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A digital elevation model (DEM) is a digital representation of elevation across a geographic area, usually stored as a georeferenced grid of values. In U.S. Geological Survey (USGS) usage, “DEM” normally means the bare-earth terrain, with trees, buildings, and other surface objects excluded. Outside that convention the term is sometimes used more loosely, so the surface a particular file represents has to be confirmed in its metadata before you rely on it.
What a DEM actually stores
A DEM records elevation at regularly spaced x/y positions. Each position carries an elevation value, and all values are referenced to a common vertical datum, which is the reference surface, such as a geoid or ellipsoid, from which heights are measured. Because the values are digital, a DEM is not tied to one file format. It may be delivered as a raster image, a set of tiles, or, in some lidar workflows, as a different structure that is converted into a grid.
The bare-earth convention in USGS usage
The USGS FAQ defines the term this way: “A Digital Elevation Model (DEM) is a representation of the bare ground (bare earth) topographic surface of the Earth excluding trees, buildings, and any other surface objects.” That definition is the reason USGS products labeled DEM show the ground itself. A forest canopy or a rooftop does not appear as a high point in those files.
Many other producers, software packages, and writers use “DEM” as an umbrella term for any gridded elevation model. A file called a DEM by one organization may therefore contain building and vegetation heights. The label alone does not settle the question.
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DEM, DSM, and DTM compared
The three abbreviations are often mixed up because they describe the same kind of grid but treat the land surface differently.
| Term | Surface represented | Typical example | Caveat |
|---|---|---|---|
| Digital elevation model (DEM) | Bare earth under USGS usage; elsewhere a general term for any elevation grid | A USGS 1-meter DEM showing ground under a forest | The label does not guarantee bare earth unless the producer says so |
| Digital surface model (DSM) | Top of everything above ground, including trees, buildings, towers, and other features | A grid showing a building roof as the highest value on a lot | Useful when above-ground features are themselves the subject |
| Digital terrain model (DTM) | Varies by country and producer; in some countries a synonym for a bare-earth DEM | In USGS lidar terminology, a vector set of terrain mass points and breaklines from which a TIN or DEM surface can be derived | Not a consistent synonym; the producer’s definition governs |
When a digital surface model is the right surface
A DSM is the appropriate choice when the height of objects matters more than the ground beneath them. Examples include estimating canopy height in a forest, checking whether a structure clears a flight path, or modeling how a city’s buildings shade one another. The same grid used for drainage or slope analysis would give misleading results in those cases, because the built and vegetated heights would distort the flow and gradient calculations.
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Why the DTM term causes confusion
In some countries “DTM” is simply another name for a bare-earth DEM. In USGS lidar terminology, however, a DTM is a vector dataset made of terrain mass points and breaklines, which is a different kind of object from a raster grid. A continuous TIN (triangulated irregular network) or a DEM surface can be derived from it. When a source says “DTM,” check whether it means a grid, a vector point-and-line set, or a bare-earth surface in another country’s convention.
How to read cell size and resolution
In a raster DEM, the resolution is usually the linear size of one cell, also called cell size or grid spacing. A 1-meter cell covers a ground square one meter on each side. Any feature smaller than the cell cannot be represented explicitly in that raster, so cell size sets the smallest feature the grid can show, not the accuracy of each value.
Cell size is a sampling scale, not a quality grade
USGS’s 1-Meter Digital Elevation Model specification describes a 1-meter-by-1-meter cell. That figure applies to that product specification only. Other DEMs may use coarser spacing, and some grids use spacing that changes with latitude. A finer cell therefore does not automatically mean the elevations are more accurate, because the vertical error of the source data can be larger than the horizontal detail the grid resolves.
Resolution and vertical accuracy are separate measures
Vertical accuracy describes how close each elevation value is to the true height. It is set by the source sensor, the processing, and the quality controls, and it is reported separately in the dataset documentation. A dataset can have fine cells and loose vertical accuracy, or coarse cells and tight vertical accuracy. Compare both before choosing a dataset for slope, flood, or volume work.
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Metadata checklist before you compare or analyze DEMs
Because the abbreviation does not fix the surface, the dataset documentation is the reliable source. Check these items before combining or comparing elevation datasets:
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- Represented surface: bare earth, surface including objects, or another treatment defined by the producer.
- Horizontal coordinate system: the projection and datum used for x/y positions.
- Vertical datum: the reference surface for elevation values. Datasets with different vertical datums differ even when their grids line up.
- Cell size, grid spacing, and pixel-area convention: including whether spacing varies with latitude.
- Vertical accuracy and quality controls: the reported accuracy figure and how it was tested.
- Source and processing: the sensor, such as lidar or photogrammetry, and the processing steps applied.
- Acquisition dates and coverage: when the data was collected for each tile and whether any areas are void or missing.
Checking these items takes a few minutes and prevents the most common errors: subtracting a canopy surface from a bare-earth grid, or mixing datasets referenced to different vertical datums.
The USGS definition quoted above is the most direct reference for the bare-earth meaning of DEM in U.S. usage, and the producer’s metadata always takes precedence for any specific file.
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