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i.rectify(1grass) | Grass 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, rectifySYNOPSIS¶
i.rectifyFlags:¶
- -c
-
- -a
-
- --verbose
-
- --quiet
-
Parameters:¶
- group=name
-
- input=name[,name,...]
-
- extension=string
-
- order=integer
-
- res=float
-
- memory=memory in MB
-
- method=string
-
DESCRIPTION¶
i.rectify uses the control points identified in i.points or i.vpoints to calculate a transformation matrix based on a first, second, or third order polynomial 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). i.points or i.vpoints must be run before i.rectify, and both programs are required to rectify an image. An image must be rectified 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.Program Prompts¶
The first prompt in the program asks for the name of the group containing the files to be rectified.Enter the group containing files to be rectified
Enter 'list' for a list of existing imagery groups
Enter 'list -f' for a verbose listing
Hit RETURN to cancel request
>
spot1.1 in mapsetname .............
spot1.2 in mapsetname .............
spot1.3 in mapsetname .............
spotclass1 in mapsetname spotrectify1.
spotreject1 in mapsetname .............
(OR<Ctrl-C> TO CANCEL)
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
>
1st Order 2nd Order 3rd Order
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)¶
The ANALYZE function has been changed to support calculating the registration coefficients using a first, second, or third order transformation matrix. The number of control points required for a selected order of transformation (represented by n) isResampling method¶
The rectified data is resampled with one of five different methods: nearest, bilinear, cubic, bilinear_f or cubic_f. The method=nearest method, which performs a nearest neighbor assignment, is the fastest of the three 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.Program Execution¶
Note: 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.NOTES¶
i.rectify uses nearest neighbor resampling during the transformation choosing the actual pixel that has its centre nearest to the point location in the image. Advantage of this method is that the pixel brightness of the image is kept as i.rectify rearranges the geometry of the image pixels. If i.rectify starts normally but after some time the following text is seen:SEE ALSO¶
The GRASS 4 Image Processing manual g.transform, r.proj, v.proj, i.group, i.points, i.vpoints, i.targetAUTHORS¶
William R. Enslin, Michigan State University, Center for Remote Sensing Modified for GRASS 5.0 by:GRASS 6.4.4 |