relief-corrected affine transformation (for the rationale for using this model and other details, see Baltsavias etal., 2001). Firstly, the used GCP object coordinates are reduced to a reference height level (the height selection is irrelevant, but here a height of 0 m was used). Then,an affine transformation between these reduced GCPs and their pixel coordinates is estimated by least squares. A minimum of three GCPs is required, whereby their spatial distribution can be relaxed, and extrapolation does not lead to errors. This model has been used with several IKONOS Geo images for orthorectification (Baltsavias et al., 2001). The second model compared to the first one has several advantages:computation time is lower, less GCPs with not-sostringent spatial distribution criteria are needed, and extrapolation errors do not occur. Thus, it was even anticipated that the second model would lead to higher orthophoto accuracy.
The first model was estimated once with all 38 GCPs and a second time with 28 GCPs, while the