## table of contents

i.rectify(1grass) | GRASS GIS User's Manual | i.rectify(1grass) |

# NAME¶

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

# KEYWORDS¶

imagery, rectify, geometry

# SYNOPSIS¶

**i.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**]

## Flags:¶

## 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 **order**=*integer***[required]**-

Rectification polynomial order (1-3)

Options:*1-3*

Default:*1* **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*

# DESCRIPTION¶

*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
used.

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 + By + 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 like TPS, 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
distortions.

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.

# NOTES¶

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.

# SEE ALSO¶

The GRASS 4 *Image* *Processing manual*

*m.transform,* *r.proj,* *v.proj,*
*i.group,* *i.target*

Ground Control Points Manager

# AUTHORS¶

William R. Enslin, Michigan State University, Center for Remote Sensing

Modified for GRASS 5.0 by:

Luca Palmeri (palmeri@ux1.unipd.it)

Bill Hughes

Pierre de Mouveaux (pmx@audiovu.com)

CMD mode by Bob Covill

# SOURCE CODE¶

Available at: i.rectify source code (history)

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