Five groups of input parameters are required for ortho-rectification:
• Aerial image (images),
• Exposure and characteristics of the camera, i.e. its coordinates in target coordinate system and
height above sea level, focal length, yaw, pitch and roll, dimensions of the camera sensor and
resolution of aerial images,
• Reference surface, i.e. digital elevation model in the target coordinate system used to normalize
the terrain undulation,
• Topographic reference map used to find corresponding ground control points and/or,
• Coordinates of ground control points in the target coordinate system.
Exampleofaninputobliqueimageinasourceproject
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 sourceproject - 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 targetproject in a real-world coordinate system (e.g. ETRS33).
The steps to follow are described below:
1Create/Modifyimagerygrouptobeorthorectified:i.group
This step is to be run in the sourceproject.
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
2Select/Modifytargetprojectandmapsetfororthorectification:i.ortho.target
This step is to be run in the sourceproject.
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
3Select/Modifytargetelevationmodelusedfororthorectification:i.ortho.elev
This step is to be run in the sourceproject.
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)
4Create/Modifycamerafileofimagerygroup:i.ortho.camera
This step is to be run in the sourceproject.
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
5Computeimage-to-phototransformation:g.gui.photo2image
This step is to be run in the sourceproject.
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
Step5:Image-to-phototransformationofanobliqueimage6Initializeparametersofcamera:i.ortho.init
This step is to be run in the sourceproject.
In Step 6, initial camera exposure station parameters and initial variances may be selected or modified.
• X: East aircraft position;
• Y: North aircraft position;
• Z: Flight height above surface;
• Omega(pitch): Raising or lowering of the aircraft’s front (turning around the wings’ axis);
• Phi(roll): Raising or lowering of the wings (turning around the aircraft’s axis);
• Kappa(yaw): Rotation needed to align the aerial photo to true north: needs to be denoted as +90°
for clockwise turn and -90° for a counter-clockwise turn.
Principleofpitchandyaw
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
7Computeortho-rectificationparametersfromgroundcontrolpoints:g.gui.image2target
This step is to be run in the targetproject.
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
Step7:Detailofgroundcontrolpointsmatchinginanobliqueimageandterrainmodel8Ortho-rectifyimagerygroup:i.ortho.rectify
This step is to be run in the sourceproject.
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.
Step8:Ortho-rectifiedobliqueimage As a result, the ortho-rectified raster map is available for
visualization and further image analysis.
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