What is a digital elevation model?

A surface made from numbers

A DEM assigns height to positions across an area.

The familiar form is a raster grid. Each cell or regularly spaced post has a horizontal position and an elevation value. Software can use that array to calculate slope, draw contours, shade relief or sample heights beneath a route.

Digital elevation model is also used as a wider family term. Some authorities reserve particular meanings for DEM, DTM and DSM. Some data begins as irregular points or breaklines and is later interpolated into a regular grid. The filename or acronym alone does not settle the structure or represented surface.

Name the surface

Ground and visible tops are different elevation products.

A bare-ground model aims to remove trees, buildings and other raised objects so the values describe terrain. A surface model may retain the tops of vegetation and structures. Both can be valid because they answer different questions. A canopy height is useful for forestry, while the trail below it needs a ground model.

Water, bridges, tunnels, cliffs and overhangs expose the limits of a single-valued surface. One horizontal position normally receives one stored height in a raster, even when the real world contains a bridge deck above a river or a tunnel below a hillside. Providers set explicit treatments for these cases.

The DTM and DSM comparison looks closely at these surface definitions and the differences in provider terminology.

Several ways to observe height

The measuring method shapes what the model can see.

• LiDARLaser returns create a point cloud that can be classified into ground and non-ground points before a surface is derived.
• Stereo imageryOverlapping aerial or satellite images use viewing geometry to recover three-dimensional positions.
• RadarSignals collected from different positions or times can supply broad elevation coverage, with the represented surface affected by the sensor and processing.
• Survey and mappingMeasured points, contours and breaklines can support a model directly or strengthen other sources.

A national product may combine techniques, vintages or resolutions across its coverage. Seamless delivery does not mean every cell came from one flight or instrument. Metadata may describe source tiles or collection dates separately.

Derived terrain

Many familiar map products begin with the same height field.

Contours connect equal modelled heights. Hill shading uses a simulated light direction to reveal form. Slope and aspect calculate steepness and direction from neighbouring values. Hydrological tools infer flow paths and catchments after the surface has received suitable treatment. A renderer can turn the grid into a mesh for a three-dimensional scene.

A route profile samples the model along a line and orders the resulting heights by route distance. This can provide a coherent profile for a planned GPX that has no elevation values, but it also inherits the model's resolution, alignment, datum and processing. Terrain-derived route height explains that sampling step in detail.

Before trusting the output

Ask what was modelled, when and at what level of detail.

1. Surface

   Confirm ground, top surface or another defined product.

   A plausible shaded image cannot reveal which objects were retained.

2. Reference

   Read horizontal and vertical reference systems.

   Misaligned coordinates or different height datums can create systematic disagreement.

3. Quality

   Separate spacing, accuracy and source date.

   These are independent properties and none can stand in for the other two.

4. Treatment

   Check water, structures, missing cells and interpolation.

   Those choices matter at bridges, cliffs, coastlines and tile boundaries.

The beacen Guide explains how to use terrain route views as planning evidence.

Before interpreting one, identify the modelled surface, reference systems, source date and processing. A DEM is suitable only when those properties match the scale and purpose of the decision.