The ortho-rectification procedure in GRASS GIS places the image pixels on the surface of the earth by matching the coordinate system of the aerial image in pixels (image coordinate system) and the coordinate system of the camera sensor in millimetres (photo coordinate system) for the interior orientation of the image, and further to the georeferenced coordinate system defined by projection parametres (target coordinate system) for exterior orientation.
To ortho-rectify aerial images the user has to follow the menu options step by step. Alternatively, all the steps can be performed separately by running the corresponding modules.
The aerial photos shall be stored in a source project - a general Cartesian coordinate system (XY). Digital elevation model and a map reference (topo sheet or other map used for ground control point matching) shall be stored in a target project in a real-world coordinate system (e.g. ETRS33).
The steps to follow are described below:
This step is to be run in the source project.
In this first step an imagery group of aerial images for ortho-rectification is created or modified. The current imagery group is displayed at the top of the menu. You may select a new or existing imagery group for the ortho-rectification. After choosing this option you will be prompted for the name of a new or existing imagery group. As a result, a new file mapset/group/name_of_group/REF is created that contatins the names of all images in a group.
IMG_0020 source_mapset IMG_0021 source_mapset IMG_0022 source_mapset
This step is to be run in the source project.
The target project and mapset may be selected or modified in Step 2. You will be prompted for the names of the projected target project and mapset where the ortho-rectified raster maps will reside. The target project is also the project from which the elevation model (raster map) will be selected (see Step 3). In Step 2, a new file mapset/group/name_of_group/TARGET is created contatining the names of target project and mapset.
ETRS_33N target_mapset
This step is to be run in the source project.
Step 3 allows you to select the raster map from the target project to be used as the elevation model. The elevation model is required for both the computation of photo-to-target parameters (Step 6) and for the ortho-rectification of the imagery group files (Step 8). The raster map selected for the elevation model should cover the entire area of the image group to be ortho-rectified. DTED and DEM files are suitable for use as elevation model in the ortho-rectification program. In Step 3 you will be prompted for the name of the raster map in the target project that you want to use as the elevation model. As a result of this step, a new file mapset/group/name_of_group/ELEVATION is created contatining the name and mapset of the chosen DEM.
elevation layer :ELEVATION mapset elevation:target_mapset location :ETRS_33N math expression :(null) units :(null) no data values :(null)
This step is to be run in the source project.
In Step 4 you may select or create a 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 (Step 5). Two new files are created in this step: a file mapset/group/name_of_group/CAMERA, contatining the name of the reference camera and a file mapset/camera/name_of_reference, contatining the camera parameters.
CAMERA NAME sony CAMERA ID 123 CAMERA XP 0 CAMERA YP 0 CAMERA CFL 16 NUM FID 4 0 -11.6 0 1 0 7.7 2 11.6 0 3 0 -7.7
This step is to be run in the source project.
The scanned image to photo coordinate transformation parameters, i.e. the "interior orientation", is computed in Step 5. In this interactive step you associate the scanned reference points (fiducials, reseau marks, etc.) with their known photo coordinates from the camera reference file. A new file mapset/group/name_of_group/REF_POINTS is created, contatining a list of pairs of coordinates in image and photo coordinate systems.
# Ground Control Points File # # target location: XY # target mapset: source_mapset # source target status # east north east north (1=ok, 0=ignore) #------------------------------------------------------------- 0 1816 -11.6 0.0 1 2728 3632 0.0 7.7 1 5456 1816 11.6 0.0 1 2728 0.0 0.0 -7.7 1
This step is to be run in the source project.
In Step 6, initial camera exposure station parameters and initial variances may be selected or modified.
In Step 6, a new file mapset/group/name_of_group/INIT_EXP is created, contatining camera parameters.
INITIAL XC 215258.345387 INITIAL YC 6911444.022270 INITIAL ZC 1101.991120 INITIAL OMEGA 0.000000 INITIAL PHI -0.168721 INITIAL KAPPA 3.403392 VARIANCE XC 5.000000 VARIANCE YC 5.000000 VARIANCE ZC 5.000000 VARIANCE OMEGA 0.000000 VARIANCE PHI 0.020153 VARIANCE KAPPA 0.017453 STATUS (1=OK, 0=NOT OK) 0
This step is to be run in the target project.
The photo to target transformation parameters, i.e. the "exterior orientation", is computed in Step 7. In this interactive step, control points are marked on one or more imagery group files and associated with the known standard (e.g. UTM) and elevation coordinates. Reasonable rectification results can be obtained with around twelve control points well distributed over the image. In this step, a new file mapset/group/name_of_group/CONTROL_POINTS is created, containing a list of pairs of coordinates of ground control points in photo and target coordinate systems.
# Ground Control Points File # # target location: ETRS_33N # target mapset: target_mapset # source target status # east north height east north height (1=ok, 0=ignore) #------------------------------ ---------------------- --------------- 98.3679932698 906.327649515 0.0 1.0 5.0 100.0 1 733.293023813 1329.61100321 0.0 2.0 6.0 100.0 1 1292.6317412 1703.76325335 0.0 3.0 7.0 100.0 1 1625.54617472 1368.11694482 0.0 4.0 6.0 100.3 1 3239.82849913 1390.97403968 0.0 7.4 6.0 100.3 1 1570.09788497 2790.06537829 0.0 3.0 11.0 100.0 1
This step is to be run in the source project.
Step 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 project for each selected imagery group file. You may select either the current window in the target project or the minimal bounding window for the ortho-rectified image.
As a result, the ortho-rectified raster map is available for visualization and further image analysis.Available at: i.ortho.photo source code (history)
Latest change: Friday Aug 02 04:59:01 2024 in commit: e671a4322778af043684130680c553bbe98d14cc
Main index | Imagery index | Topics index | Keywords index | Graphical index | Full index
© 2003-2024 GRASS Development Team, GRASS GIS 8.5.0dev Reference Manual