of the base image. The base image is defined as the orthorectified image before any post-processing occurs. It always has a UTM WGS-84 projection with a square pixel size set in the Model Grid Output section of the Output Parameters step. By default, the base image is not reprojected.

Figure 3-13: Post Processing Reprojection section of Output Parameters

Selecting Output Parameters ENVI Orthorectification Module User’s Guide


The current output projection is shown at the top. The default projection in the example is shown as follows:

UTM_Zone_13N Datum: WGS-84

The UTM zone number will vary, depending on the geographic location.

By default, the initial projection pixel size is the same (when viewed in meters) as the pixel size of the base output image. To change the projected pixel size and output number of pixels, follow these steps:

1. Click Change Projection. The Convert Map Projection Parameters dialog appears.

2. The X/Y Pixel Size fields indicate the projected pixel size in the standard units of that projection type. The Output X/Y Size fields indicate the output image size in pixels (by default). Change these values as needed, and press Enter. For best results, the projection pixel size should be greater than or equal to the base output pixel size. For detailed instructions on using this dialog, see “Converting Map Projections” in the ENVI User’s Guide.

3. Click OK in the Convert Map Projection Parameters dialog. The new projection information is shown in the Post Processing Reprojection section.

4. To reset the orthorectified image to the default UTM projection and minimum pixel size of the input images, click Reset Projection in the Output Parameters step.

ENVI Orthorectification Module User’s Guide Selecting Output Parameters


Selecting Output Parameters ENVI Orthorectification Module User’s Guide

Chapter 4

Programming


ENVI_RIGOROUS_ORTHO_DOIT . . . . . 50


50 Chapter 4: Programming

ENVI_RIGOROUS_ORTHO_DOITThis procedure automates the ENVI Orthorectification Module workflow for programmers in ENVI.

SyntaxSee the “Syntax” heading in the Overview chapter of the ENVI Reference Guide for a description of the syntax below.

ENVI_RIGOROUS_ORTHO_DOIT [, /AUTO_COLOR], DEM_FID=file IDs, DEM_POS=array, FID=file IDs [, FILETYPE=string] [, GCPS=array], OUT_NAME=string, PIXEL_SIZE=variable, R_FID=variable [, TIE_POINTS=array]

KeywordsAUTO_COLOR (optional)

Set this keyword to perform automatic color and contrast adjustment in the output mosaic. See Step 3 in “Selecting Output Parameters” on page 45.

DEM_FID

Set this keyword to an array of one or more file IDs representing digital elevation model (DEM) files. A file ID is a long-integer scalar with a value greater than 0. An invalid FID has a value of -1. The FID is provided as a named variable by one of several ENVI routines used to open or select an input file.

DEM_POS

Use this keyword to specify an array of band positions for the input DEM files, indicating the band numbers to use. DEM_POS is an array of long integers, ranging from 0 to the number of bands minus 1. Specify bands starting with zero (Band 1=0, Band 2=1, etc.). DEM_POS must consist of the same number of array elements as DEM_FID.

FID

Set this keyword to an array of one or more file IDs representing the input images used for orthorectification. The file ID (FID) is a long-integer scalar with a value greater than 0. An invalid FID has a value of -1. The FID is provided as a named variable by one of several ENVI routines used to open or select an input file.

The order of file IDs in the FID array determines the image stack order (see “Changing the Image Stack Order” on page 16 and “Setting the Image Order” on page 40). For example, if you want FID3 on top, FID1 in the middle, and FID2 on the bottom of the image stack for the output mosaic, set FID as follows: FID= [fid3, fid1, fid2].

FILETYPE (optional)Set this keyword to an ‘ENVI’ or ‘GeoTiff’ string to indicate the output file format.

GCPS (optional)

ENVI_RIGOROUS_ORTHO_DOIT ENVI Orthorectification Module User’s Guide

Chapter 4: Programming 51

Set this keyword to a string scalar with the filename of a single ENVI Orthorectification GCP file, or to a string array with the filenames of multiple GCP files.

OUT_NAME

Use this keyword to specify a string with the output filename for the resulting data.

PIXEL_SIZE

Use this keyword to specify the square pixel size. PIXEL_SIZE is a single, double-precision variable that specifies the output square pixel size in meters.

R_FID

Set this keyword to a named variable containing the file ID to access the processed data. Specifying this keyword saves you the step of opening the new file from disk.

