Constructing Topographic maps from radar images - an application of
min cost flow algorithms 

Jens Clausen and Jesper Larsen, IMM, DTU -
joint work with Soeren Noervang Madsen and Johan Mohr, EMI, DTU.

Using a technique called SAR interferometry it is possible to
construct an image of a region, in which the value of each pixel
corresponds to it's height modulo an integer multiple of two pi. This
is called the wrapped phase of the pixel. Under the assumptions that
the surface of the region is smooth, that no noise appears, and that
neighboring pixels have heights not differing substantially, it is
relatively simple to construct the true height of each point in the
region. The true height is called the unwrapped phase and the process
is called phase unwrapping. These assumptions are, however, seldomly
satisfied in practice. To overcome the difficulties arising, a
non-linear optimization model can be applied. The model can be
transformed to a very large Min Cost Flow model. In the talk we
present the problem, discuss various modifications and the
computational behaviour of the Min Cost Flow algorithm applied, and
show an example of a map generated using the technique.