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.