This is not a command in itself, but an option of the command i.ortho.photo.

## NAME

photo.2image - An imagery function that enables you to mark known reference points (fiducial marks, reseau marks, etc.) on imagery group files, and compute the image-to-photo coordinate transformation parameters. The transformation parameters are required for the GRASS Image Processing Program i.ortho.photo.
(GRASS Image Processing Program)

## SYNOPSIS

Selected from i.ortho.photo

## DESCRIPTION

photo.2image is an imagery function that enables you to mark fiducial or reseau points on an image to be ortho-rectified and then computes the image-to-photo coordinate transformation parameters. The coordinates of the fiducials or reseau marks can be choosen for a camera reference file.

During the process of marking reference points with known photo coordinates, you may compute the RMS (root mean square) error for each reference point entered. photo.2image does this by calculating the transformation equation (the same one that is calculated by i.points ), and then plugging these results into an equation for RMS error.

photo.2image offers a zoom option to locate precisely the fiducial or reseau point to be marked on an image.

To run photo.2image, a graphics monitor is required.

The procedure for marking fiducial or reseau points, entering known photo-coordinates, and analyzing the RMS error is described below.

The terminal screen displays this message:

use mouse now...

The graphics monitor displays the following screen:

        +---------------------------+---------------------------------+
        |imagery                    |                                 |
        +---------------------------+---------------------------------+
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        +---------------------------+---------------------------------+
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        |                           |                                 |
        +---------------------------+---------------------------------+
        |                           |                                 |
        +---------------------------+---------------------------------+

A pop-down menu like that shown below will be superimposed on the left half of the screen:

                  +----------------------------------------+
                  |Double click on cell file to be plotted |
                  | Double click here to cancel       |
                  +----------------------------------------+
                              +----------------+
                              |Mapset imagery  |
                              +-------+--------+
                              |gs13.1 | gs21.1 |
                              +-------+--------+
                              |gs14.1 | gs22.2 |
                              +-------+--------+

Any single file in the imagery group may be used on which to mark points, and you may mark points on more than one file in the imagery group to accumulate the suggested minimum number of 4-8 fiducial or ressue points. The imagery file selected is displayed in the upper left quadrant of the screen.

## CAMERA FILE

The camera reference file may be viewed by placing the mouse cross hairs on the words CAMERA. The camera reference file will be displayed in the lower left quadrant of the display:

    |---------------------------------------------------------------|
    |Camera Reference File                                          |
    |---------------------------------------------------------------|
    |  CAMERA NAME :        camera name                             |
    |  CAMERA ID   :        L1234                                   |
    |  CAMERA CFL  :        153.021 mm.                             |
    |  CAMERA XP   :        0.050 mm.                               |
    |  CAMERA XP   :        0.056 mm.                               |
    |  number of fid.       4                                       |
    |---------------------------------------------------------------|
    |                                                               |
    |ID        (X PHOTO) mm.                 (Y PHOTO) mm.          |
    |_______________________________________________________________|
    |1         -105.023                        110.123              |
    |2          107.987                        109.834              |
    |3          110.965                       -104.329              |
    |4         -103.932                       -110.352              |
    |_______________________________________________________________|

The following menu is displayed at the bottom of the graphics display:

    +-----+------+-----------+--------+---------+-------------------+-------------+
    |Quit | Zoom | Plot Cell | Camera | Analyze |   Input->KEYBOARD | CAMERA FILE |
    +-----+------+-----------+--------+---------+-------------------+-------------+

## 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 have the option of identifying the zoom window either by using the mouse to make a box, or by using the mouse to mark the center of the desired window, and adjusting the magnification factor. The terminal screen will display a mouse button menu to guide you in identifying the window. The section of the image within the zoom window will be displayed in the upper right hand quadrant.

## MARKING REFERENCE POINTS

To mark the known reference points (fiducial marks, reseau marks, etc.) on the image, 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.

