Specifying visibility and ground reflectance

To specify visibility and ground reflectance:

In the Atmospheric Correction window, under Workflows, click Visibility and ground reflectance.
Note: This task is available only for the ATCOR - Ground Reflectance and ATCOR - Surface Temperature workflows.
The rightmost pane of the window is refreshed to display the settings for visibility and ground reflectance.
Under Atmospheric Information, do the following:
- In the Aerosol Type list, click an appropriate aerosol type.
  For more information, see About aerosol types.
- In the Conditions list, click the standard atmosphere present when the image was acquired.
  By default, the atmospheric condition is estimated based on the image-acquisition date and center coordinates. For more information, see About standard atmospheres.
Under Satellite Viewing Geometry, do the following:
- If necessary, enter the tilt angle of the sensor in the Sensor tilt angle box.
  That is, you can only add or enter a value when the sensor provides tilt capability. Typically, the value is updated automatically based on the AcrossTrackAngle metadata tag of the image.
- If necessary, enter the azimuth angle of the satellite in the Satellite Azimuth angle box.
  That is, when the sensor has a tilt angle greater than 3 degrees, you must enter an azimuth angle. Typically, the value is updated automatically based on the SatelliteAzimuth metadata tag of the image. The azimuth angle describes the horizontal position of the satellite relative to the center of the scene; for example:
  - North = 0 degrees
  - East = 90 degrees
  - South = 180 degrees
  - West = 270 degrees
Under Visibility Map, do one of the following to specify the atmospheric conditions at the time the image was taken:
- To specify a constant-visibility value, make sure Use constant visibility is selected, and then, if necessary, type or select a value in the box to the right.
  That is, by default, the ATCOR workflows use a constant visibility of 30 kilometers; however, you can specify a value up to 180.
  
  Note: When the varying visibility map is required but insufficient reference pixels are found, the workflow automatically uses a constant visibility.
- To calculate a visibility map for varying conditions, click Calculate (spatially varying conditions), and then click Settings.
  For information on the settings, see Varying Visibility settings.
  
  The program will compute a visibility map for the scene, using Dark Vegetation pixels. These are defined based on NDVI and top-of-the-atmosphere reflectance in the red band.
- To select a file of visibility values, click Use existing file, click Browse, and then select the file you want.
  The file you select must be reprojectable to the image, and the image must be fully contained within the visibility map.
Under Reflectance Correction, to perform terrain correction during atmospheric correction, do any of the following:
- To specify the range of correction values to apply to the adjacency effect, select the Correct for adjacency effects check box.
- In the Kernel size list, select a kernel size of the correction.
  For more information, see About adjacency.
- In the Iterate list, select the number of iterations to use.
  That is, select the number of times to apply the adjacency-effects correction.
- To compensate for overcorrection caused by topographic effects, select the Perform topographic normalization check box.
  That is, digital imagery of mountainous regions can contain a radiometric distortion known as a topographic effect. You can reduce this effect by applying transformations, such as the Lambertian or non-Lambertian reflectance models.
  
  To verify or modify the normalization settings, click Settings, and then proceed to Setting up normalization.
Under Output, do any of the following, as applicable, to scale the output reflectance units:
- In the Output reflectance units list, select an appropriate unit from the following:
  - 16 Bit Reflectance: Computes the output in reflectance units from zero through 10,000 stored in a 16S channel (default).
  - Pure Reflectance: Computes the output in from zero through one stored in a 32R channel.
  - Percent Reflectance: Computes the output in reflectance units from zero through 100 percent stored in a 32R channel.
  - Scaled Reflectance, Scale Factor: Computes the output in reflectance units from zero through one multiplied by the scale factor you specify. If no reflectance factor is specified, a default of 400.0 is used for 8-bit channels, and a default of 1,000 is used for 16-bit and higher data. When you select this option, you must enter a value in the Scale factor box.
  Note: With Pure Reflectance, reflectance values greater than one (1) are possible over very bright areas in which specular reflectance is occurring; that is, all light is reflected in one direction. Such situations occur typically over roofs of buildings (mirror reflection), water (sun glint), and, to a lesser extent, over snow and very thick clouds.
- To generate additional layers during atmospheric correction, select the Compute value-added products check box.
  That is, the following raster layers are generated:
  - Soil-Adjusted Vegetation Index (SAVI): Uses the red and near-infrared bands to measure the density and vigor of green vegetation by attempting to eliminate the reflectivity of the ground beneath the canopy.
  - Leaf-Area Index (LAI): Calculates the green-leaf density.
  - Fraction of Absorbed Radiation (FPAR): Calculates the amount of photosynthetically active radiation absorbed by plant canopy.
  - Surface Albedo: Calculates wavelength-integrated surface reflectance.
  - Absorbed Solar Radiation: Calculates the shortwave solar radiation absorbed by the surface.