DTM, DEM and 3D models

Choosing the right model

A DTM (digital terrain model) is the bare ground. A DSM (digital surface model) includes what stands on it: trees, houses, walls. DEM (digital elevation model) is the general name for elevation models, so it is always worth stating which surface a file represents. A textured 3D mesh is something else: it describes the visible shape of an object and has its own measurement uses.

Over large areas, such as a solar farm, a power line, an industrial estate or a road or railway alignment, the DTM is the basis for everything else: contour lines, cut and fill volumes for levelling, shadows and drainage networks.

When you need these products

• To extract terrain profiles and cross-sections.
• To study finished levels, excavations, fills and access roads, also on hillside plots.
• As the basis for slope, visibility, shadow or drainage analyses.
• To bring the terrain or a measured object into 3D design software.

What we do in the field and in the office

If measuring is needed, we take points and breaklines (toes and crests of slopes, terrace edges, ditches), or fly with a drone or LiDAR depending on the terrain. We then clean the data, build the surface, review the gaps and check that it is consistent. If we start from existing data, we first look at its date, its resolution and its reference system.