the user, depending on the system used for the final 
visualization. For visualization a master file is 
created which controls the loading and drawing of 
the different Tiles at the correct position and the 
optimal level of detail. 
4 EXAMPLE 
The following example was carried out to test the 
applicability and the limitations of VRML for large 
textured data sets on the internet. The test area 
covers about 10000 km square in the region of 
Austria's highest mountain. This area was chosen 
because of its great differences in height between 
the lowest point (Zell am See 740m) and the highest 
one (Grossglockner 3400m). So, during navigation 
over the terrain the requirements for computing 
visibility is much higher as in flat areas. 
The DTM was exported using a grid size of 250 m. 
For texturing, a georeferenced Landsat TM scene 
with a resolution of 25 m was used. For interactive 
visualization we use a more or less standard 
computer (350 MHz, 64 MB RAM, simple graphic 
card with 3D accelerator) running under Windows 
NT. Concerning robustness of the computer system 
and fulfillment of the VRML97 standards we made 
best experiences using the free available VRML 
viewer CosmoPlayer™, a plug-in for Netscape. 
The terrain model representation of VRML only 
supports a regular grid. To conserve all structure 
information a TIN representation is also possible, 
but applying texture information from orthophotos 
would be much more complicate then. Besides, the 
grid interpolation is based on the hybrid data 
structure used in SCOP, yielding a higher quality of 
the exported elevation grid than achievable by a 
simple regular grid derived from the original data 
without taking breaklines into account. 
The main drawback of applying VRML for interactive 
visualization is the impossibility of using the full 
resolution of the high quality DTM for great areas. 
While for that reason this sort of presentation cannot 
be recommended as a substitute for plots of high 
accuracy, but it offers an excellent possibility to 
complement and extend the palette of 
photogrammetric products. 
One advantage of VRML is the easy way to link 
additional data (vector data, thematic information, 
...) apart from geometry and texture to the VRML 
model. So, complex spatial and thematic situations 
can be shown in a really vivid way through 
animation. 
In addition to the interactive visualization also 
measurement tools (coordinates, orientations, 
distances, ...) were added [Zeisler Ph., 1999]. This 
was done using the EAI (External Authoring 
Interface) [External Authoring Interface Working 
Group]. (Figure 3) 
Figure 3 Interactive visualization using VRML and coordinate measurement using the EAI.