For very large data sets of building models partitioning is 
necessary. Figure 9 shows a small area of interest (A) 
for orthophoto computation. Buildings located in the 
sector defined by projection center P-1 and area A are 
required for this purpose. TOPDB could be used for this 
task. A database with full ability for 3D queries would 
meet all requirements of partitioning. 
In order to fill hidden areas in (A), overlapping aerial 
images are necessary. P-2 ist the projection center of 
this overlapping image. In this case also buildings 
located in sector P-2, A are required. 
4. EXAMPLE 
The proposed method has been checked with data from 
Vienna. Figure 10 shows a conventional orthophoto of 
the test area. 
  
Fig. 10: Conventional orthophoto 
A program has been developed to extract data of the 
multipurpose city map. Extracted terrain data were used 
to compute a digital terrain model with the DTM-software 
SCOP. The digital building model was generated in 
TOPDB. In some cases of inconsistency 
remeasurements of building data were necessary. 
Triangulation of roof elements has been done manually, 
since this algorithm is still in work. 
Based on the digital building model the building 
orthophoto could be computed. The generated building 
mask was used for the computation of the terrain 
orthophoto. Figure 11 shows the combination of terrain 
and building orthophoto. The black area around the 
building indicates hidden surfaces. Figure 12 is a mosaic 
of two orthophotos. Therefore hidden surface areas 
could be minimized. The geometric distortion of the 
ramp are the result of an incomplete building model. 
88 
  
Fig.11: Orthophoto with accurately mapped building 
  
Fig. 12: Orthophoto mosaic 
5. CONCLUSION 
The generation of large scale digital orthophotos in urban 
areas requires an integrated solution for data acquisition, 
modelling and rectification. Rectification is based on four 
steps that are: generating of building mask, generating 
building orthophoto, generating terrain orthophoto. The 
last step is the combination of both orthophotos. It is 
also possible to image hidden surface areas by 
mosaicking orthophotos compiled from different aerial 
images. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996