# GRASS Help System

## GEOMETRIC RECTIFICATION

Rectification is the mapping of an image from one coordinate system to another. The geometry of an image extracted from tape into a GRASS LOCATION having an x,y coordinate system is not planimetric. To make an image planimetric, that is, to convert the x,y coordinate system into a standard map coordinate system (e.g., the UTM coordinate system or the State Plane coordinate system), points from a map having the standard coordinates must be associated with the the same points on an image to be rectified. This is done by marking points on an image and then inputting standard coordinates from a map. A squares regression is then calculated using the two types of coordinates for each marked point. A matrix containing transformation coefficients is the result. The coefficients are then used to calculate a standard coordinate for each x,y pixel in the image. The rectified planimetric image can then be deposited in a target database.

The GRASS programs that are used in image rectification are:

- i.points - i.rectify
- i.build.blk - i.rectify.blk

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

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Automatically created on: Mon Apr 4 09:21:46 2005