This tutorial uses Polarimetric SAR data and a Digital Elevation Model (DEM) generated from JPL's TOPSAR (interferometric SAR) data for Tarrawarra, Australia. Data Courtesy of JPL. The exercise demonstrates input and display of the SAR data and display and analysis of the DEM using standard tools within ENVI. For the DEM, these include data input; grayscale and color-density-sliced display; generation and overlay of elevation contours, use of ENVI's X, Y, and arbitrary profiles (transects) to generate terrain profiles; generation of slope, aspect, and shaded relief images; and 3-D perspective viewing and image overlay.
Files Used in This Tutorial
This dataset includes all of the typical files provided by JPL for a TOPSAR distribution tape or CD. Some of the filenames have been changed from JPL's standard naming convention because they would have non-unique ENVI header names using DOS 8.3 filenames, and thus could not be used directly by systems supporting only the DOS convention (Windows 3.1). The original filenames are indicated in the file descriptions.
You must have the ENVI TUTORIALS & DATA CD-ROM mounted on your system to access the files used by this tutorial, or copy the files to your disk.
The files used in this tutorial are contained in the TOPSAR subdirectory of the ENVIDATA directory on the ENVI TUTORIALS & DATA CD-ROM.
The files listed below are required to run this exercise. Not all files are provided so several new files will be generated during the processing steps.
Data Provided in Typical JPL TOPSAR Distribution
TS0218_C.VVI C-Band VV-polarization image, integer format. Original name TS0218_C.VVI2
TS0218_C.COR C-Band Correlation image: original name TS0218_C.CORGR
TS0218_C.DEM C-Band-Derived, integer-scaled DEM: original name TS0218_C.DEMI2
TS0218_c.INC C-Band incidence angle image: original name TS0218_C.INCGR
TS0218_L.DAT L-Band stokes matrix data
TS0218_P.DAT P-Band stokes matrix data
Files Generated During This Tutorial
TS0218LP.SYN L- and P-Band Synthesized data
TS0218LP.HDR Generated ENVI Header for Above
TS_CGAM.IMG Generated Gamma filtered image
TS_CGAM.HDR Generated ENVI Header file for Above
TS_PED.IMG Generated Pedestal Height image
TS_PED.HDR Generated ENVI Header for Above
TOPSAR.IMG Generated DEM image scaled to meters,
C-VV scaled to Sigma Zero, and incidence angle and correlation images.
TOPSAR.HDR Generated ENVI Header for Above
TOPSAR data are a polarimetric Synthetic Aperture Radar (SAR) dataset generated by an airborne SAR system flown by Jet Propulsion Laboratory (JPL).
Description of the Dataset
The data consist of fully polarimetric (quad polarized) data for both P- and L-band and a C-Band VV-polarization image. The Digital Elevation Model (DEM) provided is generated by JPL using SAR interferometry using the C-Band antenna. Also provided are the correlation image and an incidence angle image generated from the C-band data.
JPL provides several images for display including the C-Band VV, C-Band correlation image, and C-Band Incidence Angle image, which are easily displayed using ENVI TOPSAR routines. The P- and L-Band data are provided as non-viewable Stokes Matrix data and images must be synthesized as described below.
Before attempting to start the program, ensure that ENVI is properly installed as described in the installation guide.
- To start ENVI in Unix, enter " envi " at the UNIX command line.
- To start ENVI from a Windows or Macintosh system, d ouble-click on the ENVI icon.
The ENVI Main Menu appears when the program has successfully loaded and executed.
View the TOPSAR Headers
- Select Radar Tools->Topsar Tools->View AIRSAR/TOPSAR Header (this can also be selected from Radar Tools->View AIRSAR/TOPSAR Header).
Note that on some platforms you must hold the left mouse button down to display the submenus from the Main Menu.
An Enter Input Data File file selection dialog appears.
- Navigate to theTOPSAR subdirectory of the ENVIDATA directory on the ENVI TUTORIALS & DATA CD-ROM just as you would in any other application and select the file TS0218_C.VVIfrom the list and click "OK".
The AIRSAR File Information dialog appears listing information from the embedded AIRSAR Integrated Processor headers.
- Review the New Header, Parameter Header, and the Calibration header.
