i.points(1grass) | Grass User's Manual | i.points(1grass) |
NAME¶
i.points - Mark ground control points on image to be rectified.KEYWORDS¶
imagery, geometrySYNOPSIS¶
i.pointsParameters:¶
- group=string
-
DESCRIPTION¶
i.points is an imagery function that enables the user to mark points on a (raster) image to be rectified and then input the geographic coordinates of each point for calculation of a coordinate transformation matrix. i.points must be followed by use of the GRASS program i.rectify, which rectifies the image using the transformation matrix coefficients calculated by i.points. 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:Please run i.target for group [spot]
Use mouse now...
______________________________________________________________
__________________________________
| Double click on raster map layer|
| to be plotted |
| Double click here to cancel |
|_________________________________|
____________________
| Mapset demo |
|___________________|
| spotclass| spot.1|
|__________|________|
| composite| spot.2|
|__________|________|
| spot.3 | |
|__________|________|
ZOOM¶
To magnify the displayed file, the user must place the mouse cross hairs on the word ZOOM. The following menu will then be displayed at the bottom of the screen:____________________________________________
| Cancel| Box| Point| Select type of ZOOM|
|_______|_____|_______|_____________________|
MARKING POINTS¶
To mark the points on the image that correspond to the points on a standard coordinate system map, the user must place the mouse cross hairs on the corresponding location on the image to be marked and press the left hand button on the mouse. A diamond shaped symbol will be marked on the image. The user's terminal will display the following menu:___________________________________________________________
PLOT RASTER¶
In addition to acquiring reference points from a standard map, the user has the option of acquiring the reference points from a raster map layer in the target data base LOCATION_NAME. The data base raster map layer is displayed by placing the mouse cross hairs on the words PLOT RASTER. The following line is then displayed at the bottom of the graphics monitor:________________________________________________
| Cancel| Indicate which side should be plotted|
|_______|_______________________________________|
_________________________________________
| 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 | |
|______________|______________|
______________________________________
|_________________|__________|________|
ANALYZE¶
After a number of points have been marked (4 to 7), the user can check the RMS error of the points marked on the image. This is done by placing the mouse cross hairs on the word ANALYZE at the bottom of the graphics monitor. An error report resembling that shown below is superimposed on the graphics monitor:____________________________________________________________________
QUIT¶
To end the i.points program place the mouse cross hairs on the word QUIT; the marked points (including coordinates) will be saved.NOTES¶
A good rule of thumb is to mark at least 12 to 15 points which are evenly distributed over the entire imagery group file in order to obtain an accurate transformation equation for the rectification process. The RMS error may increase with more points added, but the transformation equation will be more accurate. 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.SEE ALSO¶
The GRASS 4 Image Processing manual g.mapsets, i.group, i.rectify, i.target, i.vpointsAUTHOR¶
Michael Shapiro, U.S.Army Construction Engineering Research Laboratory Last changed: $Date: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $ Full index © 2003-2011 GRASS Development TeamGRASS 6.4.2 |