Automatic generation of elevation data over Danish landscape

Automatic generation of elevation data
A terrain model reproduces the ground surface without objects such as houses and trees, as illustrated in
figure 1.1 with the green line. Terrain models are, among other things, used as grid models for orthophoto
rectification, flood analysis, and volume estimation of soil.
A surface model, on the other hand, also describes the elevation of objects, such as houses, trees etc.
That is to say, a surface model includes terrain as well as objects. In figure 1.1., the surface model is indicated
with the red line. Surface models are used for 3D animation, trace optimisation and true orthophoto
rectification, among other applications.
1.4 Standards for Elevation models
Users have required new standards for elevation data based on their uses and applications. These new
demands fall into the following three categories:
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� Accuracy
� Density
� Currentness
1.4.1 Accuracy
Different users of elevation models have different demands for how accurate the elevation model has to
be. Users in the building construction industry and environmental authorities demand an accuracy on the
centimetre level.
Users such as 3D animators or 3D graphicers do not require an accuracy on the centimetre level, but can
still use an elevation model with a high accuracy.
1.4.2 Density
Accuracy on its own is actually not good enough if the mesh size is too large. What good will it be if, at
one point on the mesh, the accuracy is within 10 centimetres and the next point, which is maybe 50 metres
away, also has an accuracy within 10 centimetres, but there is no description of the terrain between
the mesh points. A grid with a small mesh size represents the terrain better than a grid with a large mesh
size. An elevation model described with high accuracy also needs to contain a small mesh size.
1.4.3 Currentness
Elevation models and geo-spatial data in general are only snapshots or a status of the landscape or area
they represent. As the landscape changes over time, the elevation model describing the area becomes
outdated, corresponding to the degree of change. This yields unreliable data in the elevation model.
As times goes by, the elevation model slowly degenerates and becomes more and more unreliable, and
in analyses where time is a crucial parameter, outdated information will have an effect on the accuracy
and reliability of the results obtained from the analysis.
1.5 Danish digital elevation models
In Denmark there are two national digital elevation models in existence; an old one within the framework
of the National Survey and Cadastre (25 x 25 m) (KMS DTM), and one within the framework of the firm
COWI A/S (2 x 2 m) Danish Digital Height model (DDH). A new, national elevation model within the
framework of the National Survey and Cadastre is in production and will be available in the very near future.
1.5.1 KMS DTM
In connection with the production of the national vector atlas TOP10DK in the beginning of the 1990s, the
National Survey and Cadastre wanted to add an elevation model. It was decided to produce a new digital
elevation model on the basis of the 2.5m contours from the topographic maps in scale 1:25,000 which
were scanned and vectored. To achieve an even greater accuracy, the 2.5m contours have been combined
with elevation data from different themes from TOP10DK. Examination of the 2.5m contours shows
that the accuracy lies in the interval from 1m to 1.5m with a few gross errors up to 2.3 m, in spite of the