- Repeat the above for the DEM file TS0218_C.DEM, viewing the New Header, Parameter Header, and the TOPSAR DEM header in place of the above Calibration header.
Load and Display the Raw C-Band image
- Select Radar Tools->Topsar Tools->Open Topsar File and choose the file TS0218_C.VVI.
This opens the TOPSAR C-Band data and displays without converting to physical units (Sigma Zero), utilizing the embedded AIRSAR/TOPSAR header to get the required file information, and puts the image into the Available Bands List. This can also can be started from File->Open External File->Remote Sensing Formats->TOPSAR. You could also open this file using the generic file opening procedure, File ->Open Image File, however, then you would have to manually enter the file parameters.
- Make sure that the Grayscale radio button is selected, choose the image at the top of the dialog, and click on Load Band to load the image.
- Examine the geometry and characteristics of the image. Start the Cursor Location/Value dialog by clicking the right mouse button in the Main Image Display to toggle the functions menu and selecting Functions->Interactive Analysis->Cursor Location/Value.
This is a ground-range C-Band VV-polarization image scaled to integer format. A scaling factor must be applied to the data to convert to Sigma Zero (radar backscatter coefficient).
- review the image pixel values, observing the general magnitude of the numbers (integer values).
Figure 1: TOPSAR C-Band VV Image.
Load and Display the Raw DEM image
- Select Radar Tools->Topsar Tools->Open Topsar File and choose the file TS0218_C.DEM.
This opens the TOPSAR DEM data, utilizing the embedded AIRSAR/TOPSAR header to get the required file information and puts the image into the Available Bands List. You could also open this file using File ->Open Image File, however, then you would have to manually enter the file parameters.
- Display the image by clicking on the band name in the Available Bands List and then on Load Band. Select Functions->Interactive Analysis->Cursor Location/Value and review the image pixel values, observing the general magnitude of the numbers (integer values).
The image displayed is in RAW Digital Number, as stored in the DEM file.
Figure 2: TOPSAR DEM Image
- Select Functions->Link->Link Displays and use ENVI's dynamic overlay capabilities to compare the two images.
Convert the C-Band data to Sigma Zero and DEM to meters
- Select Radar Tools->Topsar Tools->Convert Topsar Data and choose the file TS0218_C.VVI.
ENVI automatically identifies all of the TOPSAR data present in this dataset based on the TOPSAR file naming convention. The VV Polarization, Correlation, Incidence Angle, and DEM images will be opened. The C-VV data will automatically be converted to Sigma Zero and the DEM to meters based upon values present in the TOPSAR headers.
- Enter the output filename TOPSAR.IMG and click OK to start the conversion.
The four images are placed in the available bands list.
- Display the C-VV Sigma Zero image by clicking on the VV Polarization band name in the Available Bands List, clicking New, and then on Load Band. Select Functions->Interactive Analysis->Cursor Location/Value and review the image pixel values, observing the general magnitude of the numbers (Sigma Zero).
- Select Functions->Link->Link Displays and use ENVI's dynamic overlay capabilities to compare the two C-VV images (Raw vs Sigma Zero).
- Display the DEM (meters) image by clicking on the DEM (m) band name in the Available Bands List, clicking New, and then on Load Band. Toggle on the Functions menu if not present by clicking the right mouse button in the main image window, then select Functions->Interactive Analysis->Cursor Location/Value and review the image pixel values, observing the general magnitude of the numbers (Elevation in meters). Note the large negative number (approximately -2911) associated with holes in the DEM and the image border. These are not valid elevations and these areas should be excluded from analysis using ENVI's masking functions.
- Select Functions->Link->Link Displays and use ENVI's dynamic overlay capabilities to compare the two DEM images (Raw vs Elevation in meters).
Both the L-Band and P-Band data are distributed by JPL in compressed Stokes Matrix format. These data can not be displayed directly by most image processing systems. ENVI provides utilities to decompress the data and synthesize to image format.
- Select Radar Tools->Polarimetric Tools->Decompress/Synthesize Images->Synthesize AIRSAR Images or Radar Tools->Decompress/Synthesize Images->Synthesize AIRSAR Images from the ENVI Main menu.
