3
This program enables the user to identify coordinate pairs of points from a
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vector map or keyboard entry and corresponding points in an image to be
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rectified. The map coordinate values of each point are used to calculate a
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transformation matrix. The operator may then use the
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<EM><a href="i.rectify.html">i.rectify</A></EM> program to rectify the image using
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the transformation matrix coefficients calculated from the control point
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file created in <EM>i.vpoints</EM>. The
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<EM> <a href="i.rectify.html">i.rectify</A></EM> program performs a first, second or
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third order transformation of the image.
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The first step is to display the unrectified image and
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corresponding vector map data. The operator would then
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mark corresponding control point locations on the image and
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map. To identify the precise location of a point to be
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marked, <EM>i.vpoints</EM> has a zoom option. In addition to
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marking control points on an image to be rectified and
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inputting their world coordinate values using the
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keyboard, <EM>i.vpoints</EM> has the option to simultaneously
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display vector map data available in the targeted database,
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and identify on the vector map the location of the
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corresponding marked points. When this option is chosen,
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the coordinate values are input automatically. Any GRASS
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map layer or vector map in the targeted database LOCATION
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can be displayed using <EM>i.vpoints</EM>. The <EM>i.vpoints</EM> program
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also has the capability of overlaying (i.e., warping) the
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vector data onto the raster image to visually check the
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accuracy of the registration based on the current set of
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active control points. During the process of marking
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points and entering map coordinates, the user can compute
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the RMS (root mean square) error for each point entered.
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The <EM>i.vpoints</EM> program does this by calculating a
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transformation equation (the same one that is calculated in
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<A HREF="i.rectify.html">i.rectify</A>
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computed for the equation. The coefficients are then used in the
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equation along with the x,y coordinates of the marked points. The
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results are plugged into an equation for RMS error. The
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interpretation of RMS error is described in the ANALYZE subsection.
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The procedures for marking control points (registration
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points), displaying vector map layers, overlaying vector maps onto
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the raster image, and calculating RMS error are described in the
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To enter the program (the <EM>i.vpoints</EM> program requires the use of a
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graphics monitor) type <EM>i.vpoints</EM>:
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The first prompt in the program asks for the imagery group to
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Enter imagery group to be registered
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Enter 'list' for a list of existing imagery groups
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Enter 'list -f' for a verbose listing
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Hit RETURN to cancel request
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might produce the following response:
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- - - - - - - - - - - - - - - - - - - - - - - - - -
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- - - - - - - - - - - - - - - - - - - - - - - - - -
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hit RETURN to continue -->
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<DD>><B>list -f</B>
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might produce the following response
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- - - - - - - - - - - - - - - - - - - - - - - - - -
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- - - - - - - - - - - - - - - - - - - - - - - - - -
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hit RETURN to continue -->
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The imagery group entered above should contain the files
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that you wish to rectify. After entering the group to be
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registered, the terminal screen displays the message:
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And the color graphics monitor displays the following
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Any single file in the imagery group may be used to mark
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points, and points can be marked on more than one file in the
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imagery group to accumulate the suggested minimum number of points
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(3 for a 1st order transformation, 6 for a 2nd order, and 10 for a
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3rd order). Any file in the imagery group can be subsequently
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rectified (using <EM><A HREF="i.rectify.html">i.rectify</A></EM>)
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based on the transformation matrix computed from these points.
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The chosen file is displayed in the upper left quadrant of the
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monitor at a default magnification based on the extent of the
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current active window.
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The raster image option on the menu at the bottom of the
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window allows the user to display any single file in the imagery
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group in the upper right quadrant of the window screen. The option
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provides the same file selection pick list as is presented when you
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first enter the <EM>i.vpoints</EM> program. When you select this option,
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the program will erase the data contained in all of the four
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quadrant windows and will reinitialize all program values.
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The vector maps option on the menu at the bottom of the screen
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allows the user to display vector map data in the upper right
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quadrant of the screen. After selecting the vector map layer to
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display, a menu selection bar appears along the bottom on the
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screen. This pick list is used to select the line color (blue,
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gray, green, red, white, or yellow) for the selected vector data
160
The refresh option on the main menu allows the user to
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"refresh" or re-draw the displayed vector data. This function will
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erase all outlines showing the limits of previously zoomed areas.
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A "yes/no" roompt will appear:
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To enlarge a raster or vector image, place the mouse cross
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hairs on the word zoom on the main menu and press the left button.
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The following menu will be displayed at the bottom. of the screen:
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CANCEL BOX POINT Select type of zoom
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You have the option to identify the map extent of the zoom window
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using either the mouse to define a box, or the mouse to mark a
178
center point from which to enlarge the image. The box option first
179
prompts you to identify a starting corner for the zoom region and
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then allows you to define the area to be zoomed using a rubber band
181
box. The prompts appear as follows...
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CANCEL Mark the first corner of region
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CANCEL Define the region
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After marking the first corner of the region to be enlarged, hold
188
down the left button and move the mouse to change the size and
189
shape of the rubber band box. After defining the area to be
190
enlarged, press the right button to accept it.
