Use AVHRR Build Geometry File to calculate the geometry values for each pixel. You may select which values to calculate: latitude, longitude, solar zenith, and/or sensor zenith angles.

ENVI uses the same solar geometry calculations as the NOAA Earth System Research Laboratory (see https://gml.noaa.gov/), except that it uses lookup tables to obtain equations for time, declination, and julian day.

Note: You can also georeference your AVHRR data by using the results from this procedure as the IGM input file in Georeferencing from Input Geometry.

  1. Open an AVHRR file by selecting File > Open As > Optical Sensors > AVHRR >data_type from the menu bar.
  2. Select Geometric Correction > Build Geometry File by Sensor > AVHRR Build Geometry File from the Toolbox. The AVHRR Input File dialog appears.
  3. Select the original AVHRR file.
  4. Click OK. The AVHRR Image Geometry Parameters dialog appears.
  5. Select which values to compute.

  6. Select the Degrees or Radians radio button to indicate the output units for zenith angle.

    The zenith angle is measured vertically from each pixel to the sensor or sun (straight above the pixel = 0 degrees).

  7. Enter an output filename.
  8. Select Double or Floating Point output data type from the Output Data Type drop-down list.
  9. Click OK. ENVI extracts the header information and calculates the geometry values for each pixel.