Their ETRS-89 coordinates
were measured by differential positioning in the static mode (baselines
of 0.5 to 1.5 km) using precise geodetic single-frequency GPS
equipment and processed with high accuracy. Their final accuracy was
better than 0.1 m (xyz) after transformation of ETRS coordinates to
National geodetic datum (S-JTSK). The crucial condition for the
selection of GCPs is their stability since the image capture time. For
obvious reasons this condition can be violated for some points.
Therefore the identification of such misleading GCPs and their
exclusion from the triangulation process is essential.s