1gm_georect(1) Grass User's Manual gm_georect(1)
2
6 In order to use and overlay different forms of geospatial data in a
7 GIS, they must use the same coordinate systems and be projected identi‐
8 cally. The Georectify Tool uses multiple GRASS modules to georectify
9 spatial data (map or image) that is not originally coded in an earth-
10 based coordinate system (e.g., latitude/longitude or UTM). The Georec‐
11 tify Tool can transform the simple x/y coordinates of the original data
12 by shifting, rotating, and warping them as needed for the original data
13 to match other georectified data from the same location on earth.
14 The basic procedure is to identify a set of ground control points
15 (GCPs) in the ungeorectified data. The georectified coordinates for the
16 points are identified. A mathematical transformation is calculated by
17 regressing the original x/y coordinates against the georectified coor‐
18 dinates for the same points. This transformation is then applied to all
19 the data.
20 The Georectify Tool can georectify both raster and vector maps. Raster
21 maps can be transformed using 1st, 2nd, and 3rd order polynomial trans‐
22 formations; vector maps are transformed only with a 1st order polyno‐
23 mial transformation. The georectified coordinates of GCPs can be
24 entered by typing or can be extracted by clicking the equivalent loca‐
25 tions with a mouse in a displayed georectified map.
26 Using the Georectify Tool
28 Map Preparation
29 The unrectified map or image (e.g., scanned image or CAD drawing) must
30 be imported into a GRASS XY location.
31 In order to be georectified, the unrectified map or image must be added
32 to a imagery group. An imagery group holds a single map/image or a set
33 of maps/images that represent different views of exactly the same
34 locale. All maps or images in a single imagery group must be exactly
35 aligned (e.g., as in the different spectral bands of a multi-channel
36 satellite image or layers of a single CAD drawing). You can create an
37 imagery group from within the Georectify Tool if desired.
38 Setup
40 Restart GRASS, selecting the location/mapset where you want the map you
41 will georectify to END UP (i.e., the projected TARGET location). Open
42 any map display (or displays) that you want to use for extracting geo‐
43 graphic coordinates by clicking with a mouse. Make sure that the compu‐
44 tational region extents and resolution are set to the values you want
45 for the newly georectified map.
46 Select Georectify from the File menu.
48 The Georectify Tool startup lets you select the type of map to georec‐
50 tify, the location/mapset of the map to georectify, the image group of
51 the map to georectify, and a map to use for marking GCPs.
52 1
54 click the radio button to georectify either raster or vector
55 maps (you can only georectify raster OR vector maps in a single
56 georectification session),
57 2
59 select the source (XY) location/mapset of the map to be georec‐
60 tified,
61 3 if needed, create an imagery group for the map to georectify.
63 (the imagery group target is automatically set to the current
64 location and mapset),
65 4
67 select the imagery group containing the map to georectify,
68 5
70 select the map/image to display for marking GCPs interactively
71 (a raster or vector in the group you want to georectify),
72 6
74 click the "Start Georectifying" button to begin marking GCPs.
75 GCP Marking
77 This opens a special map display with an ungeorectified map to use for
78 marking GCPs, and a GCP manager window. The GCP map display can be
79 zoomed and panned like a normal map display.
80 The GCP manager window holds the x/y coordinates and geographic coordi‐
81 nates of each GCP, and displays the error for each GCP (i.e., the dis‐
82 tance that the actual GCP marked deviates from its expected placement
83 using a transformation equation).
84 1
86 Click in an empty x/y entry box in the GCP manager to begin
87 marking a new GCP,
88 2
90 Click on the ungeorectified map to mark a GCP and automatically
91 enter its x/y coordinates in the entry box. (Your cursor will
92 automatically jump to the corresponding geographic coordinate
93 entry box for the same GCP),
94 3 Enter the corresponding geographic coordinates for the GCP OR
96 click on the same place in a georectified map (in a normal map
97 display) to automatically extract the geographic coordinates and
98 enter them into the geographic coordinate entry box in the GCP
99 manager window (Note: use the pointer tool in the map display
100 window)
101 GCP ManagementContinue marking GCPs until you
103 have enough for an accurate georectification. At least 3 GCPs are
104 needed for a simple 1st order (affine) transformation that can shift
105 and rotate a map; at least 6 GCPs are needed for a 2nd order (polyno‐
106 mial) transformation that will do simple warping in addition to shift‐
107 ing and rotating; at least 10 GCPs are needed for a 3rd order (polyno‐
108 mial) transformation that will do complex warping.
109 Any GCP can be edited by typing new values or by selecting the appro‐
110 priate entry box and clicking on the GCP-marking display or georecti‐
111 fied map display.
112 GCPs can be excluded and not used for calculating the transformation
113 equation or the total RMS error by unchecking the box in the "use" col‐
114 umn at the left of each GCP entry line. Only active GCPs (i.e., with
115 the "use" box checked) will be used for computing the georectification
116 and total error.
117 You can delete all inactive GCPs (i.e., with unchecked "use" boxes) by
118 clicking the eraser tool in the GCP manager tool bar.
119 Total RMS (root mean square) error is calculated from all active GCPs.
120 Click the RMS tool in the GCP manager tool bar to update the total RMS
121 error after changing or including/excluding a GCP. Excluding a GCP with
122 an especially high individual error can reduce overall RMS error, but
123 will also change the individual errors of all other GCPs. The lower the
124 RMS error, the more accurate the final georectification will be.
125 All active GCPs can be saved to a POINTS file, associated with the
126 imagery group for the map(s) to be georectified. If a saved POINTS file
127 exists, prior to a georectification session the GCP values in the
128 POINTS file will automatically be entered into the GCP manager window
129 when a georectification session is started.
130 Georectification
132 When you are satisfied with your selection of GCPs and RMS error...
133 1 Select the type of georectification you want to perform (1st,
135 2nd, or 3rd order for rasters; 1st order only for vectors).
136 Remember, at least 3 points are needed for 1st order georectifi‐
137 cation, 6 points for 2nd order, and 10 points for 3rd order.
138 2 Press the Georectification button on the GCP manager tool bar.
140 Depending on the number of cells in a raster map or nodes in a
141 vector map, georectification may take some time.
142
144 gis.m manual
145 gis.m: ANIMATION TOOL
146 gis.m: PROFILE TOOL
147 i.points
148 i.vpoints
149 i.rectify
150 g.proj
151 g.setproj
152 m.proj
153 r.proj
154 v.proj
155
157 Michael Barton, Arizona State University, Tempe, Arizona, USA
158 Last changed: $Date: 2007-11-18 21:17:33 +0100 (Sun, 18 Nov 2007) $
160 Full index
162GRASS 6.3.0 gm_georect(1)