ENVI Classic Tutorial: Atmospherically Correcting Hyperspectral Data Using FLAASH Atmospherically Correcting Hyperspectral Data using FLAASH 2 Files Used in This Tutorial 2 Opening the Uncorrected AVIRIS Image 3 Atmospherically Correcting the AVIRIS Image Using FLAASH 5 Viewing the Corrected Image 7 Comparing Images 7 References 8 Page 1 of 8
Atmospherically Correcting Hyperspectral Data using FLAASH This tutorial provides an introduction to using FLAASH to atmospherically correct a hyperspectral image. You will display the radiance image, apply an atmospheric correction, and examine the results. This tutorial applies to ENVI Classic version 5 and earlier. A newer tutorial, Preprocessing AVIRIS Data, is available for users of ENVI Classic 5.1. Please see the ENVI Classic 5.1 Help, or visit the Exelis website to access the tutorial. Note: The Atmospheric Correction Module: QUAC and FLAASH requires an additional license for your ENVI Classic installation; contact your Exelis sales representative to purchase a license. Files Used in This Tutorial The image used in this exercise was collected by the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) sensor, which is operated by NASA. The sample image covers a portion of the Jasper Ridge Biological Preserve, located in the eastern foothills of the Santa Cruz Mountains at the base of the San Francisco Peninsula, 9 km west of the Stanford University campus in San Mateo County, California. The AVIRIS data were provided courtesy of the NASA Jet Propulsion Laboratory (JPL) in Pasadena, California. This image contains approximately the same area as the Landsat TM image used for the multispectral FLAASH tutorial; however, the pixel size, image orientation, and collection dates are different. Download data files from the Exelis website. File JasperRidge98av.img (and.hdr) AVIRIS_1998_scale.txt JasperRidge98av_template.txt Description AVIRIS radiance image and header file Scale factors file for the AVIRIS data Template file Page 2 of 8
Opening the Uncorrected AVIRIS Image This exercise will demonstrate how to use FLAASH to produce an apparent surface reflectance image. 1. From the ENVI Classic main menu bar, select File > Open Image File. The Enter Data Filenames dialog appears. 2. Select JasperRidge98av.img. Click Open. The Available Bands List appears. 3. In the Available Bands List, right-click on JasperRidge98av.img and select Load True Color. The image is loaded into a display group. You may recognize several features in the scene, including a vertically oriented lake in the top center of the image, various types of vegetation in the left side of the image, and urban areas in the right side of the image. This image is a standard AVIRIS data product that JPL processed. It contains calibrated at-sensor radiance values that were scaled into 2-byte signed integers. 4. Right-click in the Image window and select Z Profile (Spectrum) to display the Spectral Profile plot window. 5. Move the Image box (inside the Image window) around the image and note how the shape of the radiance curve automatically updates in the Spectral Profile window. 6. Right-click in the Image window and select Pixel Locator. 7. Move the Pixel Locator so that you can see it and the Spectral Profile window at the same time. 8. In the Pixel Locator, enter 366 and 179 in the Sample and Line fields, respectively. Click Apply to center on this pixel location, which illustrates some of the common atmospheric features often seen in hyperspectral data: Page 3 of 8
9. Click and drag the cursor in the Spectral Profile window along the radiance spectrum, and locate the water vapor absorption features at approximately 760 nm, 940 nm, and 1135 nm. Note also the opaque atmospheric regions around 1400 nm and 1900 nm where virtually no signal is recorded at the instrument. You can also see a common CO 2 signature that consists of two absorption features near 2000 nm. 10. Keep the display group and Spectral Profile open for the next exercise. Page 4 of 8
Atmospherically Correcting the AVIRIS Image Using FLAASH 1. From the ENVI Classic main menu bar, select Spectral > FLAASH. The FLAASH Atmospheric Correction Model Input Parameters dialog appears. 2. Click Input Radiance Image. The FLAASH Input File dialog appears. 3. Select the file JasperRidge98av.img, and click OK. The input radiance image consists of 2-byte signed integer values. For FLAASH to compute the atmospheric correction, these data values must be converted into floating-point radiance values in units of μw / (cm 2 * nm * sr). In the next few steps, you will restore a scale factor file that will convert the values to floating point. 4. In the Radiance Scale Factors dialog, select the Read array of scale factors (1 per band) from ASCII file radio button, and click OK. The file selection dialog appears. 5. Select the file AVIRIS_1998_scale.txt and click Open. The Input ASCII File dialog appears. 6. Accept all of the default values, and click OK. The 1998 AVIRIS scale factors (which are valid for all AVIRIS data collected between 1995 and 2003) are 500 for the first 160 bands and 1000 for the remainder. In the FLAASH Atmospheric Correction Model Input Parameters dialog, the default path and filename for the reflectance output are displayed in the Output Reflectance File field. 