DTM

A Digital Terrain Model (DTM) represents the bare-earth surface of the terrain — with all vegetation, buildings, and other above-ground objects removed — but goes further than a basic DEM grid by incorporating supplementary vector features that describe the shape of the landscape more precisely. These features include breaklines (sharp edges like ridgelines, riverbanks, and road cuts), spot heights (surveyed elevation points), mass points, and contour lines. A DTM preserves the critical morphological features of the terrain that a simple raster grid might smooth over or miss entirely.

In practice, the terms DEM and DTM are frequently used interchangeably, and their definitions vary by country and organization. In the United States, "DEM" is the dominant term for bare-earth models. In Europe — particularly in the French, German, and Scandinavian traditions — "DTM" is preferred and explicitly implies a model that combines grid data with vector terrain features. The International Society for Photogrammetry and Remote Sensing (ISPRS) defines DTM as the most complete representation of the bare-earth surface, with DEM as the raster-only subset. Regardless of naming conventions, the key distinction from a DSM remains the same: DTMs and DEMs represent bare earth; DSMs represent the visible surface including objects.

DTM creation typically begins with a point cloud from LiDAR or photogrammetry. First, points are classified as ground or non-ground using algorithms that identify terrain returns by their position relative to neighbors — low points on locally smooth surfaces are classified as ground, while elevated outliers (trees, buildings) are filtered out. The filtered ground points form the foundation of the DTM.

Next, breaklines are manually digitized or automatically extracted from the point cloud to capture critical terrain features: stream channels, road edges, retaining walls, embankments, and ridgelines. These breaklines enforce linear features in the surface model — without them, a purely interpolated grid would smooth over sharp terrain changes, producing unrealistic drainage patterns or artificially rounded road cuts.

The final DTM is constructed by interpolating a surface through the ground points and breaklines, typically using a Triangulated Irregular Network (TIN) that is then converted to a regular grid. The TIN honors the breaklines as hard edges, producing a surface that correctly represents both gradual slopes and abrupt terrain changes. Quality DTMs also incorporate hydro-enforcement — ensuring that water flows correctly through the model by lowering stream channels and removing artificial dams caused by road crossings.

DTM creation is significantly more labor-intensive than DEM production because breakline digitization typically requires manual editing or at minimum manual quality control. Automated breakline extraction from point clouds is improving but still produces errors at complex terrain features. The distinction between DTM and DEM is inconsistent across organizations — always verify whether a product includes breaklines and hydro-enforcement before using it for precision applications. DTMs inherit the accuracy limitations of their source data: LiDAR-derived DTMs achieve centimeter accuracy, while photogrammetric DTMs in forested areas may have meter-scale errors because ground points are sparse under canopy. Temporal currency is a concern — terrain changes from construction, mining, or erosion require updated surveys.

The term "Digital Terrain Model" was coined by MIT engineers in the late 1950s as part of early computer-aided highway design research. The concept predates DEMs — the original vision was always a terrain representation that included structural features, not just a grid of heights. As computing advanced, the simpler raster DEM became dominant for large-area applications, while DTMs remained the standard in surveying, civil engineering, and national mapping agencies, particularly in Europe. France's Institut National de l'Information Géographique et Forestière (IGN) has produced DTMs with breaklines as part of its RGE ALTI product since the 2000s. The US 3D Elevation Program (3DEP) produces both raster DEMs and TIN-based DTMs with hydro-enforcement from nationwide LiDAR coverage.