# GRASS Help System

## IMAGE CORRECTION

Commonly, when analyzing multiple images, it is necessary to correct image distortions attributable to such sources as the earth's radiation. Such distortions can result in different spectral reflectance values being collected for the same image pixel, based on the time of day, month, year, etc., at which the image was photographed. GRASS programs exist which help the user to identify and eliminate such distortions from images, allowing valid comparisons to be made among images. See the paper "r.mapcalc: An Algebra for GIS and Image Processing" for an example of how radiometric corrections can be made to an image.

GRASS commands that can be used for image correction are:

- i.fft
- i.ifft
- i.zc
- r.mapcalc

For more information on a specific command, select a command and press ESC.

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Automatically created on: Wed Oct 12 14:08:41 2005