Digital Elevation Models Explained

This article continues the Elevation & Vertical Datums sub-hub. The /learn/mean-sea-level-explained and /learn/vertical-datums-explained articles cover the reference surfaces; this article covers the data products built on top — the elevation datasets that GIS practitioners, surveyors, hydrologists, flight planners, and game developers actually use.

The terminology landscape

The three closely related terms cause endless confusion:

| Term | Meaning | | ---- | ------- | | DEM (Digital Elevation Model) | Generic term. Sometimes synonymous with DTM; sometimes an umbrella over DSM + DTM. | | DSM (Digital Surface Model) | The top-of-surface elevation. Includes tree canopy, building rooftops, and bare ground where no obstruction exists. | | DTM (Digital Terrain Model) | Bare ground only. Vegetation and buildings removed in post-processing. |

In practice:

• SRTM and ASTER GDEM are essentially DSMs — they measure the first reflection the radar/optical sensor sees, which is the top of vegetation in forested areas.
• Lidar-derived DEMs can be either: the first-return surface is a DSM; the ground classification of the point cloud is a DTM.
• Many products labeled “DEM” are semantically ambiguous; documentation is needed to determine which.

For most applications:

• Hydrology / drainage / flood modeling: need DTM (water flows over ground, not over treetops).
• Aviation obstacle / line-of-sight: need DSM (planes hit trees and buildings, not just ground).
• Volume calculations / earthworks: need DTM (calculating excavation volumes from ground level).
• Visualizations / 3D rendering: DSM gives a more realistic appearance; DTM cleaner for terrain shape.

Major global DEMs

SRTM

The Shuttle Radar Topography Mission flew aboard Space Shuttle Endeavour during STS-99, February 11–22, 2000. The mission used a dual-antenna interferometric synthetic aperture radar (InSAR):

• One antenna on the shuttle's payload bay.
• A second antenna on a 60-meter deployable mast — the largest rigid structure ever flown in space at that time.
• The two antennas observed the same terrain simultaneously from slightly different angles.
• The geometry let SRTM derive interferometric phase differences that translate into elevation.

The mission collected data for 11 days, then the mast was jettisoned (still in orbit as of writing). Result: a DEM covering 60°N to 56°S — essentially all inhabited land plus most of Antarctica's perimeter.

Resolution and accuracy:

• 30 m horizontal resolution (1 arcsecond at the equator). Initially released at 90 m for non-US data for ~12 years; full 30 m global release in 2014–2015.
• ±16 m absolute vertical accuracy (LE90).
• ±6 m relative vertical accuracy (LE90).

Limitations:

• Voids in steep terrain (radar layover) — small no-data areas in mountainous regions.
• DSM behavior — measures canopy in forested areas.
• Static snapshot — frozen in February 2000; doesn't reflect post-2000 changes (deforestation, new construction, post-earthquake terrain).

Despite these limitations, SRTM remains the most widely used global DEM because it's free, globally consistent, and well-validated. Most online elevation services (Google Maps elevation, OpenTopoData SRTM endpoint) use SRTM as one input.

ASTER GDEM

The Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model is a joint product of NASA and METI (Japan's Ministry of Economy, Trade and Industry).

Lineage:

• ASTER instrument: launched 1999 on the NASA Terra satellite. Has a stereo camera (two viewing angles) that lets it derive elevation from optical imagery.
• GDEM v1: released June 2009. Notable artifacts.
• GDEM v2: released October 2011. Improved.
• GDEM v3: released August 2019. Current version, with substantial quality improvements.

Specifications:

• 30 m horizontal resolution.
• Coverage 83°N to 83°S — beats SRTM at high latitudes.
• ±17 m vertical accuracy (LE95).
• Optical / stereo-derived, so vulnerable to clouds but works in extreme latitudes where radar performance degrades.

ASTER GDEM v3 has cleaner data than SRTM at high latitudes (Arctic and Antarctic coastlines, Iceland, Patagonia). It's the better choice for high-latitude applications.

ALOS World 3D

JAXA's ALOS World 3D 30m DEM (AW3D30) is derived from the PRISM stereo instrument on ALOS (Advanced Land Observing Satellite, 2006–2011).

Specifications:

• 30 m horizontal resolution in the free version.
• 5 m commercial version (AW3D5) also available.
• Coverage 60°N to 60°S.
• ±5 m vertical accuracy in the commercial product; ~7 m in the free 30 m.

AW3D5 is the highest-resolution global commercial DEM available. It's used in engineering, infrastructure planning, and reconnaissance work where the cost is justified.

Copernicus DEM

The Copernicus DEM is ESA's product, released free-of-charge in 2020. Two flavors:

• GLO-30: 30 m global coverage.
• EEA-10: 10 m European coverage.

Both are derived from TanDEM-X raw data (see next entry) under a special agreement that lets ESA distribute the lower-resolution product for free.

Specifications:

• Vertical accuracy: 4 m (LE90, much better than SRTM).
• Recent acquisition (2010–2015 timeframe), so reflects post-SRTM terrain changes in many areas.

Copernicus DEM is increasingly the default choice for new projects that need higher accuracy than SRTM but free data. Many GIS portals now offer Copernicus DEM as the recommended global product.

