Note: A new GRASS GIS stable version has been released: GRASS GIS 7. Go directly to the new manual page here
Rectification is the mapping (transformation) of an image from one coordinate system to another. The geometry of an image extracted into a GRASS LOCATION having an x,y coordinate system is not planimetric. To create a planimetric image, that is, to convert the x,y coordinate system into a standard coordinate system (for example, the UTM coordinate system or the State Plane coordinate system), points from a map having the standard coordinates must be associated with the same points on the image to be rectified. i.points enables the user to mark points on an image and input the standard coordinates for that point. i.points then calculates a least squares regression using the two coordinate systems (x,y and standard) for the marked points. A matrix containing transformation coefficients is the output file for i.points.
During the process of marking points and entering map coordinates, the user can compute the RMS (root mean square) error for each point entered. i.points does this by calculating the transformation equation (the same one that is calculated in the GRASS program i.rectify), and then plugging these results into an equation for RMS error.
i.points offers a zoom option to locate precisely the point to be marked on an image. This program also offers the user the option of acquiring standard coordinates for a marked point from a map layer in the target data base.
i.target must be run before running i.points to enable the PLOT RASTER option to be used and to identify a target data base LOCATION_NAME and MAPSET for the rectified image. To run i.points, a graphics monitor is required.
The procedure for marking points, entering coordinates, and calculating RMS error is described below.
The first prompt in the program asks the user for the imagery group to be registered. Note that if i.target is not run before i.points, the i.points program will display the following error message:
ERROR: Target information for group [spot] missing
Please run i.target for group [spot]
After entering the group to be registered the terminal screen
displays the message:
Use mouse now...
The graphics monitor displays the following screen:
______________________________________________________________ | imagery filename (mag) | target filename (mag) | |_________________________________|___________________________| | | | | | | | | | | | | | | | | | | | | | |_________________________________|___________________________| | | | | | | | | | | | | | | | | | | | | | |_________________________________|___________________________| | QUIT ZOOM PLOT RASTER ANALYZE| | |_________________________________|___________________________|A pop-down menu like that shown below will be superimposed on the left half of the screen:
__________________________________
| Double click on raster map layer|
| to be plotted |
| Double click here to cancel |
|_________________________________|
____________________
| Mapset demo |
|___________________|
| spotclass| spot.1|
|__________|________|
| composite| spot.2|
|__________|________|
| spot.3 | |
|__________|________|
Any single raster map layer in the
imagery group may be used on
which to mark points, and the user can mark points on more
than one raster map layer in the
imagery group to accumulate the
suggested minimum number of 12 points. Any raster map
layer in the imagery group
can be rectified (using
i.rectify) based on the
transformation matrix computed from these points.
The imagery file chosen by the user is displayed in the upper left quadrant of the screen.
____________________________________________ | Cancel| Box| Point| Select type of ZOOM| |_______|_____|_______|_____________________|The user has the option of identifying the zoom region either by using the mouse to make a box, or by using the mouse to mark the two diagonal points of the desired region. The terminal screen will display a mouse button menu to guide the user in identifying the corner points of the region.
___________________________________________________________ | Point 1 marked on the image at| | | East: 1023.77 | | | North: -164.41 | | | | | | | | | | | | | | | | | |__________________________________________________________| | Enter coordinates as east north: | |__________________________________________________________|The user then enters the easting and northing (separated by a space) for the point marked on the image. If the user wishes not to enter a coordinate, he or she may simply hit RETURN to return control to the mouse; the marked point then disappears.
________________________________________________
| Cancel| Indicate which side should be plotted|
|_______|_______________________________________|
Which side of the graphics monitor is to be plotted
is indicated by placing the mouse cross hairs on the
half of the graphics monitor screen that the user would like to use,
and pressing the left mouse button. The following pop-down
menu will be superimposed on the half of the screen that was chosen:
_________________________________________
| Double click on raster (cell) map layer|
| to be plotted |
| Double click here to cancel |
|________________________________________|
______________________________
| Mapset demo |
|_____________________________|
| tm.rectified | |
|______________|______________|
| tm.classified| |
|_____________________________|
| Mapset PERMANENT |
|_____________________________|
| elevation | geology |
|______________|______________|
| slope | soils |
|______________|______________|
| aspect | |
|______________|______________|
| roads | |
|______________|______________|
After the raster map layer is displayed the following message appears
at the bottom of the graphics monitor:
______________________________________
| input method -->| keyboard| screen|
|_________________|__________|________|
If the user wishes to use the plotted raster map layer only
as a comparative reference, then the keyboard can be chosen
as the means to input coordinates corresponding to the
marked points on the image. This is done by placing the
mouse cross hairs on the word KEYBOARD and pressing the
left button on the mouse.
If the user selects the SCREEN option, then points marked on the image will automatically be associated with the coordinates from the corresponding points on the target data base map layer. In this option, when the user marks a point on the image, the following menu is displayed at the terminal:
___________________________________________________________ | Point 5 marked on the image at| | | East: 1023.77 | | | North: -164.41 | | | | | | | | | Point located at | | | East: 679132.57 | | | North: 4351080.67 | | | | | | | | | | | | | | | | | |__________________________________________________________| | use mouse now... | |__________________________________________________________|The user then uses the mouse to mark a corresponding point on the displayed image from the target data base. The coordinates for the target data base map layer are automatically saved as the coordinates corresponding to the marked point on the image.
______________________________________________________________________ | error image target | |# row col target east north east north | |____________________________________________________________________| |1 0.0 -0.9 1.0 1048.5 -144.8 679132.5 4351080.6| |2 0.4 1.0 1.3 2153.1 -567.2 684314.7 4399001.4| |3 -1.2 -0.5 .6 1452.8 -476.5 567841.4 3457682.8| |4 1.1 0.5 1.3 1034.0 -109.2 677573.8 4352626.4| |5 -2.7 14.0 14.2 1048.6 -144.9 679132.6 4351080.7| | | |____________________________________________________________________| | overall rms error: 4.46 | |____________________________________________________________________|The following menu then appears at the bottom of the graphics monitor:
____________________________________________________________________ | DONE| PRINT FILE| Double click on point to be included/excluded| |_____|____________|________________________________________________|The RMS error for the image is given under the column TITLEd "error" and subTITLEd "row" and "col". In the above report, point number 1 is 0.0 rows and -0.9 columns from the predicted location calculated from the transformation equation. The RMS error for the target raster map layer is listed under the heading "target". This is the RMS error for the east and north coordinates of the target map layer, but it is presented in the table using one general value. The overall RMS error is displayed at the bottom of the screen in meters. Points that create high RMS error are displayed in red on the graphics monitor (represented here in italics).
The location of the point marked on the imagery group file is given under the heading "image" and the subheadings "east" and "north". The location of the point in the target data base is given under the heading "target" and the subheadings "east" and "north". If the user would like to exclude or include a point, this can be accomplished by placing the mouse cross hairs on the point number to be included (if the point is absent) or excluded (if the point is displayed) and pressing the left button on the mouse twice. When a point is excluded, it is not afterwards included in the calculation of the RMS error, or included in the final transformation matrix. However, it can be retrieved within i.points at any time by double clicking with the mouse as described above.
An RMS error of less than or equal to approximately one resolution unit (pixel or cell) for the image being rectified is generally considered acceptable.
In order to use a digitizer with i.points, at least one digitizer driver besides "none" (the on-screen digitizer) must be available in the digitcap file.
This program is interactive.
g.mapsets,
i.group,
i.rectify,
i.target,
i.vpoints
gis.m: GEORECTIFY TOOL and
wxGUI: Manage Ground Control Points
Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $
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