remaining 10 served as check points. For the second model, just four GCPs, not enclosing all remaining 34 points, were used. The four GCPs had height 2–3 m and one 34 m, thus, extrapolation errors both in planimetry and height could occur.Starting from the regular DEM, the nodes weretransformed by one of the two models into pixel space and were used as anchor points to bilinearly interpolate the pixel coordinates of the remaining orthophoto pixels (the orthophoto had a pixel spacing of 1 m). Then, the grey values were interpolated by a bilinear interpolation. The orthophoto was generated for each spectral channel separately with own software, usingthe same parameters, except the input image of course(see Fig. 5).