Preprocess AVIRIS-3 Radiance Data

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Lesson content

Lesson 1 of 1

Preprocess AVIRIS-3 Radiance Data

In this quick guide, you will:

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Open and explore an AVIRIS-3 radiance image. * •

Review and exclude bands that correspond to water vapor and CO2 absorption features. * •

Use the Quick Atmospheric Correction (QUAC®) tool to mitigate atmospheric effects and calibrate the image to apparent surface reflectance.

Sample Data

Download sample data below. Then extract the contents of the .zip file to a local directory.

[AVIRIS3_Radiance.zip

1.6 GB

DownloadArrow down with horizontal line beneath it](assets/AVIRIS3_Radiance.zip)

Background

Radiance data should be corrected for atmospheric effects and calibrated to surface reflectance prior to spectral analysis. In remote sensing, materials are identified by their reflectance spectra; therefore, it is common to correct an image to reflectance as a first step toward locating and identifying features in an image.

If you plan to use spectral libraries or other reference spectra to associate pixels with known materials at the surface, then image data must be in units of reflectance.

This also ensures that the shape of image spectra are mostly influenced by chemical and physical properties of the surface materials, rather than by atmosphere and solar effects.

NASA also provides Level-2A reflectance imagery that has been corrected for atmospheric effects. However, you may choose to calibrate and correct radiance imagery on your own using a tool like FLAASH® or QUAC® (available in ENVI). This exercise uses QUAC to perform atmospheric correction and to create a reflectance image.

Open and Explore a Radiance Image

1

From the Menu bar, select File > Open. An Open dialog appears. 2. 2

Go to the location where you saved the sample data, and select the file AVIRIS3_Radiance_MountainPassCA.dat. 3. 3

Click Open. ENVI displays the image with an approximate true-color representation. 4. 4

Press the F12 key on your keyboard to see the full extent of the image.

5

Double-click the file name in the Layer Manager to open the View Metadatadialog where you can view detailed information about the image. 2. 6

Click the Spectral category on the left. A table shows the center wavelengths for each band. The "FWHM" column lists the full width half maximum values that NASA provided for the AVIRIS-3 file.

7

Close the View Metadata dialog. 2. 8

In the middle part of the Status Bar (at the bottom of the ENVI application), right-click and select Raster Data Values. When you move the cursor around the image, the Status Bar shows the pixel values for the red, green, and blue channels.

Radiance values are in units of microwatts/(centimeters2 * steradians * nanometers).

9

Click the Spectral Profile button in the Toolbar.

The Spectral Profile dialog appears, and red crosshairs are displayed in the Image window.

10

Click inside of the image to move the crosshairs to a different location. The Spectral Profile dialog updates with the spectrum of that pixel. Notice how the shape of the radiance curve changes slightly as you move the crosshairs to different materials within the scene. In general, the Spectral Profile shows some of the common atmospheric absorption features often seen in hyperspectral data.

11

Close the Spectral Profile dialog.

Review and Exclude Noisy Bands

Next, you will use the ENVI Xtreme Viewer to visually review the AVIRIS-3 radiance bands. You will identify those that contain a combination of striping, sensor artifacts, and noise from water and CO2 absorption. Then you will define a spectral subset of the radiance image that excludes those bands.

1

In the Layer Manager, right-click on AVIRIS3_Radiance_MountainPassCA.dat and select Band Animation >Using Xtreme Viewer. The Xtreme Viewer appears, beginning with Band 1. Playback controls are located along the bottom of the viewer. 2. 2

The default playback speed is 30 frames per second, which is too fast. To slow it down, click the Frames per Second drop-down list at the top of the viewer and select 3. 3. 3

Click the Play Forwards button to start the animation. 4. 4

Review each band to look for noise. For example, Band 133 shows striping noise. Click the Pause button if you need more time to review an individual band.

5

When you are finished animating the bands, close the Xtreme Viewer. The greyscale raster series is removed from the Layer Manager and Image window.