TIE_POINTS (optional)Set this keyword to a string scalar with the filename for a single ENVI Orthorectification tie points file, or to a string array with the filenames of multiple tie points files.

ExampleThis example orthorectifies two QuickBird scenes of Phoenix, Arizona, USA, which are available on the ENVI 4.7 Resource DVD. It demonstrates how to open the imagery, DEM, GCP file, and tie points file. The output initially is in a UTM WGS-84 projection, but it will be reprojected into a geographic coordinate system (latitude/longitude).

Preparing Input Data

You will copy the input data for this project to a local, writeable directory.

1. Create a new, writeable directory called workdata at the root level of your C: drive (C:\workdata).

2. On the ENVI 4.7 Resource DVD, navigate to X:\Data\rigorous_ortho, where X is the DVD drive.

3. Copy the file phoenix_DEM_subset.tif to workdata. This is a DEM of the Phoenix area, subsetted to the area encompassing the imagery.

4. On the ENVI 4.7 Resource DVD, navigate to X:\Data\rigorous_ortho\005606990010_01_P008_MUL. Copy all of the files in this directory into your workdata directory. These files comprise the QuickBird imagery of Phoenix from 11 July 2005.

5. On the ENVI 4.7 Resource DVD, navigate to X:\Data\rigorous_ortho\005606990010_01_P011_MUL. Copy all of the files in this directory into your workdata directory. These files comprise the QuickBird imagery of Phoenix from 09 October 2005.

6. Using a text editor such as Notepad, copy the following GCP content into a new file called phx_GCPs.pts, and place it in your workdata directory.

; ENVI Rigorous Orthorectification GCP File; FileName0=C:\workdata\05OCT09183407-M1BS-005606990010_01_P011.TIF

ENVI Orthorectification Module User’s Guide ENVI_RIGOROUS_ORTHO_DOIT


; FileName1=C:\workdata\05JUL11182931-M1BS-005606990010_01_P008.TIF; projection info = {Geographic Lat/Lon, WGS-84, units=Degrees}; ImageFile#, Map (x,y,elev), Image (x,y);1 -112.004007 33.417396 311.083000 3569.000000 479.0000001 -112.094899 33.385167 309.603000 245.000000 1861.0000001 -112.038770 33.388203 316.788000 2263.000000 1788.0000001 -112.074479 33.413951 296.985000 910.000000 621.0000001 -111.978822 33.380607 336.632000 4521.000000 2143.0000001 -111.930535 33.402122 327.582000 6332.000000 1181.0000000 -111.894365 33.390883 333.140000 1279.000000 1631.0000000 -111.900306 33.419145 329.810000 1058.000000 385.000000

NoteYou may be prompted, “You are about to save the document in a Text-Only format, which will remove all formatting. Are you sure you want to do this?” Click Yes.

7. Copy the following tie point information into a new file called phx_TIEs.pts, and place it in your workdata directory.

; ENVI Rigorous Orthorectification TiePoint File; FileName0=C:\workdata\05OCT09183407-M1BS-005606990010_01_P011.TIF; FileName1=C:\workdata\05JUL11182931-M1BS-005606990010_01_P008.TIF; Image1File#, Image2File#, Image1(ImageX, ImageY), Image2(ImageX, ImageY); 1 0 6737.000000 511.750000 363.750000 480.000000 1 0 6860.250000 1997.750000 489.750000 1928.000000 1 0 6835.500000 3174.000000 471.750000 3073.500000

Running the Program

Copy the following code into a new IDL file named ortho_doit_example.pro, and save it to your IDL working directory. Compile and run the program in ENVI + IDL.

pro ortho_doit_example ;Output Parameters pixel_size = 15.0d ; Set the square pixel size in meters filetype='GeoTiff' ; Set file output type to GeoTiff autocolor=0 ; Turn off auto color balancing filename1 = 'c:\workdata\Ortho_Output.tif' ; Set the output filename ;Input data inputimage1 = 'c:\workdata\05JUL11182931-M1BS-005606990010_01_P008.TIF' inputimage2 = 'c:\workdata\05OCT09183407-M1BS-005606990010_01_P011.TIF' inputdem = 'c:\workdata\phoenix_DEM_subset.tif' inputgcps = 'c:\workdata\phx_GCPs.pts' inputties= 'c:\workdata\phx_TIEs.pts'


