Products : Digital Terrain Model Extraction Lines of relief such as sloping embankments, works of art, river banks or even railway tracks are systematically lifted and integrated into the model. The D.T.M., generated from aerial exposures, is a digital presentation of the altitude of a geographical zone integrating parameters of slope and aspect. The views in 3-D are made possible because for each place defined by its co-ordinates X,Y, the value of the altitude Z is presented. Used with the digital aerial photographs, the D.T.M. plays an integral part in the production of orthophotoplans. The precision of the orthophotoplans therefore depends directly on the quality achieved by the D.T.M.