Manfred Wiggenhagen
Figure 4: Orthoimage with DEM errors Orthoimage with automatically "healed" DEM
For the production process of orthophotos and for the error correction of DEMs it is more important to find and
determine the location of height errors and to correct them in the data file. For this purpose the user can measure the
three dimensional coordinates of the gridpoint with the pointing device. This measurement should be carried out in
wireframe mode (see figure 5).
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Figure 5: Measurement of DEM error location Display of measured coordinates
2.5 Improvement of Visualization Speed and Navigation
For fast data access quadtree techniques and dynamic level of detail representations have been implemented.
Normally the graphic display function tries to display all known vertices in the current view. If the user changes the
viewing angle and viewing direction, the algorithm has to recalculate all vertices again. This time consuming
method can be improved by the use of quadtree techniques.
After the quadtree for the whole scene is calculated the visualization algorithm will detect which quadrangles are
hidden in the current view and will only recalculate the vertices in the remaining quadrangles. For flat regions the
number of quadrangles and the number of vertices per quadrangle will be reduced to a minimum. This method
increases the computational speed in such a successful way, that standard graphic boards reach acceptable refresh
rates for the recalculation of the whole scene. Elevation models with a high amount of gridpoints can be displayed
with real-time navigation over the whole terrain. To show the customer the distribution of the quadrangles and the
separation of the surface, the additional overlay can be checked in the project dialog box with the radio button
"Show QuadTree". The DEM will then be visualized with the calculated quadtree (see figure 6).
990 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.
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