Input data for the system are digital aerial or satellite 
images which have been obtained from scanning analo- 
gue photographs or directly have been recorded by air- 
borne or satellite sensors. Either ground control points 
or orientation parameters e.g. from an aerial triangula- 
tion have to be supplied for exterior orientation. 
Furthermore a digital terrain model (DTM) has to be 
given for the generation of the digital orthoimage and 
further use during monoplotting. 
The software package PHODIS-OP (Mayr, 1993) from 
Carl Zeiss is used for the generation of the digital orthoi- 
mage. PHODIS is the photogrammetric base for image 
conversion, interior and exterior orientation, orthoima- 
ge generation and mosaicing. 
Since the quality of the DTM directly effects the geo- 
metric quality of the orthoimage the HIFI-GIS interface 
(Ebner et al. 1990) was integrated which allows for direct 
access and rigorous use of high quality DTM generated 
with the DTM program package HIFI (Ebner et al. 
1988). 
For data acquisition from the orthoimage PHOCUS-M 
(Braun, 1989) from Carl Zeiss is used. It enables the 
operator to acquire threedimensional data structured 
according to an object code table. In order to ensure a 
smooth data flow between the forestrial GIS and the 
monoplotting system and vice versa an optimal adaption 
of the data structure of PHOCUS to the forestry GIS 
has been realized. The data exchange between both 
systems is carried out by interface programs. Under the 
shell of PHOCUS-M various DTM products can be 
derived and directly superimposed during monoplot- 
ting. This possibility allows for a DTM supported eva- 
luation of the scene. 
3. GENERATION OF HIGH QUALITY 
ORTHOIMAGES 
The quality of the DTM has an essential impact on the 
accuracy of the orthoimage and therefore on digital 
monoplotting in general. Therefore, it is necessary that 
the DTM data structure is able to model the terrain 
surface in a waythat topographic features like breaklines 
are considered. Furthermore it has to be ensured that 
this DTM structure is taken as the base for the rectifica- 
tion process without a loss of accuracy. The DTM pro- 
gram package HIFI uses a combined grid/tin data- 
structure and therefore is appropriate for this task. The 
HIFI-GIS interface in a first step loads this DTM 
structure into the memory of the computer. For the 
rectification process the terrain height (z) of each indi- 
vidual orthoimage pixel is computed from this DTM and 
transferred as a profile for an orthoimage row (cf. Fig. 
2). 
The accuracy improvement of the implemented proce- 
dure in comparison with the standard version of PHO- 
DIS-OP (which allows the use of a grid DTM only) is 
documented by an example.The orthoimage of a moun- 
tainous area has been computed as explained above and 
also with the standard version of PHODIS-OP. After. 
wards the orthoimages have been combined with the 
result of a photogrammetric evaluation (contours and 
breaklines). Figures 3a and 3b show the results obtained 
with the use of the high quality DTM and the standard 
DTM, respectively. The displacement of the breakline 
in figure 3b (in the order of 5-8 m) is obviously caused 
by the DTM without consideration of the breakline. 
breakline 
  
  
  
  
  
  
  
  
profile A-A 
Figure 2: A section of a transferred profile for an orthoi- 
mage row 
   
  
  
Figure 3a: A section of a high quality orthoimage, 
superimposed with a photogrammetric evaluation 
Figure 3b: Same as Figure 3a but orthoimage without 
consideration of breaklines 
222 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996 
  
Fi