Height Data - Terrain (DTM) vs Surface (DSM) Models
Digital Surface Models (DSM's)
DSM's measure the height values of the first surface on the ground. This includes terrain features, buildings, vegetaion and power lines etc. They therefore provides a topographic model of the earth's surface
Digital Terrain Models (DTM's)
DTM's provide a topographic model of the bare earth / underlying terrain of the earth's surface. They are usually derived from DSM's by digitally removing the cultural (man-made) and vegetation features described above.
As a result of these processes, the stated accuracy of a DTM will usually be slightly lower than that of the DSM from which it is derived, paricularly in area of continuous and dense building and tree coverage.