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i.rectify - Rectifies an image by computing a coordinate transformation for each pixel in
the image based on the control points.


imagery, rectify


i.rectify --help
i.rectify [-cat] group=name [input=name[,name,...]] extension=string order=integer
[resolution=float] [memory=memory in MB] [method=string] [--help] [--verbose]
[--quiet] [--ui]

Use current region settings in target location (def.=calculate smallest area)

Rectify all raster maps in group

Use thin plate spline

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

order=integer [required]
Rectification polynomial order (1-3)
Options: 1-3
Default: 1

Target resolution (ignored if -c flag used)

memory=memory in MB
Amount of memory to use in MB
Default: 300

Interpolation method to use
Options: nearest, linear, cubic, lanczos, linear_f, cubic_f, lanczos_f
Default: nearest


i.rectify uses the control points included in the source data or identified with the
Ground Control Points Manager to calculate a transformation matrix and then converts x,y
cell coordinates to standard map coordinates for each pixel in the image. The result is a
planimetric image with a transformed coordinate system (i.e., a different coordinate
system than before it was rectified). Supported transformation methods are first, second,
and third order polynomial and thin plate spline. Thin plate spline is recommended for
ungeoreferenced satellite imagery where ground control points (GCPs) are included.
Examples are NOAA/AVHRR and ENVISAT imagery which include throusands of GCPs.

If no ground control points are available, the Ground Control Points Manager must be run
before i.rectify. An image must be georeferences before it can reside in a standard
coordinate LOCATION, and therefore be analyzed with the other map layers in the standard
coordinate LOCATION. Upon completion of i.rectify, the rectified image is deposited in the
target standard coordinate LOCATION. This LOCATION is selected using i.target.

More than one raster map may be rectified at a time. Each cell file should be given a
unique output file name. The rectified image or rectified raster maps will be located in
the target LOCATION when the program is completed. The original unrectified files are not
modified or removed.

If the -c flag is used, i.rectify will only rectify that portion of the image or raster
map that occurs within the chosen window region in the target location, and only that
portion of the cell file will be relocated in the target database. It is important
therefore, to check the current mapset window in the target LOCATION if the -c flag is

If you are rectifying a file with plans to patch it to another file using the GRASS
program r.patch, choose option number one, the current window in the target location. This
window, however, must be the default window for the target LOCATION. When a file being
rectified is smaller than the default window in which it is being rectified, NULLs are
added to the rectified file. Patching files of the same size that contain NULL data,
eliminates the possibility of a no-data line in the patched result. This is because, when
the images are patched, the NULLs in the image are "covered" with non-NULL pixel values.
When rectifying files that are going to be patched, rectify all of the files using the
same default window.

Coordinate transformation
The desired order of transformation (1, 2, or 3) is selected with the order option. The
program will calculate the RMSE and check the required number of points.

Linear affine transformation (1st order 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.

Polynomial Transformation Matrix (2nd, 3d order transformation)
i.rectify uses a first, second, or third order transformation matrix to calculate the
registration coefficients. The number of control points required for a selected order of
transformation (represented by n) is
((n + 1) * (n + 2) / 2) or 3, 6, and 10 respectively. It is strongly recommended that one
or more additional points be identified to allow for an overly-determined transformation
calculation which will generate the Root Mean Square (RMS) error values for each included
point. The RMS error values for all the included control points are immediately
recalculated when the user selects a different transformation order from the menu bar. The
polynomial equations are performed using a modified Gaussian elimination method.

Thin plate spline (TPS) transformation
TPS transformation is selected with the -t flag. This method of coordinate transformation
is recommended for satellite imagery where hundreds or thousands of GCPs are included, and
for historical printed or scanned maps with unknown georeferencing and/or known localized

TPS combines a linear affine transformation with individual transformation coefficients
for each GCP, using the radial basis kernel function with the distance dist between any
two points:
dist2 * log(dist) As a consequence, localized distortions can be removed with TPS
transformation. For example, scan line sensors will have due to the changing viewing angle
larger distortions towards the end points of the scan line than at the center of the scan
line. Even higher order polynomial transformations are not able to remove these locally
different distortions, but TPS transformation can. For best results, TPS requires an even
and, for localized distortions, dense spacing of GCPs.

Resampling method
The rectified data is resampled with one of seven different methods: nearest, bilinear,
cubic, lanczos, bilinear_f, cubic_f, or lanczos_f.

The method=nearest method, which performs a nearest neighbor assignment, is the fastest of
the resampling methods. It is primarily used for categorical data such as a land use
classification, since it will not change the values of the data cells. The method=bilinear
method determines the new value of the cell based on a weighted distance average of the 4
surrounding cells in the input map. The method=cubic method determines the new value of
the cell based on a weighted distance average of the 16 surrounding cells in the input
map. The method=lanczos method determines the new value of the cell based on a weighted
distance average of the 25 surrounding cells in the input map.

The bilinear, cubic and lanczos interpolation methods are most appropriate for continuous
data and cause some smoothing. These options should not be used with categorical data,
since the cell values will be altered.

In the bilinear, cubic and lanczos methods, if any of the surrounding cells used to
interpolate the new cell value are NULL, the resulting cell will be NULL, even if the
nearest cell is not NULL. This will cause some thinning along NULL borders, such as the
coasts of land areas in a DEM. The bilinear_f, cubic_f and lanczos_f interpolation methods
can be used if thinning along NULL edges is not desired. These methods "fall back" to
simpler interpolation methods along NULL borders. That is, from lanczos to cubic to
bilinear to nearest.

If nearest neighbor assignment is used, the output map has the same raster format as the
input map. If any of the other interpolations is used, the output map is written as
floating point.


If i.rectify starts normally but after some time the following text is seen:
ERROR: Error writing segment file
the user may try the -c flag or the module needs more free space on the hard drive.

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