Digital elevation model

A digital elevation model (DEM) or digital surface model (DSM) is a 3D computer representation of height data, typically used to show the terrain and objects on it, such as those found on a planet, moon, or asteroid. When a DEM covers the entire globe, it is called a discrete global grid. These models are widely used in geographic information systems (GIS) and form the basis for most digital relief maps. A digital terrain model (DTM) differs in that it shows only the bare ground, while DEMs and DSMs can include features like treetops or building roofs.

Terminology

The terms DEM, DTM, and DSM are not used consistently across scientific literature. Generally, a DSM includes the Earth's surface and everything on it, whereas a DTM shows only the ground, free of plants and structures. Many use DEM as a catch-all term for both DSMs and DTMs, simply indicating height information without specifying surface type. Some sources treat DEM and DTM as synonyms, others equate DEM with DSM, and still others define DEM as a subset of DTM or as a rectangular grid versus a TIN-based 3D model.

Major data providers like USGS, ERSDAC, CGIAR, and Spot Image treat DEM as a generic term. Datasets such as SRTM or ASTER GDEM are originally DSMs, though SRTM in forests captures readings between a DSM and a DTM. DTMs are derived from high-resolution DSMs by filtering out buildings and other objects, a process called bare-earth extraction. In this entry, DEM is used as a general term for DSMs and DTMs.

Types

DEMs come in two main types: raster (a grid of squares, also called a heightmap) and vector-based triangular irregular network (TIN). The TIN version is considered a primary (measured) DEM, while the raster version is a secondary (computed) DEM. They can be created through methods like photogrammetry, lidar, IfSAR, InSAR, or land surveying. Remote sensing is a common data source, but land surveying is also used.

Rendering

The DEM itself is a matrix of numbers, but it is often turned into visual formats for human understanding. This can be a contoured topographic map, or it may use shading and false color—such as green for low elevations, red for mid-range, and white for high—to represent height. Visualizations may also be oblique views that simulate looking down at an angle, sometimes with vertical exaggeration to highlight subtle elevation changes, though some scientists argue this misleads viewers about the actual landscape.

Production

Mappers produce DEMs through various methods, frequently relying on remote sensing rather than direct survey. Older techniques involve interpolating digital contour maps from land surveys, a method still used in mountainous areas where interferometry struggles. Contour data or other sampled elevation datasets (from GPS or ground surveys) are not DEMs themselves but can be considered DTMs, as a DEM requires continuous elevation data for every location in the study area.

A key technique for generating DEMs is interferometric synthetic aperture radar (InSAR), which uses two passes of a radar satellite—or a single pass with two antennas, as in SRTM—to create elevation maps covering tens of kilometers with about ten-meter resolution. Stereoscopic pairs from optical images, captured from different angles during a single airplane or satellite pass, can also be used via digital image correlation.

The SPOT 1 satellite provided the first usable elevation data for much of the planet's landmass using two-pass stereoscopic correlation. Later, data came from the European Remote-Sensing Satellite, the Shuttle Radar Topography Mission, and the ASTER instrument on Terra. The HRS instrument on SPOT 5 has collected over 100 million square kilometers of stereo pairs.

In planetary science, orbital altimetry—primarily laser altimetry, but also radar altimetry—is increasingly used to create digital elevation maps of other worlds. Examples include the Mars Orbiter Laser Altimeter mapping Mars, and the Lunar Orbital Laser Altimeter and Lunar Altimeter mapping the Moon.