2. PROGRAMMING SYSTEM
Programming language is standard FORTRAN 77, the program system is
being developed and implemented on PRIME minicomputers 550 and 750.
However, an optimized version has also been produced to enable parts
of the program with a high computational load to run on the CYBER 205
vector computer.
2.1 Epipolar geometry
After orientation of the images by image correlation or conventional
methods, then the position of points in the stereo partner,
corresponding to a selected point, can only lie on a well defined line
- the epipolar line. Because of this, the search for corresponding
points is greatly accelerated. Further, the possibility of gross
errors, due to correlation between similar but not identical points,
is reduced, since the search area is restricted to this line. «This
considerably increases the reliability and has the advantage that more
points can be used. For instance, edges with components normal to the
epipolar line can be used. If the search were not then restricted to
this line, false correlations (gross errors) would be obtained at
other points along the edge.
To make full use of the epipolar geometry and simplify the procedure,
both photographs are rectified over the stereoscopic area to be
evaluated. This makes all epipolar lines parallel. If this direction
is taken as the x-direction, then all corresponding points have the
same y-value /10/.
2.2 Grid generation
The grid points used for the DTM are generated by a process of point
densification. The technique starts with a very coarse grid, which
gets finer until the required point density is reached. This is done
in a hierarchy of 3 stages. For the coarse grids a low-pass filter is
used to create an appropriate coarse image. At this level, accuracy
of point identification is low. However, as the grid gets finer the
photo resolution increases and the accuracy improves. This technique
differs from the "progressive sampling" method used by Makarovic /8/,
which densifies the points only in certain areas. At each grid level
the search process described below can be used. Care must be taken to
avoid gross errors at an early stage, since this will obviously effect
the following stages. To assist in this, a visual: control is
supported by the program.
2.3 Search process
In finding suitable points 4 steps in the search process have been
identified. At the end of the search process it would be possible to
interpolate points again in a regular grid. However, this technique
is not used here and is not further discussed.
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