- Click on Open File in the Input Stokes Matrix Files dialog, select the file TS02218_L.DAT and click Open.
Both the L and P band Stokes Matrix file names will be entered into the dialog.
- Click OK to start the Synthesize Parameters dialog.
The "standard" polarization bands, L-HH, L-VV, L-HV, L-TP (total power), P-HH, P-VV, P-HV, and P-TP (total power) will be automatically entered into the dialog.
Figure 3: Synthesize Parameters dialog.
- If additional polarizations are desired, enter the Transmit and Receive Ellipticity and Orientation angles into the appropriate text boxes in the upper left part of the dialog and click on Add Combination.
- Select "Byte" as the Output Data Type from the pulldown menu originally labeled "Floating Point", enter the output filename TS0218LP.SYN, and click OK to synthesize the images.
- Select one or more of the synthesized bands to display as grayscale or RGB images, load using the Available Bands List, and display.
- Compare the L-Band and C-Band VV data to the C-Band VV data using image linking and dynamic overlays.
Figure 4: TOPSAR P-Band VV Image.
This section of the tutorial describes some of the analysis options for general analysis of SAR data as well as selected polarimetric analysis capabilities. For additional SAR processing information see the ENVI User's Guide.
General Concepts for processing SAR with ENVI
Most standard ENVI processing functions are inherently radar capable including all display capabilities, stretching, color manipulations, classification, registration, filters, geometric rectification, etc. Specific Radar menu items are also included under the Radar Tools menu for specific ENVI routines that are particularly useful for radar processing. Many of these can also be accessed from their functional areas on the ENVI main menu. ENVI provides standard and advanced tools for analysis of detected radar images as well as advanced SAR systems such as JPL's fully polarimetric AIRSAR and SIR-C systems. ENVI can process ERS-1/ERS-2, JERS-1, RADARSAT, SIR-C, X-SAR, and AIRSAR data as well as any other detected SAR data set. In addition, ENVI is designed to handle radar data distributed in the CEOS format, and should be able to handle data from other radar systems that distribute their data in this format.
Remove Speckle using Adaptive Filters
Adaptive filters provide a means of removing radar speckle from images without seriously affecting the spatial characteristics of the data. The Gamma Filter and other adaptive filters provide considerable improvement over the unfiltered data. To perform the Gamma Filter on the AIRSAR data:
- Select Radar Tools->Adaptive Filters->Gamma, choose the input image TS0218_C.VVI and use the default filter size (3x3) and Number of Looks (1), enter the output filename TS_CGAM.IMG, and click OK.
A Laplacian Filter can be used to enhance edges in SAR data and other data types. This is a convolution filter with a kernel (for a 5 x 5 filter) of:
Of course, applying the kernel in this fashion strongly enhances the edges and causes loss of most of the radiometric information.
- From the ENVI Main Menu, select Filter ->Convolution ->Laplacian.
One solution to this problem is to "add-back" part of the original image.
- To apply add-back to a filtered image, select Filter->Convolution->Laplacian, and enter the add-back factor (between 0 and 1 where 1 is 100%) in the Convolution Parameters dialog.
Compare Images using Linked Images and Dynamic Overlay
Compare images generated above to each other and the original data using ENVI's dynamic overlay capabilities. To compare the images using Dynamic Overlay:
- Display the two images in separate displays and contrast stretch as described above.
- Click the right mouse button in one of the displays to toggle the Functions menu and select Functions ->Link ->Link Displays.
- Click "OK" to link the two images and compare by dragging the dynamic overlay with the left mouse button.
ENVI provides a full suite of tools for analyzing Polarimetric SAR data. These include generation and display of specific polarization images, a phase image, and pedestal height image as well as extraction of polarization signatures and generation of a scattering classification image.
Display Multifrequency, Polarimetric SAR Images
The synthesized images produced above form the basis for image analysis of polarimetric data.
- Display one of the synthesized images by selecting the band [L-HH}:TS0218LP.SYN in the Available Bands List and clicking on Load Band.
- Start an animation by choosing Functions->Interactive Analysis->Animation. Click on the check boxes for the two total power images to deselect and then OK to start the animation. Observe the different frequencies and polarizations and compare the SAR response for various materials to determine a 3-band combination that will maximize "spectral contrast".
