The file must be a GDB-supported type and contain at least one DEM channel (data to process). The file containing the channel to convert. In the following table, the key aspects of converting a DEM datum are summarized. To convert between ellipsoid and MSL, OrthoEngine calculates the difference between the geoid and the ellipsoid at the point in question, and then applies the difference to compute the transformed elevation. Therefore, you must ensure that the elevation reference of the DEM matches that of the imagery. The difference between elevations relative to the ellipsoid and those relative to the geoid can be significant in some areas, up to 107 meters. Orbit information is always referenced to an ellipsoid, and the ellipsoid number is taken from the projection information in the file.Ī DEM extracted from satellite imagery using the preceding math models is based on an ellipsoidal model of the Earth, not the geoid. With OrthoEngine, there are two math models that are based on orbital information instead of GCPs: the radar-specific model, and the rational-functions model when used with the IKONOS GEO Ortho Kit product, CARTOSAT, KOMPSAT-2, KOMPSAT-3, PALSAR, PRISM, QuickBird, OrbView, and WorldView imagery. With most math models, the model is based on ground control points (GCP), which are also based on the geoid. Elevation values can be referenced to a number of different surfaces, but with mapping operations, you typically use elevations above mean sea level (MSL), which is based on the geoid.

The elevation reference in the digital elevation model (DEM) must match that of the imagery you want to orthorectify.

Vertical datum conversion