Automatic generation of elevation data over Danish landscape

3 Experience and investigation strategy
Scale
Opinions also differ about the influence of scale. [Krzystek et al., 1992] shows that the same accuracy
can be achieved by different scales, regardless of which scale is in question. However, this is only valid
for images in medium or small scale. As regards large scale, it is stated that the landscape type gives
problems. [Schenk, 1996]. It can also be seen from the OEEPE presentations that an accuracy of 0.1‰ of
the flight altitude cannot always be counted on.
Mesh size
The sources disagree on the influence of mesh size on the accuracy. [Krzystek et al., 1992] mentions that
the mesh size has an influence; the greater the mesh size, the better the result, whereas [Catalunya,
Brussels and the undersigned OEEPE, 1998] found no such influence.
Landscape type
The sources agree that landscape types pose a fundamental problem. A generation only succeeds in
open areas with solitary objects which Match-T can eliminate on its own [Ackermann et al., 1992; a)
Krzystek et al., 1995]. In addition, the investigations show [Ackermann, 1992; a) Krzystek et al. 1995] that
housing estates appear promising. The investigation [Seyfert, 1995] shows clearly that the landscape
type has an influence. Here, it is shown that built-up areas give three times as poor results as open fields.
Likewise, [Thorpe et al., 1996] have shown that generation over built-up areas gives a poor result. In addition,
the presentations from OEEPE show that there are problems with vegetation and buildings.
Terrain type
The sources agree that the terrain type has an influence. According to [Ackermann et al., 1992; Ackermann
1992; Eide et al., 1993; a) Krzystek et al., 1995], an accuracy of better than 0.1‰ of the flight altitude
can be achieved over a flat to slightly hilly landscape with solitary objects, as Match-T according to
[Krzystek, 1991; Ackermann 1992; a) Krzystek et al., 1995; b) Krzystek et al., 1995] can handle these objects.
[Ackermann,1996] mentions that photogrammetric DEM generation over open terrain will run into
problems. Opinions differ about what accuracy can be achieved over, for instance, mountainous terrain.
In the investigation by [Eide et al., 1993], it is shown that in mountainous terrain, an accuracy of 0.55‰ of
the flight altitude can be achieved, while, for instance [a) Krzystek et al., 1995], show that an accuracy of
between 0.2‰ and 0.35‰ can be achieved.
Texture
The sources agree that poor texture has a negative influence on the accuracy. Several sources [Krzystek
1991; b) Krzystek et al., 1995; Inpho (OEEPE), 1998] mention that there are problems with areas with
poor texture, and thus lack of interest points for generation of grid points.
Summation
As seen from the above, there is divergent information about, for instance, the influence of pixel size,
scale and mesh size, or the impact of landscape type and texture. This divergent information forms the
basis for the choice of topic for the investigation with Match-T under Danish conditions with regard to a
nationwide elevation model.
3.3 The investigation strategy of this project
On the basis of finding out how good a result can be achieved by using Match-T, the lessons learned
from various investigations and the workshop in Stuttgart, a series of investigations have been set up with
relevance to a Danish terrain. The investigations are chosen with regard to optimising the accuracy, and
at the same time minimising the data quantity and editing work. The data quantity is evaluated in relation
to the scale and resolution (pixel size) of the aerial photos. The editing is evaluated, among other things,
on the basis of the influence of landscape type on the accuracy. Furthermore, a closer analysis of the impact
of the mesh size has been included.
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