Chapter 4: Programming 53

;Loading Images and DEMs into ENVI and setting sensor type ; Open image 1 envi_open_data_file, inputimage1, /tiff, r_fid=fid1 sensor_type = envi_sensor_type('quickbird') envi_change_head, fid1, sensor_type=sensor_type ; Open image 2 envi_open_data_file, inputimage2, /tiff, r_fid=fid2 sensor_type = envi_sensor_type('quickbird') envi_change_head, fid2, sensor_type=sensor_type ; Open the DEM envi_open_file, inputdem, r_fid=dfid ;Pass data to the Rigorous Ortho module envi_doit, 'envi_rigorous_ortho_doit', $ r_fid=ofid, $ ;Returned File ID of the output image fid=[fid1,fid2], $ ;File IDs of the input images dem_fid=dfid, $ ;File ID of the DEM image dem_pos=1, $ ;Band to use in the DEM image gcps=inputgcps, $ ;GCP filename tie_points=inputties, $ ;Tie point filename pixel_size=pixel_size, $ ;Square output pixel size filetype=filetype, $ ;Filetype autocolor=autocolor, $ ;Auto color adjustment option out_name= filename1 ;Output image filename ;Reproject output the orthorectified UTM file to a geographic coordinate system envi_open_data_file, filename1, r_fid=ofid, /tiff ;Output projection parameters filename2 = 'c:\workdata\Ortho_Output_Reprojected' envi_file_query, ofid, dims=dims, nb=nb, fname=fname pos=lindgen(nb) o_proj=envi_proj_create(/geographic) ;Convert the file map projection envi_convert_file_map_projection, $ r_fid=rfid, $ ;Returned File ID of the output image fid=ofid, $ ;File IDs of the input image pos=pos, $ ;Bands to be included in output image dims=dims, $ ;dimensions of the output image o_proj=o_proj, $ ;Projection of the output image resampling=0, $ ;Set interpolation to nearest neighbor warp_method=2 , $ ;Set warping method to triangulation out_name=filename2 ;The output name of the reprojected file

end

The process will take several minutes to complete. Output will consist of the following files:

• Ortho_Output.tif: orthorectified base image in a UTM WGS-84 projection

• Ortho_Output_project_summary.txt: summary file listing data used in the project

ENVI Orthorectification Module User’s Guide ENVI_RIGOROUS_ORTHO_DOIT


• Ortho_Output_Reprojected: Orthorectified image in ENVI format, reprojected to a geographic coordinate system

• Ortho_Output_Reprojected.hdr: Header file for above.

The resulting images are added to the ENVI Available Bands List. To validate the results, open the two images (Ortho_Output.tif and Ortho_Output_Reprojected) in separate display groups. From one of the Display group menu bars, select Tools → Link → Geographic Link. In the Geographic Link dialog, use the toggle buttons to turn both images On. Select a location in either image to move to the corresponding location in the other image.


Chapter 5

Tips and Tricks for the ENVI Orthorectification Module


Tips and Tricks . . . . . . . . . . . . . . . . . . . . . . . . . 56


56 Chapter 5: Tips and Tricks for the ENVI Orthorectification Module

Tips and TricksFollowing are some helpful tips and tricks for using the ENVI Orthorectification Module.

• Ensure that your working directories do not have read-only permission. The ENVI Orthorectification Module creates temporary files during the orthorectification process and needs write access in order to save them to your working directory.

• It is recommended that you copy input images and DEMs to a local drive rather than access them through a network.

• Input DEMs should cover a contiguous map region about 10% or greater than the total image area (to account for relief displacement), and elevations should be expressed in meters above sea level.

• The recommended minimum screen resolution for using the ENVI Orthorectification Wizard is 1152 x 864.

• The ENVI Orthorectification Module requires input images to be in their native format, filenames, and directory structure, exactly as they are delivered by the data provider. For example, you cannot spatially subset a QuickBird GeoTIFF image, rename the subset, save it as a TIFF image, and read it into the ENVI Orthorectification Module. The imagery must include all associated metadata and ephemeris data. See “Introduction to the ENVI Orthorectification Module” on page 6 for a list of supported image formats and accompanying metadata.

• For UltraCam data, you must separate RGB images, color-infrared (CIR) images, and panchromatic images, in different directories. All must be at the same level as the Lvl03 directory.