- When finished, click Cancel to end the animation.
- Load a multifrequency, multipolarizaiton AIRSAR image into the display as an RGB image using your selected bands from above, or load the P-HH, L-HH, and the C-VV image as RGB and observe the color differences associated with various materials. Experiment with different contrast stretches using both the Main Display and the Zoom Windows to maximize color differences in the image.
Examine Polarization Signatures
Polarization signatures are 3D representations of the complete scattering characteristics for a single pixel or ROI in terms of the orientation angle and ellipticity angle of the transmitted and received radar waves. These signatures can be used to observe the scattering characteristics of materials and surfaces and to determine what polarization images to generate to maximize image contrast between materials as viewed in the SAR data.
- Position the cursor over an area that appeared to change significantly in the animation by clicking at the desired location in the Main Image Display window using the middle mouse button.
- Choose Interactive Analysis->Polarization Signatures->AIRSAR, click on the Open File button, and choose the L-Band Stokes Matrix data TS0218_L.DAT and click Open.
Both the L-Band and P-Band Stokes Matrix filenames will be entered into the Input Stokes Matrix Files dialog.
- Click OK to start the Polarization Signature Viewer window and choose Options->Extract Current Pixel to extract an L-Band polarization signature for the current pixel. Choose Frequency->P to display the corresponding P-Band polarization signature, then click in the signature in the left portion of the plot using the left mouse button to toggle different surface plotting options. Be sure to display and compare the cross-polarized signature.
- Choose Options->Show Statistics to view polarization signature information and statistics. Click Cancel to quit this function.
- Click and drag using the left mouse button in the image on the right side of the plot to display interactive 2-D, 3-D cursor. Read the cursor values in the lower left corner of the plot. Observe the maximum contrast orientation and ellipticity angles in the plot and optionally synthesize and display an image with these parameters as described above.
- Select Polsig_Data->Option, where Option is one of the available display methods. Be sure to display and compare the cross-polarized signature. Select File->Cancel to exit the function.
Pedestal Height Image
- Generate a pedestal height image for the AIRSAR data by choosing Radar Tools->Polarimetric Tools->Pedestal Height Image->AIRSAR, clicking OK in the Input Stokes Matrix Files dialog, entering the output filename TS_PED.IMG and clicking OK.
- Display the image by clicking on the band name in the Available Bands List and clicking on Load Band.
This image provides a measure of the amount of multiple scatter of the radar wave for every pixel by averaging the following four polarization combinations: Orientation 0 degrees, Ellipticity -45 degrees; Orientation 90 degrees, Ellipticity -45 degrees; Orientation 0 degrees, Ellipticity 45 degrees; Orientation 90 degrees, Ellipticity 45 degrees. A higher pedestal height value in the image indicates greater multiple scattering, generally a rougher surface.
- Select Interactive Analysis->Cursor Location/Value to start the Cursor Location/Value window and examine the pedestal height values for this image.
This portion of the tutorial describes ENVI's tools for processing and analyzing Digital Elevation Models. While this tutorial refers specifically to the TOPSAR DEM, all of these methods and tools are fully applicable to any DEM data set.
Display the DEM Converted to Meters
- If not already displayed, display the band DEM (m) from the generated file TOPSAR.IMG. Use Radar Tools->TOPSAR Tools->View AIRSAR/TOPSAR Header to examine the DEM header parameters.
- Select Functions->Interactive Analysis->Cursor Location/Value and examine the pixel values of the DEM.
This DEM has had a scaling function applied to convert these data to physical parameters; the elevation in meters. The formula for this conversion is on page 21 of the AIRSAR Integrated Processor Documentation, version 0.01, May 1995. The required information for this conversion is obtained from the TOPSAR DEM header, in this case, consisting of an ELEVATION INCREMENT of 0.1 and the ELEVATION OFFSET of 365.6. Also note that after this conversion, bad pixels (borders, radar shadows in the DEM, etc) will have the value -2911.099. You probably will want to generate a mask for these areas using ENVI masking capabilities to set these values to zero (see the ENVI User's Guide for details).
