Automatic generation of elevation data over Danish landscape

Automatic generation of elevation data
2.4.3 Parameters for DEM generation
2.4.3.1 Correlation
The starting point of a correlation done in Match-T is a FBM and, as in Socet Set, a combination of ABM
and FBM is also used here. Although the algorithms used in Match-T have been in use for more 15 years,
there are no alternatives in order to derive DTM’s efficiently for large areas [Heuchel, 2005].
The correlation and the DEM generation is done stepwise down through the levels of the pyramid [Inpho
a), 1995].
The correlation of the interest points presupposes that the following information is given:
30
� A temporary indication of the terrain elevation as starting value
� The epipolar geometry
� Delimitation in the row-direction by parallax bound
and the correlation is done on the basis of the following values:
� The existence of a gradient
� The size of the gradient
� The correlation coefficient
The correlation is only done in the x-direction, as the images are normalised. The movement in the xdirection
is delimited by the size of the parallax bound. In Match-T, the parallax bound is changed by
choice of the landscape type: flat = 3 pixels, hilly = 8 pixels and mountainous = 15 pixels. By choice of, for
instance, a mountainous terrain, the fitting in of the correlation can therefore be moved 15 pixels in the xdirection.
When the best fit for the correlation is found, taking into consideration the existence of gradients,
and the size of these in the two images, a correlation coefficient is calculated, see appendix A, section
A.1.2.2.1. The position of the correlation is where the greatest correlation coefficient can be obtained.
By means of the pyramid levels and an approximated starting value for the terrain elevation, a quick and,
in principle, independent pre-knowledge of the terrain can be deduced from the preceding pyramid level.
2.4.3.2 The DEM generation
The result of the introductory correlation is a cloud of points on each pyramid level. These clouds of
points include gross errors, such as correlation errors, or elevation errors, created by houses, trees, etc.
The DEM generation is done stepwise down through the pyramid levels, and the terrain surface is reconstructed
by means of the finite element principle. The results for each pyramid level are used as temporary
values for the calculation of the next pyramid level, see figure 2.9. The result of a correlation and
DEM generation can be seen in figure 2.10.