A procedure automatically resolves vertical differences commonly found when merging and combining Digital Elevation Models (DEMs) from different collection and production systems. These systems may employ technologies used in single pass Interferometric Synthetic Aperture Radar (IFSAR), LIght Detection And Ranging (LIDAR), and photogrammetry, as well as DEMs derived from contour based elevation data and GPS point data. This procedure employs sophisticated software checking routines for automatically identifying horizontal and vertical datums used by the differing systems as well as any geoid models employed by them. Normally, all of these sources use different vertical datums and may use different horizontal datums. A preferred embodiment of the present invention automatically recognizes the vertical datums, including those that are associated with the geoid models and the 3-D datums that conventional GIS does not support at present.