52 
together with an orthophoto as texture source more 
than 99 percent of the data is needed to store photo 
texture in an uncompressed way. Experiments 
prove that lossy compression techniques, such as 
JPEG, shrink the original texture files to some 5 
percent of its former size without significant loss of 
quality. 
Although uncompressed during visualization the 
texture can be stored in small files thus enabling 
faster loading from disk. Decompression in memory 
is much faster than disk reading. The resulting loss 
of information and accuracy is acceptable, as the 
images are used mainly for visualization and 
animation purposes. 
3 IMPLEMENTATION 
More than 25 years ago a modular program system 
(SCOP) for the generation and management of 
high-quality DTM data with a hybrid data structure 
was developed at our institute in Vienna (Institute of 
Photogrammetry and Remote Sensing 
[http://www.ipf.tuwien.ac.at]) in very close cooperation 
with INPHO GmbH in Stuttgart (Germany) 
[http://www.inpho.de/scop.htm]. In the course of the 
years the functionality of SCOP has continuously 
been extended and improved. 
The DTM is integrated in SCOP as the central data 
base, and so it is possible to derive in a very flexible 
way (using additional modules) numerous follow-up 
products. Some important modules are: 
ISOLINES (derives isolines from any digital 
surface described in the data structure of 
SCOP, this can be elevation models, slope 
models or difference models) 
DOP (generation of digital orthophotos based 
on the high quality DTM data structure) 
PERSPECT (produces static 3D views of the 
DTM in form of a perspective representation or 
a parallel projection) 
PROFILE (interpolates single heights and height 
profiles - longitudinal, parallel, cross-sections - 
for any polygon position within the DTM area) 
TDM (management and archiving of huge 
amounts of terrain data using a relational data 
base system with efficient geometric queries) 
As a consequence of the increasing demand for 
interactive visualization an additional module to 
SCOP has been developed and implemented. This 
new Animated Terrain Model (ATM) tool provides an 
easy way for preparing and exporting SCOP DTM 
data for interactive 3D visualization. 
If the exported area is also covered - maybe only 
partly - by an orthophoto or an orthophoto mosaic 
this image data can be mapped as texture 
information over the animated terrain model. Areas 
without texture information are visualized as gray 
shading. In addition the predefinition of viewpoints 
and even a whole camera path for an automatic 
flight through the model can be done graphically in 
ATM (Figure 2). 
Figure 2 User interface of ATM showing coded DTM 
and defined flight path. 
The data is then exported using the data format 
VRML97. VRML is an acronym for "Virtual Reality 
Modeling Language". It is the international standard 
(ISO/IEC 14772) file format for describing interactive 
3D worlds and objects on the Internet [Carey R., 
Bell G., 1997]. It is in fact the 3D analogy to HTML. 
This means that VRML serves as a simple, multi 
platform language for publishing 3D Web pages. 
For visualizing such data a VRML viewer is 
necessary. This viewer can either be a standalone 
program or it can be integrated as plug-in into a 
HTML browser. There are a lot of such viewers on 
the market (CosmoPlayer™, CASUSPresenter™, 
VRwave™, WorldView , ...) for every current 
operating system, most of them as freeware [The 
VRML Repository]. Therefore, no additional cost 
may arise for the final user. The VRML world 
enables the users to interactively examine and 
visualize their data. 
As this data format is used on the Internet it is 
basically designed for a small amount of data. 
Therefore, special attention has been given to 
managing large data set, that have to be dealt with if 
DTMs or orthophotos are examined and visualized 
interactively. In order to achieve this goal all 
requirements for efficient visualization mentioned 
above have been realized in ATM. 
The whole DTM model is split up into Tiles, each 
one stored in different levels of detail. The image 
pyramid of the orthophoto is also divided using the 
same Tile limits. The number of grid points and the 
number of pixels within one Tile as well as the 
degree of data reduction for the LODs can be set by