i.ortho.rectify - GRASS GIS manual
Author: GRASS Development Team
Description: i.ortho.rectify: Orthorectifies an image by using the image to photo coordinate transformation matrix.
Keywords: imagery, orthorectify

## NAME

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

## KEYWORDS

imagery, orthorectify

## SYNOPSIS

i.ortho.rectify
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]

### Flags:

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

### Parameters:

group=name [required]
  Name of input imagery group
input=name[,name,...]
  Name of input raster map(s)
extension=string [required]
  Output raster map(s) suffix
resolution=float
  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
method=string
  Interpolation method to use
  Options: nearest, linear, cubic, lanczos, linear_f, cubic_f, lanczos_f
  Default: nearest
angle=name
  Raster map with camera angle relative to ground surface

## DESCRIPTION

i.ortho.rectify 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 i.ortho.rectify 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 project (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 project 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.

i.ortho.rectify can be run directly, specifying options in the command line or the GUI, or it can be invoked as OPTION 8 through i.ortho.photo. If invoked though i.ortho.photo, 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 project 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 project
          2.   Determine the smallest window which covers the image
          >

If you choose option 2, you can also specify a desired target resolution.

i.ortho.rectify 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 project 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 i.ortho.rectify.

## SEE ALSO

i.ortho.photo, i.ortho.camera, g.gui.photo2image, g.gui.image2target, i.ortho.init, i.rectify

## AUTHORS

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

## SOURCE CODE

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

Latest change: Wednesday Feb 05 22:17:19 2025 in commit: 12b9f28dd321e2717a4576a0393bbf39629cf09e

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© 2003-2025 GRASS Development Team, GRASS GIS 8.4.3dev Reference Manual