1i.rectify(1) Grass User's Manual i.rectify(1)
2
6 i.rectify - Rectifies an image by computing a coordinate transforma‐
7 tion for each pixel in the image based on the control points
8
10 imagery
11
13 i.rectify
14 i.rectify help
15 i.rectify [-ca] group=string [input=string[,string,...]] exten‐
16 sion=string order=integer
17 Flags:
19 -c Use curr. region settings in target location (def.=calculate small‐
20 est area)
21 -a Rectify all images in group
23 Parameters:
25 group=string
26 Name of imagery group
27 input=string[,string,...]
29 Name of input raster map(s)
30 extension=string
32 Output file extension (inputfile(s) + extension)
33 order=integer
35 Rectification polynom order (1-3)
36
38 i.rectify uses the control points identified in i.points or i.vpoints
39 to calculate a transformation matrix based on a first, second, or
40 third order polynomial and then converts x,y cell coordinates to stan‐
41 dard map coordinates for each pixel in the image. The result is a
42 planimetric image with a transformed coordinate system (i.e., a differ‐
43 ent coordinate system than before it was rectified).
44 i.points or i.vpoints must be run before i.rectify, and both programs
46 are required to rectify an image. An image must be rectified before it
47 can reside in a standard coordinate LOCATION, and therefore be analyzed
48 with the other map layers in the standard coordinate LOCATION. Upon
49 completion of i.rectify, the rectified image is deposited in the target
50 standard coordinate LOCATION. This LOCATION is selected using i.tar‐
51 get.
52
54 The first prompt in the program asks for the name of the group contain‐
55 ing the files to be rectified.
56 Enter the group containing files to be rectified
57 Enter 'list' for a list of existing imagery groups
58 Enter 'list -f' for a verbose listing
59 Hit RETURN to cancel request
60 >
61 This is the same imagery group that was selected in i.points or
62 i.vpoints and the group that contains the cell files with the marked
63 points and their associated map coordinates. You are then asked to
64 select the cell file(s) within the group to be rectified:
65 Please select the file(s) to rectify by naming an output file
66 spot1.1 in mapsetname .............
67 spot1.2 in mapsetname .............
68 spot1.3 in mapsetname .............
69 spotclass1 in mapsetname spotrectify1.
70 spotreject1 in mapsetname .............
72 (enter list by any name to get a list of existing cell files)
73 AFTER COMPLETING ALL ANSWERS, HIT <ESC> TO CONTINUE
74 (OR<Ctrl-C> TO CANCEL)
75 More than one cell file may be rectified at a time. Each cell file
76 should be given a unique output file name.
77 Next, you are asked to select one of two windows regions:
79 Please select one of the following options
80 1. Use the current window in the target location
81 2. Determine the smallest window which covers the image
82 >
83 The i.rectify program will only rectify that portion of the image or
84 cell file that occurs within the chosen window region, and only that
85 portion of the cell file will be relocated in the target database. It
86 is important therefore, to check the current mapset window in the tar‐
87 get LOCATION if choice number one is selected.
88 If you are rectifying a file with plans to patch it to another file
90 using the GRASS program r.patch, choose option number one, the current
91 window in the target location. This window, however, must be the
92 default window for the target LOCATION. When a file being rectified is
93 smaller than the default window in which it is being rectified, zeros
94 are added to the rectified file. Patching files of the same size that
95 contain 0/non-zero data, eliminates the possibility of a no-data line
96 the patched result. This is because, when the images are patched, the
97 zeros in the image are "covered" with non-zero pixel values. When rec‐
98 tifying files that are going to be patched, rectify all of the files
99 using the same default window.
100 Select the order of transformation desired with the order option:
102 1st Order 2nd Order 3rd Order
103 The program will immediately recalculate the RMSE and the number of
104 points required.
105 Linear affine transformation (1st order transformation)
107 x' = ax + by +c y' = Ax + Bt +C
108 The a,b,c,A,B,C are determined by least squares regression based on the
110 control points entered. This transformation applies scaling, transla‐
111 tion and rotation. It is NOT a general purpose rubber-sheeting, nor is
112 it ortho-photo rectification using a DEM, not second order polynomial,
113 etc. It can be used if (1) you have geometrically correct images, and
114 (2) the terrain or camera distortion effect can be ignored.
115 Polynomial Transformation Matrix (2nd, 3d order transformation)
117 The ANALYZE function has been changed to support calculating the regis‐
118 tration coefficients using a first, second, or third order transforma‐
119 tion matrix. The number of control points required for a selected
120 order of transformation (represented by n) is ((n + 1) * (n + 2) / 2)
121 or 3, 6, and 10 respectively. It is strongly recommended that one or
123 more additional points be identified to allow for an overly- determined
124 transformation calculation which will generate the Root Mean Square
125 (RMS) error values for each included point. The RMS error values for
126 all the included control points are immediately recalculated when the
127 user selects a different transformation order from the menu bar. The
128 polynomial equations are performed using a modified Gaussian elimina‐
129 tion method.
130 Program Execution
132 Note: The rectified image or rectified cell files will be located in
133 the target LOCATION when the program is completed. The original unrec‐
134 tified files are not modified or removed.
135
137 i.rectify uses nearest neighbor resampling during the transformation
138 choosing the actual pixel that has its centre nearest to the point
139 location in the image. Advantage of this method is that the pixel
140 brightness of the image is kept as i.rectify rearranges the geometry of
141 the image pixels.
142 If i.rectify starts normally but after some time the following text is
144 seen:
145 GIS ERROR: error while writing to temp file
146 the user may try the flag -c (or the module needs more free space on
147 the hard drive).
148
150 GRASS Tutorial: Image Processing
151 g.transform, r.proj, v.proj, i.group, i.points, i.vpoints, i.target
153
155 William R. Enslin, Michigan State University, Center for Remote Sensing
156 Modified for GRASS 5.0 by:
158 Luca Palmeri (palmeri@ux1.unipd.it)
159 Bill Hughes
160 Pierre de Mouveaux (pmx@audiovu.com)
161 CMD mode by Bob Covill
162 Last changed: $Date: 2006/07/03 16:37:28 $
164 Full index
166GRASS 6.2.2 i.rectify(1)