Describe the differences and relationship between a Digital Terrain Model (DTM) and a Digital Elevation Model (DEM)
Detailed spatial information on terrain, one of key geographical factors (if not the most crucial), is fundamental for diverse geographic analysis and simulation. Such information includes not only the basic information of elevation but also the derivations for depicting terrain characteristics from diverse perspectives, so called terrain metrics. In GIS, the terrain metrics are recorded in digital terrain models (DTMs, the representative of data models in GIS), being represented as continuous field or discrete objects. They are derived by digital terrain analysis (a representative subdomain of spatial analysis) on the DTM recording elevation (or saying, digital elevation model, short as DEM) and/or other DTMs. Diverse terrain metrics have been proposed, including the topographic attributes typically as contuous field or discrete objects. They could also be classified as those for characterizing terrain at the geometric, structural, or process level, respectively. Corresponding research on DTA has been conducted on how to depict terrain characteristics with higher accuracy, higher efficiency, and/or easier-to-use for diverse applications. This entry introduces these concepts as well as the remarks and issues within research and applications.