• For the best orthorectification results, try to minimize the RMS error by refining the positions of the GCPs with the largest errors, or by removing them. You can reduce errors by adding more GCPs. If you only have a few GCPs, place them near the image corners or widely scatter them throughout the image.

• Be sure that your GCPs and tie points are of a quality that is consistently better than the initial accuracy of the sensor model; this varies with data type and vendor. For example, mid-latitude QuickBird scenes of a relatively flat area may provide an accurate orthorectification without the use of GCPs or tie points.

• Files in the ENVI GCP format (described in the topic “ENVI ASCII Files” in ENVI Help) will not work with the ENVI Orthorectification Wizard; they must be in the Rigorous Orthorectification GCP file format.

• Use the Residual Image option along with the Residual Vectors option, Residual Exaggeration slider, and RMSE field to evaluate which GCPs have the highest residual values. You can then correct or delete GCPs as needed to improve the overall error for the model. The Residual Image option does not become available until you have selected at least four GCPs.

• Images must have rational polynomial coefficients (RPCs) for automatic tie point generation to work. If your images do not have RPCs, you will need to manually add tie points.

Tips and Tricks ENVI Orthorectification Module User’s Guide

Chapter 5: Tips and Tricks for the ENVI Orthorectification Module 57

• If you do not specify a cutline for a given image, ENVI uses the image frame by default. The image frame is the area of usable data within an image, excluding any background pixels.

• Restoring a project during Step 3 (tie point selection) means that all GCPs will be turned on but some tie points may still be turned off, depending on how you set them before saving the project.

• You can set the image order in Step 4 of the worflow, or from the Layout Manager.

• Auto Color Adjust results (in Step 5 of the workflow) will vary when running the same workflow more than once because the color adjustment is based on a randomized statistical calculation.

• From the ENVI Orthorectification Wizard menu bar, select Options → View Project Summary to review images, DEMs, GCPs, tie points, and cutlines currently used in the project.

ENVI Orthorectification Module User’s Guide Tips and Tricks

58 Chapter 5: Tips and Tricks for the ENVI Orthorectification Module

Tips and Tricks ENVI Orthorectification Module User’s Guide

Appendix A

Glossary

This glossary defines terms used in the ENVI Orthorectification Module.

base image In the final step of the orthorectification workflow, the base image is the orthorectified image before any post-processing (such as reprojection) occurs.

cutlines Line segments that specify the locations in image overlap regions where joining occurs.

GCP Ground control point; a point on the ground whose location is known through a horizontal coordinate system and vertical datum. A GCP relates a point in a remote sensing image (x,y) to a geographic point on the earth (latitude, longitude, height, for example).

orthorectify To remove the effects of terrain relief displacement and imaging geometry from remote sensing imagery.

RMS error Root mean square error, or RMSE; a statistical measure that represents the difference between measured and predicted data points. In ENVI, RMS error is often used to evaluate a set of GCPs for georeferencing:

Where en is the error residual for the nth point and is the magnitude of the difference between actual and projected ground locations (x,y,z).

In the ENVI Orthorectification Module, RMSE is measured in meters.

en( )2

n 0=

N

∑

n----------------------------

1 2⁄


60 Appendix A: Glossary

RPC Rational polynomial coefficients; used to build interior and exterior orientation in photogrammetry.

tie points The location of a single feature across two or more overlapping images, used in image-to-image registration.

ENVI Orthorectification Module User’s Guide

Index
Ccopyrights, 2
EENVI_RIGOROUS_ORTHO_DOIT, 50

Gglossary, 59

Llegalities, 2

Oorthorectifying

rigorous, 6aerial cameras, 9cutlines, 40GCPs, 25glossary, 59image order, 40

layout manager, 14output, 45project summary, 21residual errors, 30saving and restoring projects, 20selecting bands for display, 21selecting images and DEMs, 22selecting input data, 23starting, 23supported image formats, 6tie points, 33wizard, 20workflow, 10

Rrigorous orthorectification, 6

aerial cameras, 9cutlines, 40GCPs, 25glossary, 59image order, 40layout manager, 14output, 45project summary, 21residual errors, 30


62

saving and restoring projects, 20selecting bands for display, 21selecting images and DEMs, 22selecting input data, 23starting, 23supported image formats, 6tie points, 33

wizard, 20workflow, 10

Ttrademarks, 2

ENVI Orthorectification Module User’s Guide Index

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