[GRASSLIST:5055] Re: difficulties with i.ortho.photo
thomas.dewez at brunel.ac.uk
Mon Nov 25 04:25:12 EST 2002
Here is a quick diagram to explain what's required for proceeding to
the interior orientation of your air-photos (that's the photogrammetric
jargon for what you want to do).
The marks on the side of the photos are called fiducial marks (FM).
If you want a good interior orientation, you'd better know the order in
which they were calibrated. Look at the direction of the axes X and
Y. In the diagram, the Y axis has got positive values as it goes
down. This may not be the case on your images and depends on
the lab where the camera calibration was done. My diagram is
proper to the camera Leica LMK9 with serial number 265-002A
belonging to the Greek Army. You should see the serial number of
the camera in the instrument strip (black band where you can see
the altimeter, watch and photo counter of the camera). As a crude
indication, the instrument strip bears the so-called principal distance
(also known as focal length of the camera). The actual calibrated
principal distance will be slightly different than this one. You should
use the calibrated value.
Note that the order in which you enter the fiducials will determine
virtually the direction in which the plane was flying. Follow the order
of the calibration certificate. There should be a small hand-
annotated diagram with the fiducials numbering scheme.
The centre of symmetry is the point marked PP. Ideally it should fall
bang on the intersection of the radii at the centre of the picture, but
they never do. The excentricity as it is called is usually of the order
of a few micrometers. The origin of the axes is at the intersection of
the radii traced from the fiducial marks.
Your best bet to solve you problem is to get hold of the calibration
certificate of the camera that was used to shoot the photos. The
photo retailer should provide it to you for free. Also, unless it is for
illustration purposes, the photographs must scanned on a proper
photogrammetric scanner otherwise you'll end up with unexplained
distortions in your orthophotos.
Does this help?
> I am using i.ortho.photo to ortho-rectify aerial images of Mayotte
> (Comoros Archipelago). The associated man pages are not very clear
> about a few points.
> When i register a camera, what is this "point of symmetry" that i have
> to locate on the photo ? What is the origin and direction of X and Y
> on the photo ?
> When i must enter fiducial and reseau marks, where is the origin ? is
> it the same as the point of symmetry that i talked about above or is
> the point of symmetry the new origin ?
> Well, if someone has experience in using i.ortho.photo and explain me
> a bit, i'd be very grateful.
> Thanks in advance,
> Guillaume Criloux.
Dept Geography & Earth Sciences
Brunel University (West London)
Uxbridge UB8 3PH, Mddx
e-mail: thomas.dewez at brunel.ac.uk
More information about the grass-user