The comparison was based on selected images in all flights
which, as taken at different times, show differences in surface
colours. The requested thresholds by the algorithms were
empirically derived and then applied constant for all examined
images. The found tie points are marked with blue circles,
where the size is representative for the scale (cf. Figure 7).
The algorithm SURF was regarded as most suitable for this
purpose. The number of extracted point features was high
(Figure 8) and well distributed over the whole image.