For this exercise, you will create a new image that excludes the following noisy bands:

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Band 1 * •

Bands 129 - 136 * •

Bands 193 - 208

6

From the Menu bar, select File > Save As > Save As (ENVI, NITF, TIFF, DTED). The Data Selection dialog appears. 2. 7

Click the Spectral Subset button. The Spectral Subset dialog appears. All bands are selected by default. 3. 8

Press and hold the Ctrl key on your keyboard while following the steps below:

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Click on Band 1to deselect it. * •

Scroll down. * •

One at a time, deselect Bands 129 through 136. * •

Scroll down. * •

One at a time, deselect Bands 193-208.

9

Release the Ctrl key. The Number of items selected should be 259. You deselected 25 bands.

If you make a mistake and need to start over, click the Select All button. Then follow the steps above to deselect bands.

Here is a quick video demonstration:

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Click OK to dismiss the Spectral Subset dialog. The Spectral Subset section of the Data Selection dialog reports "259 of 284 bands." 2. 11

Click OK to dismiss the Data Selection dialog. The Save File As Parameters dialog appears. 3. 12

Keep the default selection ofENVI for Output Format. 4. 13

Keep the default value of -9999 for Data Ignore Value. 5. 14

Enter an Output Filename of RadianceSpectralSubset.dat. 6. 15

Enable the Display result option. 7. 16

Click OK. The Save File As Parameters dialog is dismissed, and ENVI creates the spectral subset. 8. 17

In the Layer Manager, right-click on AVIRIS3_Radiance_MountainPassCA.dat and click Remove.

Next, you will use Quick Atmospheric Correction (QUAC®) to mitigate atmospheric effects and to create a surface reflectance image.

Create a Surface Reflectance Image

QUAC and other atmospheric correction options are available as standalone tools and are also included in the Preprocessing Workflow. For this exercise, you will use the workflow.

QUAC is the simplest atmospheric correction tool to use in ENVI. The input scene should contain a variety of spectrally diverse materials—at least 10—such as water, soil, vegetation, and man-made structures. It performs best when the imagery is uniformly illuminated, such as in clear-sky conditions or when airborne sensors fly under complete cloud cover.

1

In the Toolbox, expand the Workflows folder and double-click Preprocessing Workflow. The workflow begins with the Select Data panel. 2. 2

Click the Browse button next to Input Raster. The Data Selection dialog appears. 3. 3

Select RadianceSpectralSubset.dat and click OK.

4

Click Next. The workflow proceeds to the Atmospheric Correction panel. 2. 5

From the Method drop-down list, select QUAC - Quick Atmospheric Correction.

See the Atmospheric Correction quick guide to learn more about this topic.

6

From the Sensor Type drop-down list, select AVIRIS.

7

Click the Next button. The workflow proceeds to the Export Final Result panel. QUAC processing takes several seconds because of the large file size and number of bands. When it is finished, the QUAC image is added to the Layer Manager and displayed in the Image window.

8

In the Output Raster field, enter an output file name of Reflectance.dat. 2. 9

Click the Finish button. The Preprocessing Workflow closes.

The QUAC image looks similar to the radiance image. How can you tell that this is a surface reflectance image? A spectral profile will verify that it is.

10

Right-click on Reflectance.dat in the Layer Manager and select Profiles > Spectral. The Spectral Profile dialog appears, and red crosshairs appear in the Image window. The reflectance curve looks different than the radiance curves you saw in the previous exercise; for example:

The straight line from about 1,800 to 1,900 nm represents missing reflectance data, i.e., the bands that were excluded from the reflectance image. Also notice the gap from 1,350 to 1,400 nm. This corresponds to Bands 129 to 136 that were excluded from the reflectance image, plus an additional band (137) that QUAC automatically identified as bad.

11

Look at the Data Values along the Y-axis. QUAC creates an image where the reflectance values are multiplied (scaled) by 10,000. A value of 2000, for example, means 0.2 or 20% reflectance. 2. 12

Close the Spectral Profile dialog.

This concludes the exercise. You can use the surface reflectance image for different spectral applications.

If you will be conducting analyses that compare image spectra to reference spectra—such as a spectral library or spectrometer data—then reflectance values should ideally range from 0 to 1. See the Scale and Mask Reflectance Images quick guide for instructions on how to do this.

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