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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
3.2.1 Structure of Terrain Object) and split the image into the
corresponding texture parts. These texture parts are mapped
as texture images on top of the geometry of the appropriate
terrain part. Due to the fact that the terrain parts at the north
and east boundary are smaller than the other parts, the
texture, which still has the 512 * 512 pixel size, overlaps the
geometry. This is not a problem, because the overlapping
texture parts are not displayed in the virtual universe. Fig. 4
shows the textured terrain of our example.
3.3.5 Memory Issues
However, it does not matter for the memory consumption if
the texture is actually visible or not. That means overlapping
parts are not useful, but they consume needed texture
memory. That is the reason, why we divided the terrain into
parts.
Looking at the original size of the fetched image (1224 *
1036) in our example we would have needed a Java 3D
corresponding texture size of 2048 * 2048 = 4194304 pixel.
Dividing the terrain into parts we need 9 texture images with
a dimension of 512 * 512 pixel. That adds to a number of
2359296 pixel. Thus we could save a significant amount of
texture memory.
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Fig. 5: Memory consumption of textures
3.3.6 Outlook
Techniques for reloading texture parts with a better
resolution when the user zooms into the 3D scene for
analysing details will be a big improvement for the
visualisation in the future.
3.4 Summary
The visualisation part of the paper described the creation of
the terrain object and the corresponding texture. The
presented techniques can be used for terrain visualisations in
3D GIS example applications.
4. INTEGRATION OF DEM IN EXAMPLE
APPLICATIONS
In the last part of the paper we return to our Baden-
Württemberg example. We describe two use cases using the
presented terrain visualisation techniques to include the DEM
data of Baden-Württemberg in 3D GIS example applications.
521
4.1 DEM Viewer
The goal of the DEMViewer is the visualisation of textured
digital terrain models. The user hast the possibility to
individually design the appearance of the texture. Depending
on the user request the visualised region can be very small or
large. Depending on the size of the region the appropriate
DEM resolution (50m, 5m or 1m) in the database will be
chosen.
The user loads his region of interest into the 2D map of
GISterm. The individually created 2D map is used as texture
for the terrain object created by the DEMViewer. Calling the
function “Create digital elevation model” in the 3D menu of
GISterm starts the DEM Viewer. The DEM Viewer fetches the
currently displayed bounding region of the 2D map and the
texture image of the map. Afterwards it selects an appropriate
DEM with the means of the Height-Service. The
DEMViewer creates a virtual universe including a textured
DEMLayer3D out of the selected data using the terrain
visualisation techniques. Fig. 6 shows a virtual universe in
GISterm created with the DEMViewer.
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Fig. 6 50m DEM in DEMViewer
Now the user has the possibility to navigate inside the 3D
scene and examine the loaded terrain object. The main view
of the 3D-Service displays the object content seen from the
current viewer position, which can be changed by navigation.
The navigation handling of the 3D-Service is realized with
the means of the mouse. The left mouse button leads to
rotation of the viewer around the objects in the 3D scene
(rotation), while the middle mouse button moves the viewer
closer to or farther from the objects (zooming). Finally the
right mouse button results in panning. The resulting. viewer
positions from the navigation handling are displayed in the
overview window, which displays the entire scene together
with the viewer position represented by a camera.
Due to the amount of computer memory the size of the
loaded terrain part is limited. Usually the user is interested in
neighbouring parts of the displayed terrain. Thus the user
needs to reload new terrain parts, which were not displayed
before. For this purpose the overview window has a button
for interactive reloading of terrain parts. Clicking that button
opens a submenu with arrow-buttons for all cardinal points.
Choosing one of the arrow-buttons deletes the cürrent terrain
part in the scene and loads the neighbouring part in the arrow
direction. The newly displayed part is partly overlapping the
former terrain part.
