Automatic generation of elevation data over Danish landscape

3 Experience and investigation strategy
the flight altitude with a small improvement using 15 μm images. The problem areas are forest edges,
built-up areas and areas with dense vegetation of bushes and trees, while the open country needs very
little editing. Resolution and mesh size have little or no influence on the accuracy.
Keywords: accuracy, resolution, mesh size, landscape type.
3.2.1.5 Investigation, Aalborg University
In the investigation done at Aalborg University, images with the resolution 15 μm, 30 μm and 60 μm, and
mesh sizes of 5 x 5 m, 12.5 x 12.5m and 25 x 25m were used. The terrain type was estimated from the
elevation difference between the control points which was ~ 250m. This elevation difference was estimated
to be neither flat nor mountainous, so the terrain type ”hilly” was chosen, which meant that the shift
by the correlation could only take place over 8 pixels. The result for the July images was an accuracy of
1m - 2m. In addition, the result showed that neither the resolution of the images nor the mesh size had
any great influence on the accuracy. A closer analysis of the individual landscape types showed that
problems arose in built-up or overgrown (forest edges) areas, while the open country and fields had an
accuracy < 1m (< 0.5‰ of the flight altitude). In the October images, the accuracy was significantly improved
by ~ 0.5m over the July images. Specifically, the result was better in the overgrown areas, as the
automatic generation here comes closer to the terrain because of the sparse vegetation in the autumn.
Conclusion: Neither the resolution of the images nor the mesh size have any great influence on the accuracy.
A better result is achieved by using images with less leaf cover, that is, October images rather than
July images.
Keywords: accuracy, resolution, mesh size, landscape type, season.
3.2.2 Summation of the investigations from the OEEPE workshop
A common result of the investigations is that image resolution has little influence on accuracy. There are
problems in areas with vegetation and buildings, while open fields give the best results. A significantly
better result is achieved by using October images rather than July images, as the leaf cover is less in October.
If the results of the five investigations are compared, it should be noted that Inpho’s investigation is
divided by �2. Even if Inpho’s results are multiplied by �2, they are still the best at ~ 0.35m – 0.7 m,
whereas the remaining participants get results of between 0.7m and 2m which corresponds to 0.36‰ –
1.02‰ of the flight altitude.
3.2.3 Summation of sources and OEEPE workshop
A comparison of the results, from the article sources and the results from the OEEPE investigations leads
to the conclusion that there is some divergence. In no way are the same good results achieved in the
source material as in the OEEPE investigations. For instance, the accuracy over open, flat terrain varies
from 0.04‰ – 0.07‰ of the flight altitude in [Ackermann et al., 1992], to 0.7‰ of the flight altitude in [Seyfert,
1995]. However, several sources show that an accuracy around 0.1‰ of the flight altitude is realistic
[Eide et al., 1993; Brussels, OEEPE, 1998].
Common to several of the articles and the OEEPE results is that the pixel size is estimated to have little
or no influence on the accuracy of images in resolution 15 μm and 30 μm, while 60 μm images according
to [Eide et al., 1993] and Brussels give a poorer result. However, the OEEPE investigation from Aalborg
university shows that even 60 μm images give the same result as images in resolution 15 μm and 30 μm.
The results of all the sources discussed are collated in the following table. For each keyword an indication
is made, whether it has an influence (+) or not ( / ). In cases where the conclusion is ambiguous, this is
marked (?).
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