Provided by: grass-doc_6.4.3-3_all 
NAME
i.points - Mark ground control points on image to be rectified.
KEYWORDS
imagery, geometry
SYNOPSIS
i.points
i.points help
i.points group=string [--verbose] [--quiet]
Parameters:
group=string
Name of imagery group to be registered
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:
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.
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|
|_______|_____|_______|_____________________|
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.
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:
___________________________________________________________
| 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.
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|
|_______|_______________________________________|
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:
______________________________________
| 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.
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:
______________________________________________________________________
| 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.
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.vpoints
gis.m: GEORECTIFY TOOL and wxGUI: Manage Ground Control Points
AUTHOR
Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $
Full index
© 2003-2013 GRASS Development Team