1i.points(1) Grass User's Manual i.points(1)
2
6 i.points - Mark ground control points on image to be rectified.
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9 imagery
10
12 i.points
13 i.points help
14 i.points group=string [--verbose] [--quiet]
15 Parameters:
17 group=string
18 Name of imagery group to be registered
19
21 i.points is an imagery function that enables the user to mark points on
22 a (raster) image to be rectified and then input the geographic coordi‐
23 nates of each point for calculation of a coordinate transformation
24 matrix. i.points must be followed by use of the GRASS program i.rec‐
25 tify, which rectifies the image using the transformation matrix coeffi‐
26 cients calculated by i.points.
27 Rectification is the mapping (transformation) of an image from one
29 coordinate system to another. The geometry of an image extracted into
30 a GRASS LOCATION having an x,y coordinate system is not planimetric.
31 To create a planimetric image, that is, to convert the x,y coordinate
32 system into a standard coordinate system (for example, the UTM coordi‐
33 nate system or the State Plane coordinate system), points from a map
34 having the standard coordinates must be associated with the same points
35 on the image to be rectified. i.points enables the user to mark points
36 on an image and input the standard coordinates for that point.
37 i.points then calculates a least squares regression using the two coor‐
38 dinate systems (x,y and standard) for the marked points. A matrix con‐
39 taining transformation coefficients is the output file for i.points.
40 During the process of marking points and entering map coordinates, the
42 user can compute the RMS (root mean square) error for each point
43 entered. i.points does this by calculating the transformation equation
44 (the same one that is calculated in the GRASS program i.rectify), and
45 then plugging these results into an equation for RMS error.
46 i.points offers a zoom option to locate precisely the point to be
48 marked on an image. This program also offers the user the option of
49 acquiring standard coordinates for a marked point from a map layer in
50 the target data base.
51 i.target must be run before running i.points to enable the PLOT RASTER
53 option to be used and to identify a target data base LOCATION_NAME and
54 MAPSET for the rectified image. To run i.points, a graphics monitor is
55 required.
56 The procedure for marking points, entering coordinates, and calculating
58 RMS error is described below.
59 The first prompt in the program asks the user for the imagery group to
61 be registered. Note that if i.target is not run before i.points, the
62 i.points program will display the following error message:
63 ERROR: Target information for group [spot] missing
64 Please run i.target for group [spot]
65 After entering the group to be registered the terminal screen displays
66 the message:
67 Use mouse now...
68 The graphics monitor displays the following screen:
69 ______________________________________________________________
70 | imagery filename (mag) | target filename (mag) |
71 |_________________________________|___________________________|
72 | | |
73 | | |
74 | | |
75 | | |
76 | | |
77 | | |
78 | | |
79 |_________________________________|___________________________|
80 | | |
81 | | |
82 | | |
83 | | |
84 | | |
85 | | |
86 | | |
87 |_________________________________|___________________________|
88 | QUIT ZOOM PLOT RASTER ANALYZE| |
89 |_________________________________|___________________________|
90 A pop-down menu like that shown below will be superimposed on the left
91 half of the screen:
92 __________________________________
93 | Double click on raster map layer|
94 | to be plotted |
95 | Double click here to cancel |
96 |_________________________________|
97 ____________________
98 | Mapset demo |
99 |___________________|
100 | spotclass| spot.1|
101 |__________|________|
102 | composite| spot.2|
103 |__________|________|
104 | spot.3 | |
105 |__________|________|
106 Any single raster map layer in the imagery group may be used on which
107 to mark points, and the user can mark points on more than one raster
108 map layer in the imagery group to accumulate the suggested minimum num‐
109 ber of 12 points. Any raster map layer in the imagery group can be
110 rectified (using i.rectify) based on the transformation matrix computed
111 from these points.
112 The imagery file chosen by the user is displayed in the upper left
114 quadrant of the screen.
115 ZOOM
117 To magnify the displayed file, the user must place the mouse cross
118 hairs on the word ZOOM. The following menu will then be displayed at
119 the bottom of the screen:
120 ____________________________________________
121 | Cancel| Box| Point| Select type of ZOOM|
122 |_______|_____|_______|_____________________|
123 The user has the option of identifying the zoom region either by using
124 the mouse to make a box, or by using the mouse to mark the two diagonal
125 points of the desired region. The terminal screen will display a mouse
126 button menu to guide the user in identifying the corner points of the
127 region.
128 MARKING POINTS
130 To mark the points on the image that correspond to the points on a
131 standard coordinate system map, the user must place the mouse cross
132 hairs on the corresponding location on the image to be marked and press
133 the left hand button on the mouse. A diamond shaped symbol will be
134 marked on the image. The user's terminal will display the following
135 menu:
136 ___________________________________________________________
137 | Point 1 marked on the image at| |
138 | East: 1023.77 | |
139 | North: -164.41 | |
140 | | |
141 | | |
142 | | |
143 | | |
144 | | |
145 |__________________________________________________________|
146 | Enter coordinates as east north: |
147 |__________________________________________________________|
148 The user then enters the easting and northing (separated by a space)
149 for the point marked on the image. If the user wishes not to enter a
150 coordinate, he or she may simply hit RETURN to return control to the
151 mouse; the marked point then disappears.