X and Y Elevation Profiles
ENVI provides tools for extracting elevation profiles along either the X or Y directions. Move these two plots off to the side of the image display so you can see them while moving the cursor in the display.
- Select Functions->Profiles->X Profile and then Functions->Profiles->Y Profile to extract the profiles for the two directions.
Figure 5: Spatial Profiles.
- Click and drag using the left mouse button to generate new elevation profiles when the left mouse button is released. The position of the red bar in the X and Y profiles marks the position of the center of the Zoom window along the profile. For continuous updating, click and drag using the middle mouse button.
- Close the two profiles by choosing Basic Tools->Display Controls->Close All Plot Windows.
Arbitrary Elevation Transect
ENVI also allows extraction of elevation profiles along multiple arbitrary transects.
- Select Functions->Profiles->Arbitrary Profile (Transect) to extract a profile along an arbitrary transect.
- Click using the left mouse button in the Main Display Window to draw each segment of the desired transect. You can hold down the left mouse button and draw if desired.
- Click the right mouse button once to close the last line segment and a second time to extract the profile.
Figure 6: Arbitrary Profiles (Transects).
- Draw one or more profiles using the Arbitrary Profile tool. Each additional profile will be assigned a new color and numbered.
Figure 7: Arbitrary Profile (Transects) plots.
- Profiles can also be drawn in the Scroll or Zoom windows by selecting the appropriate window in the Spatial Profiler dialog.
ENVI's standard plot controls can be used to read the elevation off of the plot.
- Click the left mouse button in the plot and drag along the profile. The Zoom Window will track the profile in the Main Image Display.
- Zoom in on portions of the plot using the middle mouse button to draw a box and zoom back out by clicking the middle mouse button below the plot axis.
- Close the profiles by selecting File->Cancel in the Spatial Profiler dialog.
Color Density Slice the DEM
Density slicing provides a means of visually enhancing radar differences based on image brightness.
- Click the right mouse button in the Main Image display containing the DEM in meters to toggle the Functions menu and choose Functions ->Display Enhancements->Color Mapping -> Density Slice.
- Click Clear Ranges in the Density Slice dialog to clear the default ranges (necessary because of the holes and mask values).
- Enter the value "0" in the "Min" text box. Select Options->Add New Range to start the Add Density Slice dialog.
- Enter 0.0 for the Range Start, 700.00 for the Range End, and 10 for # of Ranges, then click Apply in the Density Slice dialog to density slice the image.
- Load the grayscale DEM into a second image by clicking on the band name, the New button in the bottom right of the Available Bands List, and then Load Band.
- Use Dynamic Overlays to compare to the grayscale images.
Overlay of Elevation Contours
ENVI can generate elevation contours from the DEM data and overlay either on the DEM itself, or on another co-registered image.
- If the DEM image is still displayed, and the Color Density Slice is turned on, turn it off by selecting File->Cancel. If the DEM is not displayed, display the DEM (m) band as a grayscale image by clicking on the band name for the meters DEM in the Available Bands List followed by Load Band.
- Choose Functions->Overlays->Image Contouring in the Main Image Display window and choose DEM (m) as the input contour band.
- Click on Clear Levels in the Contour Plot dialog, enter 0.0 in the "Min" text box, and choose Options->Add New Levels. Enter 0 for the Level Start, 50 for the level increment, and 15 for the # of Levels, then click OK. Click Apply to plot the DEM contours on the DEM image. Examine the relation between contour lines and image brightness.
- Now display one of the synthesized images by selecting the band [L-HH}:TS0218LP.SYN in the Available Bands List and clicking on Load Band.
The contour lines will be overlain on the SAR band data.
Figure 8: Contour lines overlaid on DEM image.
Figure 9: Contour lines overlaid on TOPSAR C-Band Data.
3-D Perspective Viewing and Image Overlay
ENVI can create 3-D perspective views from DEM data and also overlay co-registered image data. TOPSAR provides the ideal data for this procedure because the standard dataset includes registered DEM and SAR data.
- Redisplay the meters DEM image "DEM (m)" by clicking on the band name and then the Load Band button in the Available Band List. Also redisplay the L-HH SAR band by clicking on the band name, clicking on New, and then Load Band at the bottom of the Available Band List.
