Automatic generation of elevation data over Danish landscape

3 Experience and investigation strategy
3 Experience and investigation strategy
The purpose of this chapter is to establish a strategy for the investigation of Match-T under Danish conditions.
In order to profit from the experience gained from other investigations of Match-T, such investigations
and their results are presented. On the basis of this source study, problems and divergent results/statements
will be discussed, the problems stressed and listed as ”keywords”.
In the source study, the terminology of the source has been used, DEM for Digital Elevation Model and
DTM for Digital Terrain Model.
3.1 Background
Automatic generation of elevations from digital images has for several years been included as a fixed
work routine at the photogrammetric firms in Denmark. In this project, a further investigation of the possibilities
and limitations of this method is undertaken, taking the programme package Match-T as point of
departure.
For more than 15 years, a series of investigations has been made into the accuracy, completeness and
landscape types for automatically generated elevations from digital images. Moreover, OEEPE has conducted
a workshop, where the results of automatically generated elevations were presented. On the
background of the lessons learned here, areas relevant to a Danish use of Match-T, as regards problems,
accuracy versus scale, pixel size, sources of error etc., are estimated. For each source, some central
themes will be noted as keywords. The sources are discussed chronologically.
3.2 A source study of experiences with Match-T
In an article about Match-T from 1991 [Krzystek, 1991], 5 tests are presented. These show that with
15 μm images, an accuracy of at least 0.08‰ of the flight altitude can be achieved. Using images with a
resolution of 30 μm, an accuracy of 0.1‰ can be achieved. A single one of the five tests gives a poorer
result. This is due to the images’ content of areas with poor texture. This problem is of immediate importance,
both for the automatic and the manual determination of elevations.
Keywords: accuracy, resolution, texture.
In another article [Krzystek et al., 1992], the accuracies of Match-T and control points measured with
analogous photogrammetry respectively are compared. This investigation takes as its point of departure
the fact that an automatically generated elevation model should have an accuracy of 0.1‰ of the flight
altitude. This requires Match-T to automatically find and destroy gross errors which are caused by correlation
errors. Moreover, Match-T is expected to eliminate single objects such as trees and houses, and
handle areas with a low or insufficient degree of information (none or few interest points). These three
problems are handled by Match-T with the same mathematical method, see also Chapter 2, section
2.4.3.2. The investigation also discusses what influence the distance between grid points, that is, the
mesh size, has on the result. The smaller the mesh size, the fewer interest points for determination of the
grid point. Equally, a large mesh size means a greater number of interest points to determine a grid point.
The investigation uses 15 μm and 30 μm images in three different scales (1:7,000 – 1:14,000). The conclusion
is obvious: large mesh size, and subsequently, more interest points, give a better result in flat areas.
In the case of a steeper terrain, a grid with a large mesh size will reproduce the terrain in considerably
less detail, and imply a reduction in accuracy. The accuracy with mesh sizes of over approx. 5m is
under 0.1‰ of the flight altitude, regardless of scale or resolution, while the accuracy for 15 μm images is
considerably better. The investigation shows that as a whole, Match-T can produce a DTM of high accuracy,
and subsequently, handle areas with a low degree of information.
Keywords: accuracy, scale, resolution, mesh size, terrain type.
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