INVESTIGATIONS OF DIFFERENT INTEREST OPERATORS FOR DTM-GENERATION
BY EPIPOLAR LINE CORRELATION
Johannes Piechel
Universität Karlsruhe, Inst. f. Photogrammetrie u. Topographie
7500 Karlsruhe, W.-Germany
ABSTRACT
A regular grid pattern for DTM-generation may not produce the most
ideal points for manual or automatic measurement. The paper
describes, and compares, techniques which produce more suitable points
In the region of the original grid points. Even simple methods select
DTM-points which are better defined and correlated than the original
ones. It has been found that the best results are produced by
maximizing grey level differences in the search area.
1. INTRODUCTION
The objective of the work described in this paper is the production of
high-quality digital terrain models (DTM's) from stereo pairs of
aerial photographs. A fast method of calculation is sought which
produces reliable and accurate heights. This will be achieved by an
off-line program which is currently under development. The program
uses techniques of automatic correlation and is based on the
DISC-system of Claus /4/. This takes full advantage of epipolar
geometry, but has been greatly extended and modified. These
extensions include: two-dimensional windows, hierarchical point
densification, least squares adjustment, visual controls, and interest
operators.
The interest operators are functions used for finding suitable
DTM-points, and it is these which are the subject of the current
paper. Starting from a regular grid in the image space, many points
are not suitable for measurement, due to a lack of structure within
the window. Consequently, the window is shifted until the interest
operator indicates that suitable detail is located near the center of
the window.
The following points should be noted:
1) It would be possible to use the interest operator to accept or
reject a raster point, but the current proposal uses the optimizing
method.
2) In contrast to other methods /2,3,6,9/ the position of the
feature is not determined within the window, but is arranged to be
close at the center.
3) No attempt is made to determine if the feature has topographical
significance.
4) In using the interest operator to find a new feature close to the
initial raster point, a search is made in only one direction. This
direction is the same as that used to find the corresponding point
in the stereo partner (the epipolar direction as explained below).
AS a result the final set of DTM-points remains semiregular.
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