Manfred Wiggenhagen 
  
  
and 
the 
it in | 
  
Figure 6: Quadtree calculation for fast preselection of vertices 
Inside of the calculated quadrangles the density of displayed grid points can be changed with the so-called "level of 
- | detail factor" (Berg, Dobrindt, 1995). A factor greater than 1.0 will increase the density to the total sum of stored 
| grid points. For flat and homogeneous regions a level of detail factor of about 0.5 increases the computation speed 
= with acceptable detail presentation of the earth surface (see figure 7). With real time navigation by the pointing 
device (mouse) and the keyboard, the user can inspect the terrain from any point of the virtual world (see some 
| examples in Table 1). 
  
  
  
  
  
  
  
  
Event Result 
FTO) e Turning the camera around z-axis 
<> 
e Movement of pointing device 
x | e Turning the camera around x- or y-axis 
3 e Movement of pointing device 
Y 
JE e Movement of camera in viewing direction 
7^ e Coordinate measurement 
Table 1: Real time navigation with pointing device 
the 
ng 3 SUMMARY OF RESULTS 
are A new OpenGL based visualization tool has been developed to compare several digital terrain models, inspect the 
the quality of the digital data sets and to validate corresponding orthoimages. The powerful implementation of OpenGL 
hod graphic functions in combination with low cost graphic accelerator boards enables the user to investigate regularly 
esh spaced digital terrain models in real-time. 
yed 
the 
[ton 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 991