Digital elevation model

A digital elevation model (DEM) is a 3D computer representation of elevation data, used to depict terrain or objects on top of it, such as on a planet, moon, or asteroid. When covering the entire globe, it is called a global DEM and is built on a discrete global grid. These models are a key tool in geographic information systems (GIS) and form the basis for most digital relief maps.

Terminology

There are two related terms: digital surface model (DSM) and digital terrain model (DTM). A DSM includes all surface features like tree canopies and building roofs, while a DTM shows only the bare ground. The term DEM is often used broadly to cover both DSMs and DTMs, though usage varies in scientific literature.

Some sources treat DEM and DTM as synonyms, others as distinct, and many data providers (such as USGS) use DEM as a catch-all. For instance, datasets like SRTM are originally DSMs, but in forests they record readings between a DSM and a DTM. DTMs are typically derived from high-resolution DSMs by filtering out objects like buildings—a process called bare-earth extraction.

Types

DEMs come in two main types: raster grids (also called heightmaps) and vector-based triangular irregular networks (TINs). A TIN is considered a primary, measured DEM, while a raster is a secondary, computed one. They are created using techniques such as photogrammetry, lidar, InSAR, land surveying, or remote sensing.

Rendering

The data itself is a matrix of numbers, but it is usually visualized to be understood. Common visualizations include contoured topographic maps, false-color maps (e.g., green for low elevations, red for high), and oblique views that simulate looking at the terrain from an angle. In oblique views, vertical exaggeration is sometimes used to highlight subtle differences, though some scientists argue this can mislead viewers.

Production

Production often relies on remote sensing rather than direct survey. Older methods interpolate digital contour maps from ground surveys, a technique still used in mountainous areas where interferometry is less effective. Note that contour lines or sampled elevation points are not DEMs, though they may be considered DTMs; a DEM provides continuous elevation across an area.

Satellite mapping is a powerful method. Interferometric synthetic aperture radar (InSAR) uses two passes of a radar satellite (or a single pass with two antennas, as in SRTM) to generate elevation maps tens of kilometers wide with about ten-meter resolution.

Stereoscopic pairs from optical images, using digital image correlation, also work. The SPOT 1 satellite provided the first large-scale usable elevation data via two-pass stereoscopic correlation, followed by ERS, SRTM, and ASTER. SPOT 5’s HRS instrument has acquired over 100 million square kilometers of stereo pairs.

In planetary science, orbital altimetry—both laser and radar—is used to create digital elevation maps of other worlds. Examples include the Mars Orbiter Laser Altimeter (MOLA) for Mars, and the Lunar Orbital Laser Altimeter (LOLA) and Lunar Altimeter (LALT) for the Moon.