## DESCRIPTION

i.ortho.photo allows the user to ortho-rectify imagery group files. An imagery group consists of several scanned aerial photographs (raster maps) of a common area. Imagery groups can be created or modified using the GRASS Image Processing Program i.group, or using the first menu option described below. i.ortho.photo guides the user through the steps required to ortho-rectify the raster maps in a single imagery group.

The first menu in i.ortho.photo provides the user with the following options:

Initialization Options:
  1) Select/Modify imagery group
  2) Select/Modify imagery group target
  3) Select/Modify target elevation model
  4) Select/Modify imagery group camera
Transformation Parameter Computation:
  5) Compute image-to-photo transformation parameters
  6) Initialize exposure station parameters
  7) Compute photo-to-target transformation parameters
Ortho-rectification Option:
  8) Ortho-rectify imagery group raster maps
RETURN to exit
  >

To ortho-rectify aerial images the user has to follow the menu options step by step. Generally the user builds a target location containing a projected DEM and a map reference (topo sheet or other map). Another xy-location will contain the aerial photo. The entire rectification process is started within the xy-location. First the target to the projected location has to be set (i.target), then the image references (fiducial points) will be defined. After describing the camera parameters, reference points from aerial image to the topo sheet from the target location will be set. The local elevation will be taken from the target DEM. Finally the aerial image will be rectified.

The options to follow are described as follows:

1. Select/Modify imagery group
  The current imagery group is display at the top of the previous menu. You may select another (new or existing) imagery group for the ortho-rectification program using option (1). After choosing option (1) you will be prompted for the name of a new or existing imagery group. Option (1) using the GRASS Image Processing Program i.group, for creation or modification of imagery groups. For more information on imagery group creation or modification please the the GRASS manual page for i.group.
2. Select/Modify imagery group target
  The target location and mapset may be selected or modified using option (2). After choosing option (2) you will be prompted for the names of the target location and mapset, where the ortho-rectified raster maps will reside. The target location is also the location from which the elevation model (raster map) will be selected -- see option (3). Option (2) uses the GRASS Image Processing Program i.target, for selection or modification of the imagery group target location and mapset. For more information on imagery group selection or modification please the the GRASS manual page for i.target.
3. Select/Modify target elevation model
  Option (3) allows you to select the raster map from the target location to be used as the elevation model. The elevation model is required for both the computation of photo-to-target parameters (option 6) and for the ortho-rectification of the imagery group files (option 8). The raster map select for the elevation model should cover the entire area of the image group to be ortho-rectified. Currently, the elevation model raster map is expected to be in units of meters. DTED and DEM files are suitable for use as the elevation model in the ortho-rectification program. After selection option (3) you will be prompted for the name of the raster map in the target location that you want to use as the elevation model.
4. Select/Modify imagery group camera
  Using option (4) you may select or create an camera reference file that will be used with the current imagery group. A camera reference file contains information on the internal characteristics of the aerial camera, as well as the geometry of the fiducial or reseau marks. The most important characteristic of the camera is its focal length. Fiducial or reseau marks locations are required to compute the scanned image to photo-coordinate transformation parameter (option 5). For a more detailed description of option (4) please see the GRASS manual page for i.photo.camera.
5. Compute image-to-photo transformation parameters
  The scanned image to photo-coordinate transformation parameters are computed using option (5). In this interactive option you associate scanned reference points (fiducials, reseau marks, etc.) with their known photo coordinates from the camera reference file. Complete documentation for this option is available under the manual entry i.photo.2image.
6. Initialize exposure station parameters
  If option (6) is selected, initial camera exposure station parameters and initial variances may be selected or modified. Complete documentation for this option is available under the manual entry i.photo.init.
7. Compute photo-to-target transformation parameters
  The photo to target transformation parameters are compute using option (7). Here control points are marked on one or more imagery group files and associated with there known standard (e.g. UTM) and elevation coordinates. Complete documentation for this option is available under the manual entry i.photo.2target.
8. Ortho-photo imagery group files
  Option (8) is used to perform the actual image ortho-rectification after all of the transformation parameters have been computed. Ortho-rectified raster files will be created in the target location for each selected imagery group file. You may select either the current window in the target location or the minimal bounding window for the ortho-rectified image. Complete documentation for this option is available under the manual entry i.photo.rectify.

## NOTES

i.ortho.photo currently requires the elevation model to be in meters, and the target location to be a standard (e.g. UTM) coordinate system.

## REFERENCES

- Wolf, P.R. (1983): Elements of Photogrammetry, With Air Photo Interpretation and Remote Sensing. 2nd ed. McGraw Hill, 562 pages
- Rocchini, D., Metz, M., Frigeri, A., Delucchi, L., Marcantonio, M., Neteler, M. (2012). Robust rectification of aerial photographs in an open source environment. Computers & Geosciences 39 (2): 145-151.

## SEE ALSO

i.photo.camera, i.photo.2image, i.photo.2target, i.photo.init, i.photo.rectify

## AUTHOR

Mike Baba, DBA Systems, Inc.

Updated rectification and elevation map to FP 1/2002 Markus Neteler

Last changed: $Date: 2012-01-08 13:48:21 -0800 (Sun, 08 Jan 2012) $