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Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.


i.ortho.rectify - Orthorectifies an image by using the image to photo coordinate transformation matrix.


imagery, orthorectify


i.ortho.rectify --help
i.ortho.rectify [-cap] group=name [input=name[,name,...]] extension=string [resolution=float] [memory=memory in MB] [method=string] [angle=name] [--overwrite] [--help] [--verbose] [--quiet] [--ui]


Use current region settings in target location (def.=calculate smallest area)
Rectify all raster maps in group
Enable panorama camera correction
Allow output files to overwrite existing files
Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog


group=name [required]
Name of input imagery group
Name of input raster map(s)
extension=string [required]
Output raster map(s) suffix
Target resolution (ignored if -c flag used)
memory=memory in MB
Maximum memory to be used (in MB)
Cache size for raster rows
Default: 300
Interpolation method to use
Options: nearest, linear, cubic, lanczos, linear_f, cubic_f, lanczos_f
Default: nearest
Raster map with camera angle relative to ground surface

Table of contents

DESCRIPTION rectifies an image by using the image to photo coordinate transformation matrix created by g.gui.photo2image and the rectification parameters created by g.gui.image2target. Rectification is the process by which the geometry of an image is made planimetric. This is accomplished by mapping an image from one coordinate system to another. In the parameters computed by g.gui.photo2image and g.gui.image2target are used in equations to convert x,y image coordinates to standard map coordinates for each pixel in the image. The result is an image with a standard map coordinate system, compensated for relief distortions and photographic tilt. Upon completion of the program the rectified image is deposited in a previously targeted GRASS LOCATION.

Images can be resampled with various different interpolation methods: nearest neighbor assignment, bilinear and bicubic interpolation. The bilinear and bicubic interpolation methods are also available with a fallback option. These methods "fall back" to simpler interpolation methods along NULL borders. That is, from bicubic to bilinear to nearest.

The process may take an hour or more depending on the size of the image, the speed of the computer, the number files, and the size and resolution of the selected window.

The rectified image will be located in the target LOCATION when the program is completed. The original unrectified files are not modified or removed.

The optional angle output holds the camera angle in degrees to the local surface, considering local slope and aspect. A value of 90 degrees indicates that the camera angle was orthogonal to the local surface, a value of 0 degrees indicates that the camera angle was parallel to the local surface and negative values indicate that the surface was invisible to the camera. As a rule of thumb, values below 30 degrees indicate problem areas where the orthorectified output will appear blurred. Because terrain shadowing effects are not considered, areas with high camera angles may also appear blurred if they are located (viewed from the camera position) behind mountain ridges or peaks. can be run directly, specifying options in the command line or the GUI, or it can be invoked as OPTION 8 through If invoked though, an interactive terminal is used to determine the options.

Interactive mode

You are first asked if all images within the imagery group should be rectified. If this option is not chosen, you are asked to specify for each image within the imagery group whether it should be rectified or not.

More than one file may be rectified at a time. Each file should have a unique output file name. The next prompt asks you for an extension to be appended to the rectified images.

The next prompt will ask you whether a camera angle map should be produced and if yes, what should be its name.

After that you are asked if overwriting existing maps in the target location and mapset should be allowed.

The next prompt asks you to select one of two windows:

      Please select one of the following options
      1.   Use the current window in the target location
      2.   Determine the smallest window which covers the image

If you choose option 2, you can also specify a desired target resolution. will only rectify that portion of the image that occurs within the chosen window. Only that portion will be relocated in the target database. It is therefore important to check the current window in the target LOCATION if choice number one is selected.

Next you are asked to select an interpolation method.

      Please select one of the following interpolation methods
      1. nearest neighbor
      2. bilinear
      3. bicubic
      4. bilinear with fallback
      5. bicubic with fallback

The last prompt will ask you about the amount of memory to be used by



Mike Baba, DBA Systems, Inc.
Updated rectification and elevation map to FP 1/2002 Markus Neteler
Bugfixes and enhancements 12/2010 Markus Metz


Available at: i.ortho.rectify source code (history)

Latest change: Wednesday Oct 04 09:39:09 2023 in commit: 0bd5c8d434532a31f890a307ce4c08273b8d70eb

Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.

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