Provided by: grass-doc_6.4.3-3_all 
NAME
i.photo.2target - Creates control points on an image to be ortho-rectified.
KEYWORDS
imagery, orthorectify
SYNOPSIS
i.photo.2target
i.photo.2target help
i.photo.2target group=string [map=name] [target=name] [--verbose] [--quiet]
Parameters:
group=string
Name of imagery group
map=name
Name of image to be rectified which will be initially drawn on screen
target=name
Name of a map from target mapset which will be initially drawn on screen
DESCRIPTION
i.photo.2target is an imagery function that enables you to mark control points on an image
to be ortho-rectified and then input the coordinates of each point for calculation of
rectification parameters. Rectification is the mapping 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.
The ortho-rectification parameters are computed in two phases. The first phase computes a
transformation matrix between image (row,col) coordinates and photo (x,y) coordinates
relative to the perspective center. The transformation matrix is computed explicitly in
the option i.photo.2image . The second phase, i.photo.2target, enables you to mark
control points on the image and then input the standard coordinates (Easting,Northing, and
elevation) to determine the parameters for a three dimensional projective transformation.
i.photo.init
may be run before running i.photo.2target to modify the initial camera exposure station
parameters, and/or modify the standard deviation of these parameters. i.photo.init
is not required but generally is helpful.
During the process of marking control points and entering standard coordinates, you may
compute the RMS (root mean square) error for each control point entered. i.photo.2target
does this by calculating the transformation equation, and then plugging these results into
an equation for RMS error.
i.photo.2target offers a zoom option to locate precisely the point to be marked on an
image. This program also offers you the option of acquiring standard coordinates for a
marked point from a map layer in the target database, or from a digitizer.
To run i.photo.2target, a graphics monitor is required.
The procedure for marking points, entering coordinates, and calculating RMS error is
described below.
The terminal screen displays the following message:
use mouse now...
The graphics monitor displays the following screen:
+-----------------------------+--------------------------------+
|imagery filename (mag) | target filename (mag) |
+-----------------------------+--------------------------------+
| | |
| | |
| | |
| | |
| | |
| | |
| | |
+-----------------------------+--------------------------------+
| | |
| | |
| | |
| | |
| | |
| | |
+-----------------------------+--------------------------------+
|QUIT ZOOM PLOTCELL ANALYZE | |
+-----------------------------+--------------------------------+
A pop-down menu like that shown below will be superimposed on the left half of the screen:
+------------------------------------------+
| Double click on raster map to be plotted |
| Double click here to cancel |
+------------------------------------------+
+--------------------+
| Mapset PERMANENT |
+---------+----------+
|gs13.1 | gs14.1 |
+---------+----------+
|gs21.1 | gs22.2 |
+---------+----------+
Any single file in the imagery group may be used to mark points, and you can mark points
on more than one file in the imagery group to accumulate the 12 points suggested minimum.
Any file in the imagery group can be rectified (using i.photo.rectify ) based on the
rectification parameters computed from these control points.
The imagery file you select is displayed in the upper left quadrant of the screen.
ZOOM
To magnify the displayed file, you 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 |
+-------+-----+-------++--------------------+
You may identifying the zoom window either by using the mouse to make a box, or by using
the mouse to mark the center of the window and entering a magnification factor. The
terminal screen will display a mouse button menu to guide you in identifying the window.
It is also possible to start zoom by box with middle mouse button and pan by right mouse
button.
MARKING POINTS
To mark the points on the image that correspond to the points on a standard coordinate
system map, you 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 terminal will display the following menu:
+------------------------------------+--------------------------+
|Point 1 marked at IMAGE COORDINATES | |
|IMAGE X: 1023.77 | |
|IMAGE Y: -164.41 | |
| | |
+------------------------------------+--------------------------+
|Enter CONTROL COORDINATES as east,north,elevation: |
+---------------------------------------------------------------+
You then enter the easting, northing, and elevation for the point marked on the image. If
you wish not to enter a coordinate, simply hit RETURN to return control to the mouse; the
marked point then disappears.
Entered point can be canceled by right mouse button click.
