What is a DEM (Digital Elevation Model)?

A DEM is a raster grid — a regular matrix of cells — where each cell stores a single ground elevation value. Think of it as a photograph of the terrain where, instead of a colour, every pixel holds a height above a reference datum.

Because the grid is regular and georeferenced, you can read the elevation at any location, and derive slope, drainage, contours, volumes and much more from it.

DEMs come from several sources: LiDAR point clouds, photogrammetry from aerial or drone imagery, radar and satellite data, or traditional survey. In each case the raw measurements are turned into a regular elevation grid.

Drone surveys are now a very common source: images are processed into a point cloud, and the ground points are gridded into a DEM covering the site at high resolution.

"DEM" is often used as a general term for an elevation grid. In practice it is useful to distinguish a DSM (Digital Surface Model), which includes buildings and vegetation, from a DTM (Digital Terrain Model), which is bare earth with objects removed.

Which one you want depends on the task: earthworks and drainage need the bare-earth DTM, while volumes of piles or line-of-sight work may need the surface DSM.

A DEM is the base layer for a huge range of work: measuring distances and areas that follow the ground, computing cut/fill and stockpile volumes, cutting cross-sections, generating contour lines, and running slope, hydrology, viewshed and sun-and-shadow analysis.

The practical challenge is size. A modern DEM of a real site can be hundreds of megabytes or several gigabytes, which is why software that streams the data and computes on it efficiently matters.