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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
looks for available data in the database, beginning with the
highest possible resolution. One aspect we have to consider
is, that the database has no or only fragmented data for the
requested region. Possible reasons are:
e The acquisition of the laser scanner data is still
going on. That means the requested data is not yet
fully available.
e The processing of the laser scanner data is not yet
fully completed.
In this case the Height-Service will search for a lower
resolution digital elevation model. Now we are ready for the
access of the DEM data.
2.6.2 Accessing the DEM data
The Height-Service includes methods for accessing the data
from the selected DEM. The class diagram shows the most
important methods.
TERRE]
1
<<Interface>> |
HelghtProvider TELE IOINCITENETISIMUIUEUSTUTUMEM
3 rod BoundingBox | InvalidBoundingBoxException
1 200.”
HelghtOperations
boundingBox
terWidthNorth{)
os
: ««Interface»»
|. HeightinterpolateFunction
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"NearasiNeighborinterpolateFuncfion |
[ LinearinterpolateFunction |
Fig. 2: Height-Service class diagram
The methods:
® getCoordinates (double []
eastValues, double[] northValues)
e getHeightValues ()
are accessing the grid structured base points of the DEM,
while the methods:
e getNearestNeighbourInterpolatedHei
ght (double east, double north)
e getLinearInterpolatedHeight (double
east, double north)
are able to interpolate height values between the base points.
2.7. Summary
Reviewing the data handling section of this paper we began
with a look at the data basis. Then we created PNG-image
files from the raw data and stored them in the database using
the raster tiling approach. Finally we considered the access to
the data in the database via the Height-Service interface. That
means we are now ready for the next part of the paper: the
visualisation.
519
3. VISUALISATION OF THE DATA
This part of the paper concentrates on the visualisation of the
DEM data. The presented concepts of this part are not limited
to the visualisation of our Baden-Württemberg example. The
techniques can be used for all kinds of rasterised terrain
visualisations.
Most 3D applications concerning environmental aspects need
digital elevation models as a data basis. The visualisation of
these models leads to a three-dimensional terrain-object
presentation in a virtual reality.
3.1 Creation of the Virtual Universe
For the integration into the base GIS GISterm, the virtual
universe is created with the help of the 3D-Service, which
uses the means of Java 3D. This virtual universe includes
common functions and properties needed for every 3D-GIS
application:
e Management of semantically grouped object
content in different 3D layers.
e Means for orientation and navigation.
e Possibility for changing common scene properties,
like the disbanding of the height axis, the change of
the lighting or the background colour.
For the visualisation of the digital elevation models we need
a 3D Layer, holding the terrain object generated from the
DEM data out of the database. This is described in the next
section.
3.2 Creating Three-Dimensional Object Content
For visualising the DEM data in the virtual universe, a 3D
Layer called DEMLayer3D is created. This Layer holds the
actual 3D object content. The DEMLayer3D is added into
the virtual scene created by the 3D-Service, which visualises
the terrain in the 3D-viewer component.
3.2.1 Structure of Terrain Object
This part of the paper describes the structure of this terrain
object. Due to texture size issues explained deeply in the next
section the terrain objects need to be divided into several
parts. The number of terrain parts depends on the size of the
texture parts. By dividing the terrain size by the texture size
the number of needed terrain parts is calculated. The size of
the terrain parts corresponds with the size of the texture parts.
The arrangement of the terrain parts starts in the lower left
corner increasing into east and north direction. The terrain
parts at the upper north and east border are smaller than the
texture size. The following image shows the arrangement of
the terrain parts.
Fig. 2: Arrangement of terrain parts