195
The point method for enlarging an image will display a mouse menu
196
to guide you in selecting the appropriate enlargement. To enlarge
197
or reduce the magnification factor, place the cursor on the "+" or
198
"-" box and press the left button on the mouse.
201
You may zoom either the raster or the vector display.
203
Upon accepting the new region limits, the raster or vector data are
204
redisplayed in either the lower left (raster) or lower right
207
The extent of the zoomed area is outlined on the unzoomed image in
208
the main window area.
210
While the main menu is displayed, you can mark
211
corrosponding control points on the raster and vector
212
images or enter map coordinates from the keyboard. If you
213
are using coordinates taken from a reference map, circle
214
these points and then use whatever means you have available
215
to identify as precisely as possible the coordinate values
216
for these points. Digitizing software is recommended,
217
especially GRASS v.digit. Once you have
218
determined the standard coordinates (for example, UTM's) of
219
each circled point, you are ready to mark the points on the
222
To mark the points on the image, that correspond to the points
223
on the standard coordinate map, place the mouse cross hairs on the
224
point on the image to be marked (you will probably have to ZOOM to
225
find the exact spot) and press the left hand button on the mouse.
226
A diamond shaped symbol will be marked on the image. The text
227
monitor will display the following screen:
229
Point 1 marked on the image at
233
Enter coordinates as east north:
238
After a number of points have been marked (a minimum of 4 for
239
a 1st order transformation, 7 for a 2nd order, and 11 for a 3rd
240
order), the RMS error of the points marked on the image can be
241
checked. This is done by placing the cross hairs on the word
242
ANALYZE on the main menu at the bottom of the monitor. The
243
following error report is superimposed on the monitor:
247
# col row target east north east north
249
1 -0.9 0.0 1.0 1048.5 -144.8 679132.5 4351080.6
250
2 1.0 0.4 1.3 2153.1 -567.2 684314.7 4399001.4
254
Overall rms error 76.85
257
The RMS error for the image being rectified is recorded under
258
the column "error" and subTITLEd "row' and "col". In the above
259
report, the marked point number 1 is 0.0 rows and -0.9 columns from
260
the predicted location calculated by the transformation equation.
261
The RMS error for the target database map is recorded under
262
the heading "error" and the subheading "target". This is the RMS
263
error for the east and the north coordinate values of the target
264
map, but it is represented in the table using one general value.
265
The overall RMS error for the image is displayed at the bottom
266
of the screen in meters. Points that generate a high RMS error are
267
displayed in red on the monitor.
268
The x,y coordinate values of the point marked on the image
269
being rectified are recorded under the heading "image" and the
270
subheadings "east" and "north".
271
The standard coordinate values of the point in the target
272
database are recorded under the heading "target" and the
273
subheadings "east" and "north".
274
If the user would like to exclude or include a point, this can
275
be accomplished by placing the mouse cross hairs on the point
276
number to be included (if the point is absent) or excluded (if the
277
point is displayed) and then pressing the left button on the mouse
278
twice. When a point is excluded, it is not included in the
279
calculation of the RMS error, or included in the final
280
transformation matrix. However, it can be retrieved within
281
<EM>i.vpoints</EM> at any time by double clicking with the mouse as
284
The following menu appears at the bottom of the monitor:
286
DONE PRINT FILE OVERLAY DELETE ON Transformation -
287
-> 1st ORDER Double click on point to be DELETED
289
Selecting DELETE ON will toggle the option to DELETE OFF, the
290
toggle option is used to allow the user to physically remove a
291
control point from the POINTS file instead of just flagging it as
292
an non-active reference point.
296
Overlay allows the user to overlay the vector map(s) onto the
297
raster image. Overlay can be used to warp (register) and display
298
the selected vector map data on top of the raster image contained
299
in the upper left window of the color screen. An inverse
300
coordinate transformation is performed using the currently active
301
order of transformation (i.e., first, second, or third).
303
Overlay vectors on raster image NO YES
305
By selecting the 1st ORDER option, the user may select the order of
306
transformation desired:
308
Select order of transformation --> 1st Order 2nd Order 3rd Order
310
The program will immediately recalculate the RMSE and the number of
315
To exit the <EM>i.vpoints</EM> program, place the mouse cross hairs on
316
the word QUIT at the bottom of the monitor and all of the marked
317
points (including coordinates) will be saved.
322
<A HREF="http://grass.itc.it/gdp/imagery/grass4_image_processing.pdf">Image
323
Processing manual</A></EM>
327
<A HREF="g.mapsets.html">g.mapsets</A><BR>
328
<A HREF="i.group.html">i.group</A><BR>
329
<A HREF="i.points.html">i.points</A><BR>
330
<A HREF="i.rectify.html">i.rectify</A><BR>
331
<A HREF="i.target.html">i.target</A>
333
<em><a href="gm_georect.html">gis.m: GEORECTIFY TOOL</a></em>
338
William R. Enslin, Michigan State University Center for Remote Sensing<br>
339
Radim Blazek (update to GRASS 5.7)<BR>
340
Hamish Bowman (finish GRASS 6 porting)
342
<i>Last changed: $Date: 2007-10-29 12:12:37 +0100 (Mon, 29 Oct 2007) $</i></p>