[GRASSLIST:5055] Re: difficulties with i.ortho.photo

Thomas Dewez thomas.dewez at brunel.ac.uk
Mon Nov 25 04:25:12 EST 2002

Hi Guillaume,

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?

> Hello,
> 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.

Thomas Dewez
PhD Student
Dept Geography & Earth Sciences
Brunel University (West London)
Uxbridge UB8 3PH, Mddx
United Kingdom

Phone: +44-(0)1895-203215
Fax:     +44-(0)1895-203217

e-mail: thomas.dewez at brunel.ac.uk

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