i.rectify(1grass) GRASS GIS User's Manual i.rectify(1grass)
NAME
i.rectify - Rectifies an image by computing a coordinate transforma-
tion 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:
-c
Use current region settings in target location (def.=calculate
smallest area)
-a
Rectify all raster maps in group
-t
Use thin plate spline
--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
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, lanc-
zos_f
Default: nearest
DESCRIPTION
i.rectify uses the control points included in the source data or iden-
tified with the Ground Control Points Manager to calculate a transfor-
mation 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 coordi-
nate 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 LOCA-
TION. Upon completion of i.rectify, the rectified image is deposited in
the target standard coordinate LOCATION. This LOCATION is selected us-
ing 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 rec-
tified raster maps will be located in the target LOCATION when the pro-
gram 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 relo-
cated 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 us-
ing the GRASS program r.patch, choose option number one, the current
window in the target location. This window, however, must be the de-
fault 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 re-
quired 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 re-
gression based on the control points entered. This transformation ap-
plies 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 modi-
fied Gaussian elimination method.
Thin plate spline (TPS) transformation
TPS transformation is selected with the -t flag. This method of coordi-
nate 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 distor-
tions.
TPS combines a linear affine transformation with individual transforma-
tion 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 re-
moved 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 re-
sults, TPS requires an even and, for localized distortions, dense spac-
ing 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 assign-
ment, 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 de-
termines 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 aver-
age of the 16 surrounding cells in the input map. The method=lanczos
method determines the new value of the cell based on a weighted dis-
tance average of the 25 surrounding cells in the input map.
The bilinear, cubic and lanczos interpolation methods are most appro-
priate 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 ar-
eas in a DEM. The bilinear_f, cubic_f and lanczos_f interpolation meth-
ods can be used if thinning along NULL edges is not desired. These
methods "fall back" to simpler interpolation methods along NULL bor-
ders. 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)
Accessed: unknown
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GRASS 7.8.7 i.rectify(1grass)
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