152 PLOT RASTER
154 In addition to acquiring reference points from a standard map, the user
155 has the option of acquiring the reference points from a raster map
156 layer in the target data base LOCATION_NAME. The data base raster map
157 layer is displayed by placing the mouse cross hairs on the words PLOT
158 RASTER. The following line is then displayed at the bottom of the
159 graphics monitor:
160 ________________________________________________
161 | Cancel| Indicate which side should be plotted|
162 |_______|_______________________________________|
163 Which side of the graphics monitor is to be plotted is indicated by
164 placing the mouse cross hairs on the half of the graphics monitor
165 screen that the user would like to use, and pressing the left mouse
166 button. The following pop-down menu will be superimposed on the half
167 of the screen that was chosen:
168 _________________________________________
169 | Double click on raster (cell) map layer|
170 | to be plotted |
171 | Double click here to cancel |
172 |________________________________________|
173 ______________________________
174 | Mapset demo |
175 |_____________________________|
176 | tm.rectified | |
177 |______________|______________|
178 | tm.classified| |
179 |_____________________________|
180 | Mapset PERMANENT |
181 |_____________________________|
182 | elevation | geology |
183 |______________|______________|
184 | slope | soils |
185 |______________|______________|
186 | aspect | |
187 |______________|______________|
188 | roads | |
189 |______________|______________|
190 After the raster map layer is displayed the following message appears
191 at the bottom of the graphics monitor:
192 ______________________________________
193 | keyboard| screen|
194 |_________________|__________|________|
195 If the user wishes to use the plotted raster map layer only as a com‐
196 parative reference, then the keyboard can be chosen as the means to
197 input coordinates corresponding to the marked points on the image.
198 This is done by placing the mouse cross hairs on the word KEYBOARD and
199 pressing the left button on the mouse.
200 If the user selects the SCREEN option, then points marked on the image
202 will automatically be associated with the coordinates from the corre‐
203 sponding points on the target data base map layer. In this option,
204 when the user marks a point on the image, the following menu is dis‐
205 played at the terminal:
206 ___________________________________________________________
207 | Point 5 marked on the image at| |
208 | East: 1023.77 | |
209 | North: -164.41 | |
210 | | |
211 | | |
212 | Point located at | |
213 | East: 679132.57 | |
214 | North: 4351080.67 | |
215 | | |
216 | | |
217 | | |
218 | | |
219 | | |
220 |__________________________________________________________|
221 | use mouse now... |
222 |__________________________________________________________|
223 The user then uses the mouse to mark a corresponding point on the dis‐
224 played image from the target data base. The coordinates for the target
225 data base map layer are automatically saved as the coordinates corre‐
226 sponding to the marked point on the image.
227 ANALYZE
229 After a number of points have been marked (4 to 7), the user can check
230 the RMS error of the points marked on the image. This is done by plac‐
231 ing the mouse cross hairs on the word ANALYZE at the bottom of the
232 graphics monitor. An error report resembling that shown below is
233 superimposed on the graphics monitor:
234 ______________________________________________________________________
235 | error image target |
236 |# row col target east north east north |
237 |____________________________________________________________________|
238 |1 0.0 -0.9 1.0 1048.5 -144.8 679132.5 4351080.6|
239 |2 0.4 1.0 1.3 2153.1 -567.2 684314.7 4399001.4|
240 |3 -1.2 -0.5 .6 1452.8 -476.5 567841.4 3457682.8|
241 |4 1.1 0.5 1.3 1034.0 -109.2 677573.8 4352626.4|
242 |5 -2.7 14.0 14.2 1048.6 -144.9 679132.6 4351080.7|
243 | |
244 |____________________________________________________________________|
245 | overall rms error: 4.46 |
246 |____________________________________________________________________|
247 The following menu then appears at the bottom of the graphics monitor:
248 ____________________________________________________________________
249 | DONE| PRINT FILE| Double click on point to be included/excluded|
250 |_____|____________|________________________________________________|
251 The RMS error for the image is given under the column TITLEd "error"
252 and subTITLEd "row" and "col". In the above report, point number 1 is
253 0.0 rows and -0.9 columns from the predicted location calculated from
254 the transformation equation. The RMS error for the target raster map
255 layer is listed under the heading "target". This is the RMS error for
256 the east and north coordinates of the target map layer, but it is pre‐
257 sented in the table using one general value. The overall RMS error is
258 displayed at the bottom of the screen in meters. Points that create
259 high RMS error are displayed in red on the graphics monitor (repre‐
260 sented here in italics).
261 The location of the point marked on the imagery group file is given
263 under the heading "image" and the subheadings "east" and "north". The
264 location of the point in the target data base is given under the head‐
265 ing "target" and the subheadings "east" and "north". If the user would
266 like to exclude or include a point, this can be accomplished by placing
267 the mouse cross hairs on the point number to be included (if the point
268 is absent) or excluded (if the point is displayed) and pressing the
269 left button on the mouse twice. When a point is excluded, it is not
270 afterwards included in the calculation of the RMS error, or included in
271 the final transformation matrix. However, it can be retrieved within
272 i.points at any time by double clicking with the mouse as described
273 above.
274 QUIT
276 To end the i.points program place the mouse cross hairs on the word
277 QUIT; the marked points (including coordinates) will be saved.
278
280 A good rule of thumb is to mark at least 12 to 15 points which are
281 evenly distributed over the entire imagery group file in order to
282 obtain an accurate transformation equation for the rectification
283 process. The RMS error may increase with more points added, but the
284 transformation equation will be more accurate.
285 An RMS error of less than or equal to approximately one resolution unit
287 (pixel or cell) for the image being rectified is generally considered
288 acceptable.
289 In order to use a digitizer with i.points, at least one digitizer
291 driver besides "none" (the on-screen digitizer) must be available in
292 the digitcap file.
293 This program is interactive.
295
297 The GRASS 4 Image Processing manual
298 g.mapsets
300 i.group
301 i.rectify
302 i.target
303 i.vpoints
304
306 Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
307 Last changed: $Date: 2007-06-14 14:18:14 +0200 (Thu, 14 Jun 2007) $
309 Full index
311 © 2003-2008 GRASS Development Team
313GRASS 6.3.0 i.points(1)