CI P A 2003 x/x"' International Symposium, 30 September - 04 October, 2003, Antalya, Turkey 
249 
Abbaye d'Aulne 
Orthofotoplan XI 
Frank Benoit & Dennis Devriendt, 2001 
Figure 4. Example of an Orthophotplan 
3. PROBLEMS IN THE METHODOLOGY AND 
POSSIBLE SOLUTIONS 
The methodology described in the previous paragraphs suffers 
some problems. These problems can be divided in two main 
categories, according to the solution. 
3.1 Problems that can be solved only with other 
photographs 
When parts of the wall on a photo are covered with vegetation, 
this results in a false DEM and according orthophoto. Although 
the surface beneath the vegetation can be interpolated, a more 
accurate DEM will be possible is the vegetation is removed 
before the photographs are taken. 
Problems due to the central projection of the photograph and 
the relief of the object, e.g. occlusions, can only be solved by 
making photographs from other points of view. 
As mentioned before, a number of stereopairs were taken with a 
certain inclination of the optical axis. This leads to a different 
resolution over the image. No quantitative analysis was done on 
the effect of tilt on the quality of the orthoimages. Although we 
can say that the absolute orientation is harder in the upper parts 
of tilted images, and the matching algorithm gives poor results 
in those areas, it is certainly possible to use tilted images up to a 
certain limit of the angles. In figures 5 and 6 the effect of tilt on 
the image geometry and the resolution becomes clear. 
Figure 5. Left image of a tilted stereopair 
Mr* 
Figure 6. The orthophoto derived from a tilted stereopair