Does Reference3D include a DEM or a DTM? By the way, what is exactly the difference between a DEM and a DTM?
Usually, the acronym DTM refers to the altitude of the ground itself : a DTM would consider the soil under the buildings, and around the feet of the trees. On the contrary, a DEM includes the maximum altitude everywhere : roofs of the buildings, top of the trees (often known as top of the canopy). The processing of stereoscopic images (space or aerial) naturally produces a DEM, because neither the operator nor the matching software can see the ground itself through the roofs and leaves. If a DTM is really requested, a very heavy correction process has to be undergone, to rub out the trees and buildings, and by no way one can guaranty the accuracy of the result without field checking. For these economical and accuracy reasons, and also because the elevation information has its own value (eg for airplanes), Reference3D includes a DEM, not a DTM.