PLOT CELL
In addition to acquiring control points from a standard map, you have the option of
acquiring the points from a cell-map in the target database. The database map is
displayed by placing the mouse cross hairs on the words PLOT CELL. The following line is
then displayed at the bottom of the monitor:
+-------+---------------------------------------+
|Cancel | Indicate which side should be plotted |
+-------+---------------------------------------+
Which side of the monitor is to be plotted is indicated by placing the mouse cross hairs
on the half of the monitor screen that you 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 map to be plotted |
| Double click here to cancel |
+------------------------------------------+
+-----------------------------+
| Mapset user1 |
+--------------+--------------+
|tm.rectified | |
+--------------+--------------+
|tm.classified | |
+--------------+--------------+
| Mapset PERMANENT |
+--------------+--------------+
|elevation | geology |
+--------------+--------------+
|slope | soils |
+--------------+--------------+
|aspect | |
+--------------+--------------+
|roads | |
+--------------+--------------+
|streams | |
+--------------+--------------+
|airfields | |
+--------------+--------------+
After the map is displayed the following message appears at the bottom of the monitor:
+-----------------+----------+--------+
|input method -> | keyboard | screen |
+-----------------+----------+--------+
If you wish to use the plotted map only as a comparative reference, the keyboard can be
chosen as the means to input coordinates corresponding to the marked control points. This
is done by placing the mouse cross hairs on the word KEYBOARD and pressing the left button
on the mouse.
If you select the SCREEN option, points marked on the image will automatically be
associated with the coordinates from the corresponding points on the target database map,
and a corresponding elevation from the cell-file selected for elevation data. In this
option, when you mark a point on the image, the following menu is displayed on the
terminal:
+------------------------------------+--------------------------+
|Point 5 marked at IMAGE COORDINATES | |
|IMAGE X: 1023.77 | |
|IMAGE Y: -164.41 | |
| | |
| | |
|Control Point location | |
|East: 679132.57 | |
|North: 4351080.67 | |
|Elevation: 1010.00 | |
| | |
| | |
| | |
+------------------------------------+--------------------------+
|use mouse now... |
+---------------------------------------------------------------+
The coordinates for the target database map are automatically saved as the coordinates
corresponding to the marked control point on the image.
ANALYZE
After a number of points have been marked (4 to 7), you 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 monitor. An error report resembling that shown below is
superimposed on the monitor:
+--------------------------------------------------------------------------+
| error |
|# east north target east north east north elev. |
+--------------------------------------------------------------------------+
|1 0.0 -0.9 1.0 48.5 4.8 79132.5 351080.6 10.0 |
|2 0.4 1.0 1.3 53.1 7.2 84314.7 399001.4 239.3 |
|3 -1.2 -0.5 0.6 52.8 6.5 67841.4 457682.8 209.5 |
|4 1.1 0.5 1.3 34.0 9.2 77573.8 352626.4 432.5 |
|5 -2.7 14.0 14.2 48.6 -144.9 79132.6 351080.7 985.0 |
| |
+--------------------------------------------------------------------------+
| overall rms error: 4.46 |
+--------------------------------------------------------------------------+
The following menu then appears at the bottom of the 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 "east"
and "north". In the above report, point number 1 is 0.0 and -0.9 meters (east and north)
from the predicted location calculated from the transformation equation. The RMS error
for the target map is listed under the heading "target". This is the RMS error for the
east and north coordinates of the target map 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 monitor (represented here in
italics).
The image coordinates 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 control point
in the target database is given under the heading "control" and the subheadings
"east","north", and "elev". If you would like to exclude or include a control point, this
can be accomplished by placing the mouse cross hairs on the control 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 rectification parameters.
However, it can be retrieved within i.photo.2target at any time by double clicking with
the mouse as described above.
QUIT
To end the i.photo.2target program place the mouse cross hairs on the word QUIT; the
marked control points (including coordinates) will be saved.
NOTES
During the course of marking control points and computing the ortho-rectification
parameters, a matrix inversion error may occur. This is caused by trying to invert a non-
singular normal equation matrix. When this situation arises, the status of all previously
selected control points are modified, the control points are excluded. Running
i.photo.init
for the selected imagery group with accurate camera exposure station parameters should
remedy the situation. The excluded control points may again be included as described in
the section ANALYZE.
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 parameters for
the rectification process. The RMS error may increase with more points added, but the
transformation parameters will be more accurate over the entire image.
An RMS error of less than or equal to approximately one resolution unit (pixel) for the
image being rectified is generally considered acceptable.
SEE ALSO
i.ortho.photo, i.photo.camera, i.photo.2image, i.photo.init, i.photo.rectify
AUTHOR
Mike Baba, DBA Systems, Inc.
Last changed: $Date: 2010-12-22 02:05:10 -0800 (Wed, 22 Dec 2010) $
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