7. In the Output Reflectance File field, type the full path of the directory where you want to write the output reflectance file. For the filename, type JasperRidge98av_flaash.img. To navigate to the desired output directory before defining the output filename, click the Output Reflectance File button. 8. In the Output Directory for FLAASH Files field, type the full path of the directory where you want to write all other FLAASH output files. You may also click the Output Directory for FLAASH Files button to navigate to the desired directory. 9. In the Rootname for FLAASH Files field, type a root name that will be added as a prefix to the FLAASH output files. In the next step, ENVI Classic will automatically add an underscore character to the root name that you enter. FLAASH output files consist of a column water vapor image, cloud classification map, journal file, and (optionally) a template file. All files are written to the FLAASH output directory, and the root name is added as a prefix to the individual standard filenames. 10. Click the Restore button, located on the bottom right of the FLAASH Atmospheric Correction Model Input Parameters dialog. Page 5 of 8
11. Select the file JasperRidge98sav_template.txt, and click Open. This file provides some scene information for the AVIRIS image, along with FLAASH model parameters. The FLAASH Atmospheric Correction Model Input Parameters dialog should look similar to the following. 12. Click Advanced Settings at the bottom of the dialog to explore the advanced options that are available. The parameters in the Advanced Settings dialog allow you to adjust additional controls for the FLAASH model. The default setting for Automatically Save Template File is Yes, and the default for Output Diagnostic Files is No. While you may find it excessive to save a template file for each FLAASH run, the template file is the only way to determine the model parameters that were used to atmospherically correct an image after the run is complete, so it is important to be able to access it. The ability to output diagnostic files is offered solely as an aid for Exelis Technical Support to help diagnose problems. 13. Click Cancel to dismiss the Advanced Settings dialog. 14. In the FLAASH Atmospheric Correction Model Input Parameters dialog, click Apply to begin FLAASH processing. You may cancel the processing at any point, but be aware that there are some FLAASH processing steps that cannot be interrupted, so the response to the Cancel button may not be immediate. Page 6 of 8
Viewing the Corrected Image When FLAASH processing is complete, the output reflectance image, column water vapor image, and cloud classification map, are added to the Available Bands List. You should also see the journal file and template file in your FLAASH output directory. 1. Click Cancel in the Atmospheric Correction Model Input Parameters dialog to dismiss the dialog. 2. Examine, then close, the FLAASH Atmospheric Correction Results dialog. 3. From the Available Bands List, right-click on JasperRidge98av_flaash.img (the reflectance file you just created), and select Load True Color to <New>. The image is loaded into a new display group. Comparing Images In the following steps, you will compare the Z Profile values of the uncorrected radiance image with the FLAASH-corrected reflectance image. You should already have the display group and Spectral Profile open from the original radiance image. 1. Right-click in any of the two Image windows and select Link Displays. The Link Displays dialog appears. 2. Ensure that Display #1 and Display #2 both say Yes, and click OK. 3. Right-click in the Image window for Display #2 (reflectance image), and select Z Profile (Spectrum). 4. In the Image window for Display #2, click and drag around the image and note the shape of the reflectance curves. Note that some bands in the reflectance image have been designated as ENVI Classic "bad" bands and are not displayed in the plot window. The bad bands list in the ENVI header file is automatically set by FLAASH according to the strength of the reflectance signal. 5. Right-click in any Image window and select Pixel Locator. The Pixel Locator dialog appears. 6. Enter 366 and 179 in the Sample and Line fields, respectively. Click Apply. Compare the shape of the radiance curve (#1 Spectral Profile) with the shape of the reflectance curve (#2 Spectral Profile) for the same pixel location. This is one way to verify that the atmospheric correction was successful. Page 7 of 8
References Abreau, L. W., and G. P. Anderson, eds. The MODTRAN 2/3 Report and LOWTRAN 7 Model. Air Force Research Laboratory, Hanscom AFB, MA. 01731-3010, prepared by Ontar Corp. under Contract No. F19628-919C-0132. January 1996. Kaufman, Y. J., A. E. Walk, L. A. Refer, B.-C. Gao, R. -R. I, and L. Fling. The MODIS 2.1-mm Channel--Correlation with Visible Reflectance for Use in Remote Sensing of Aerosol. IEEE Transactions on Geosciences and Remote Sensing, Vol. 35, pp. 1286-1298. 1997. Copyright Notice: ENVI Classic is a registered trademark of Exelis Inc. QUAC and FLAASH are registered trademarks of Spectral Sciences, Inc. Page 8 of 8