- Select Functions->Interactive Analysis->3-D Surface Plots, click on the Subset by Image button in the Spatial Subset dialog, and drag the red outlining box using the left mouse button so that it doesn't include any border pixels. Click OK in the Subset Function dialog and then OK again in the Spatial Subset dialog.
A wire-frame 3-D surface of the DEM is drawn in a Surface Plot window.
Figure 10: Wireframe perspective view.
- Choose Edit->Plot Parameters from the Surface Plot menu bar and enter the values 75 and 45 for the AX and AZ parameters respectively. Click on Apply at the bottom of the dialog to redraw the 3-D surface.
- Select Surface->Shaded Surface from the Surface Plot menu bar and choose Display 1 (the DEM) as the overlay image in the Overlay dialog.
This overlays the DEM as a grayscale image on the 3-D perspective
- Select Surface->Shaded Surface from the Surface Plot menu bar and choose Display 2(the SAR image) as the overlay image in the Overlay dialog.
This overlays the L-HH SAR image on the DEM as a grayscale image.
Figure 11: C-Band VV data draped on TOPSAR-generated perspective view.
- Adjust the perspective view by entering different AX and AY values in the Plot Parameters dialog.
The AX parameter specifies the angle of rotation about the X-Axis, in degrees toward the viewer. Valid AX values are greater than 0 degrees and less than 90 degrees and tilt the surface towards the viewer. The AZ parameter specifies the angle of rotation counterclockwise about the Z-Axis, in degrees. Valid AZ values are greater than 0 degrees and less than 360 degrees and control the viewing direction.
- Also vary the overlay resolution by changing the Surface Resize parameter in the Plot Parameters dialog. Higher values display improved resolution, with a value of 1.0 corresponding to the full resolution image.
Generation of Slope, Aspect, and Shaded Relief
ENVI provides tools for processing DEMs to extract parametric information including slope and aspect, and to generate lambertian (shaded relief) surfaces. A plane is fit to a 3-pixel by 3-pixel box centered over each pixel and the slope and aspect of the plane calculated. The slope is measured in degrees, from 0 to 90. Aspect angle is measured with 0 degrees to the north with increasing angles in a clockwise direction. A root mean square (rms) error image is also generated indicating the planarity of the 9 pixel box.
- Select Radar Tools->TOPSAR Tools->Topographic Modeling from the ENVI Main menu. Click on the meters DEM band name DEM (m) in the Topo Model Input dialog and click OK. Alternatively choose Utilities->Data Specific Utilities->Digital Elevation->Topographic Modeling.
- Click on the Compute Elevation and Azimuth button in the Topo Model Parameters dialog and select 11 November 1996, 1200 hours as the time and enter -37 degrees, 21 minutes and 145 degrees 17 minutes respectively for the latitude and longitude. and a pixel size of 10 meters. Click Ok to return to the Topo Model Parameters dialog.
- Enter the output filename TS_MODEL.IMG and click OK to generate the slope, aspect, shaded relief, and RMS images.
- Click sequentially on each of the band names in the new image, New, and then Load Band to display the four parameter images in separate image windows. Use ENVI's dynamic overlays and the Cursor Location/Value dialog to examine the relations between the DEM and the parameter images.
Figure 12: Shaded Relief Image generated using TOPSAR DEM.
The final output from any image processing within ENVI is usually a map-oriented, scaled image-map for presentation or visual analysis and interpretation. Radar data can be used in map composition like any other data set. The TOPSAR data are inherently map-registered, so creating an output map is simply a matter of adding all of the standard map composition elements such as pixel, map, and geographic (latitude/longitude) grids; scale-bars; declination diagrams and north arrows; text and symbols; polygons, polylines, and geometric shapes (circles, rectangles); map keys and legends; and image insets. For additional information on map composition. See the Map Composition tutorial or the ENVI User's Guide. An example of a TOPSAR map composition is shown below.
Figure 13: TOPSAR Shaded Relief Image Map.
ENVI provides tools for end-to-end polarimetric SAR processing as well as a suite of tools for working with standard DEMs and DEMS generated from interferometric SAR data. These are fully integrated into the ENVI processing environment and may be utilized along with other non-SAR-specific routines to provide optimized analysis and map output.