If you wish to use the camera reference file only as a comparative reference, then the keyboard can be chosen as the means to input photo coordinates corresponding to the marked reference 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.

The following menu is displayed on the graphics terminal:

    +------------------------------------+--------------------------+
    |Point 1 marked at IMAGE COORDINATES |                          |
    |east:  1023.77                      |                          |
    |north:  506.56                      |                          |
    |                                    |                          |
    |                                    |                          |
    |                                    |                          |
    |                                    |                          |
    |                                    |                          |
    +------------------------------------+--------------------------+
    |Enter PHOTO COORDINATES as x and y:                            |
    +---------------------------------------------------------------+

You then enter the known (x,y) photo coordinates, relative to the perspective center, for the reference point marked on the image. If you do not wish to enter a coordinate, simply hit RETURN to continue; the marked reference point will disappear.

If you select the CAMERA FILE option, then reference points marked on the image will be associated with selected photo coordinates from the displayed camera reference file. In this option, when you mark a point on the image, the following menu is displayed on the graphics terminal:

------------------------------------------------------------- CANCEL Double click on the fiducial mark to be referenced -------------------------------------------------------------

If you would like to select the photo coordinates of a displayed reference mark, this can be accomplished by placing the mouse cross hairs on the reference point to be selected and pressing the left button on the mouse twice. After a reference point is selected from the display, you are prompted with "Look ok? (Y/N)". If you respond with no, the reference point is ignored. If you respond with yes, the following is displayed on the terminal:

    +----------------------------------------+--------------------------+
    |Point 1 marked at IMAGE COORDINATES     |                          |
    |east: 1023.77                           |                          |
    |north: 1065.41                          |                          |
    |                                        |                          |
    |                                        |                          |
    |Point 1 referenced at PHOTO COORDINATES |                          |
    |X: -105.023                             |                          |
    |Y:  110.122                             |                          |
    |                                        |                          |
    |                                        |                          |
    |                                        |                          |
    |                                        |                          |
    |                                        |                          |
    +----------------------------------------+--------------------------+
    | use mouse now...                                                  |
    +----------------------------------------+--------------------------+

The photo coordinates and the corresponding image coordinates are automatically saved in the photo_points file associated with the imagery group.

## ANALYZE

After a number of points have been marked (4 to 8), 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 the one shown below is superimposed on the monitor:

    +--------------------------------------------------------------------+
    |       error           image           photo                        |
    |                                                                    |
    |#     row      col   photo    east    north   x               y     |
    |1     0.0     -0.9    1.0     1048.5  -144.8  -105.023    110.122   |
    |2     0.4      1.0    1.3     2153.1  -567.2   107.987    109.834   |
    |3    -1.2     -0.5    0.6     1452.8  -476.5   110.965   -104.329   |
    |4     1.1      0.5    1.3     1034.0  -109.2  -103.932   -110.352   |
    |                                                                    |
    +--------------------------------------------------------------------+
    |    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 "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 photo coordinates is listed under the heading "photo". This is the RMS error for the x and y coordinates of the photo coordinates but it is presented in the table using one general value. The overall RMS error is displayed at the bottom of the screen in milimeters. Points that create high RMS error are displayed in red on the 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 photo coordinates is given under the heading "photo" and the subheadings "x" and "y". If you 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 photo.2image at any time by double clicking with the mouse as described above.

## QUIT

To end the photo.2image program place the mouse cross hairs on the word QUIT; the marked reference points (including coordinates) will be saved.

## NOTES

A good rule of thumb is to mark at least 4 to 8 points which are evenly distributed over the perimeter of the 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) for the image being rectified is generally considered acceptable.

## SEE ALSO

i.ortho.photo
photo.camera
photo.2target
photo.init
photo.rectify

## AUTHOR

Mike Baba, DBA Systems, Inc.

Last changed: $Date: 2002/01/25 05:45:34 $