The final coordinate mapping is derived by adding the individual geometric corrections in a vectorial fashion. Consequently, the transformation from raw image space to geometrically rectified space necessitates the use of a sophisticated resampling algorithm because the raw image pixels must be resampled from the irregular grid resulting from the vector mapping to a regular grid to produce the SI. This type of two-dimensional interpolation is mathematically challenging and particularly difficult to solve satisfactorily. The intensity values (FNs) in the LI are resampled to the geometrically corrected space of the SI using a modified Shepard method, an algorithm found in the NAG (1990) library. The interpolant for the modified Shepard method is continuous and also possesses first derivative continuity. The method is local in that the output flux value is dependent only upon data in a surrounding neighborhood.