Specifying elevation and illumination conditions

To specify elevation and illumination conditions:

From the Atmospheric Correction workflows Tree List, select Illumination conditions. This task is available only for the ATCOR Ground Reflectance and ATCOR Surface Temperature workflows. The Illumination conditions panel appears.
Define the Elevation Information Setup by specifying a Constant Height value or by selecting a digital elevation model (DEM) file.
- If you do not have a DEM file, accept the default Constant Height option and verify the value representing a constant elevation for the area covered by the image. This value is automatically populated using the gmted2010 file and the image center.
- Otherwise, select the DEM File option and click the down arrow beside the field to select a DEM file available in the current session, or click Browse to select a file.
When you select a DEM file, the DEM File window automatically appears, listing the information for the selected DEM, populated using the file’s metadata. The selected file must be reprojectable. You may also click the DEM Settings button to access this window.
Verify the DEM Settings, making any necessary changes.
Accept the default selection of the Clip DEM to image extents check box.
When this option is selected, the application clips the DEM to the extents of the currently loaded image so that the terrain derivatives (slope, aspect, sky view, and illumination map) are computed only in the area that overlaps the image.

If you clear the check box, the application uses the entire DEM to compute the derivatives, which considerably slows processing time.
Specify options in the DEM Derivatives section to derive slope, aspect, and sky view information.
Do one of the following:
- Select Calculate to automatically calculate the slope, aspect, and sky view values.
- Select From File to import the values from a file. Click the down arrow to select the file that contains the slope, aspect, and sky view values, or click Browse to select a file. Select individual layers that contain the Slope, Aspect, and Skyview information.
Note: The Slope, Aspect, and Skyview layers can also be computed using the TERSETUP algorithm. It is strongly advised that you select a DEM that covers an area close to the image extents to speed up processing; the larger the DEM, the slower the processwill be. If you have a single DEM for an area and intend to process many overlapping images, you can pre-compute DEM derivatives using TERSETUP and use those derivatives to correct each image.
Specify options in the Illumination Map section to calculate the illumination and shadow information.
Do one of the following:
- Select Calculate to automatically calculate the illumination map and shadows.
- Select From File to import the values from a file, or click Browse to select a file. Click the down arrow to select the file that contains the illumination values, then select the layer that contains the Illumination map channel.
- Optionally select the bitmap layer that contains the shadow data.
  
  The shadow file contains a layer that includes both self-shadowing and cast shadow effects. In the layer, the shadows are represented by a pixel value of 0 and the rest of the pixels are given a value of 1. The self-shadowing effect is caused by pixels from areas facing away from the sun which are on slopes steeper than the solar elevation angle. Cast shadows are areas in shadow due to objects blocking the sun. The shadow file is used as a mask so that shadows are excluded from the atmospheric correction process.