NAME

i.rectify - An imagery function that rectifies an image by computing a coordinate transformation for each cell (pixel) in the image using the transformation coefficient matrix created by the GRASS program i.points.
(GRASS Image Processing Program)

SYNOPSIS

i.rectify

DESCRIPTION

i.rectify rectifies an image by using the transformation coefficient matrix created by i.points. Rectification is the process by which the geometry of an image is made planimetric. This is accomplished by mapping (transforming) an image from one coordinate system to another. In i.rectify, the coefficients computed by i.points are used in an equation to convert x,y coordinates to standard map coordinates for each cell in the image. The result is an image with a standard map coordinate system. Upon completion of the program the rectified image is deposited in a previously targeted GRASS LOCATION_NAME and MAPSET.

The first prompt in the program asks the user for the name of the group containing the raster map layers to be rectified.

The user is then asked to select the map layer(s) within the group to be rectified: 

Please select the file(s) to rectify by naming an output file

             spot.1 in demo       .........
             spot.2 in demo       .........
             spot.3 in demo       .........
             spotclass in demo    spotrectify..
             spotreject in demo   .........

(Enter list by any name to get a list of existing rast files)

    AFTER COMPLETING ALL ANSWERS, HIT  TO CONTINUE
                  (OR  TO CANCEL)
More than one raster map layer may be rectified at a time. Each raster map layer should have a unique output file name.

Next the user is asked to select one of two geographic region settings: 

Please select one of the following options
1.   Use the current region in the target location
2.   Determine the smallest region which covers the image
i.rectify will only rectify that portion of the image that occurs within the chosen geographic region setting. Only that portion will be relocated in the target database. It is therefore important to check the current geographic region settings in the target location if choice number one is selected.

NOTES

i.rectify uses nearest neighbor resampling during the transformation.

i.rectify uses a linear affine transformation:

x' = ax + by +c
y' = Ax + Bt +C
The a,b,c,A,B,C are determined by least squares regression based on the control points entered. This transformation applies scaling, translation and rotation. It is NOT a general purpose rubber-sheeting, nor is it ortho-photo rectification using a DEM, not second order polynomial, etc. It can be used if (1) you have geometrically correct images, and (2) the terrain or camera distortion effect can be ignored.

Use i.ortho.photo for photos for which you have camera information and a DEM, or i.rectify2 if you want a 2nd or 3rd order polynomial transformation.

i.rectify will run in the background and notify the user by mail when it is finished. The process may take an hour or more depending on the size of the image, the number of files, and the size of the geographic region definition.

The rectified (raster) image will be located in the target LOCATION when the program is completed. The original unrectified raster map layers are not modified or removed.

This program is interactive and requires no command line arguments.

SEE ALSO

GRASS Tutorial: Image Pro cessing

i.group
i.points
i.target

AUTHOR

Michael Shapiro, U.S. Army Construction Engineering Research Laboratory