Corrections are made at level of reference and parallax effects are not removed. Level 3 is achieved using a DEM, for instance derived from SPOT stereopairs and ground control points to produce an orthoimage map. Several softwares are already available and many more are in the experimental or implementation phase. The technique involves the matching of a pair of images in order to determine parallax difference from which a DEM is constructed. Orbital information, ground control points and geometric model are needed.
Image correlation is based on feature points or edges structured along epipolar lines.
The satellite imagemaps constitute a completely new type of low-budget map. They are based on the use of precision corrected, edge enhanced single images.