TanDEM-X

The German Aerospace Center (DLR) mission TanDEM-X flew from 2010 to 2016 with two satellites (TanDEM-X and TerraSAR-X) flying in close formation, producing bistatic InSAR measurements.

Result: a 12 m resolution global DEM with very high accuracy (~2 m vertical). Distributed commercially through Airbus Defence and Space — typical pricing in the hundreds of euros per scene. Used for high-end applications (planetary geology, military reconnaissance, infrastructure engineering).

Other global DEMs

• GMTED2010 (USGS Global Multi-resolution Terrain Elevation Data 2010): a global DEM aggregating multiple sources at 30 / 15 / 7.5 arcsecond resolutions. Useful for global-scale work.
• GEBCO (General Bathymetric Chart of the Oceans): the standard global bathymetric grid (sea floor). GEBCO 2024 release at ~450 m resolution.
• ETOPO1 / ETOPO2022: NOAA combined topography and bathymetry at 1 arcminute resolution.

Higher-resolution sources

Lidar

Light Detection and Ranging (lidar) uses laser pulses to measure distance to the ground. Airborne lidar produces point clouds with millions to billions of measurements per square kilometer.

From a lidar point cloud:

• First-return surface: DSM (top of canopy / buildings).
• Last-return surface: ground-level for most areas.
• Ground-classified points: filtered to bare ground → DTM.

Lidar resolution and accuracy:

• Resolution: typically 1–5 points per square meter (1 m DEM resolution practical).
• Vertical accuracy: ±0.1–0.3 m typical for modern systems.

National lidar programs:

• USGS 3DEP (3D Elevation Program): nationwide US lidar acquisition. Goal: complete US coverage at Quality Level 2 (QL2, ~2 points/m², ~0.2 m vertical accuracy) by 2030. Coverage as of 2025 is approximately 75% complete.
• UK Environment Agency lidar: 1 m DTM/DSM for England, freely available. Detailed coverage of flood-risk areas, partial elsewhere.
• Finland MMM lidar: nationwide 2 m DEM, free.
• Netherlands AHN (Actueel Hoogtebestand Nederland): ~50 cm DEM of the entire country.
• Norway, Sweden, Denmark, France, Germany: various national lidar programs at various stages.

InSAR (Interferometric SAR)

Beyond TanDEM-X and SRTM, ongoing satellite InSAR missions continue to map elevation:

• Sentinel-1 (ESA, 2014–present): C-band SAR, used for monitoring elevation changes (subsidence, earthquakes, glacier flow) rather than absolute elevation.
• NISAR (NASA-ISRO SAR, launched 2024): dual-band L+S-band SAR for global monitoring.
• BIOMASS (ESA, launched 2025): P-band SAR for forest biomass and canopy height.

Resolution vs accuracy: the false-precision trap

A 30 m DEM resampled to 5 m resolution looks like a higher-resolution product but has the original DEM's accuracy, not the resampled grid's apparent precision. Common pitfalls:

• Bilinear resampling of SRTM (30 m) to 5 m: pixels appear at 5 m precision but represent SRTM's ±16 m vertical uncertainty.
• Aspect ratio resampling: anisotropic DEMs (UTM projection 1 arcsecond ≠ 1 arcsecond at high latitudes) need careful resampling.

Always pair resolution with accuracy when describing a DEM. “30 m, ±16 m” tells the full story; “30 m” alone implies higher quality than delivered.

Coordinate systems and datums

Most global DEMs are in WGS 84 latitude/longitude with EGM96 or EGM2008 orthometric heights:

• SRTM, ASTER GDEM: WGS 84 horizontal, EGM96 vertical.
• Copernicus DEM, AW3D30: WGS 84 horizontal, EGM2008 vertical.
• TanDEM-X: WGS 84 horizontal, ellipsoidal heights (not orthometric).

National lidar products typically use the national vertical datum:

• US 3DEP: NAD 83 / NAVD88.
• UK EA: OSGB36 / ODN (Newlyn).
• Continental Europe: ETRS89 / EVRF.

Converting between datums requires both horizontal and vertical transformations. See /learn/datum-transformations for the horizontal part and /learn/vertical-datums-explained for the vertical.

File formats

| Format | Description | Common use | | ------ | ----------- | ---------- | | GeoTIFF | TIFF with georeferencing metadata | The dominant DEM format | | HGT (SRTM) | Raw signed-16-bit big-endian | SRTM data files | | HDF5 / NetCDF | Hierarchical scientific data | Climate science, large datasets | | LAS / LAZ | Point cloud (pre-rasterization) | Lidar source data | | DEM (USGS) | USGS legacy text format | Older USGS products | | BIL / BSQ / BIP | Binary image formats | ESRI conventions | | ASCII Grid (.asc) | Plain text grid | Simple interchange |

Modern workflows use Cloud-Optimized GeoTIFF (COG) — a GeoTIFF variant with internal tiling that supports HTTP range-request access. COG is the format for cloud-native geospatial data.

Common uses

Hydrology and drainage: extract stream networks, watersheds, flow accumulation, flood inundation models. Requires DTM.

Slope and aspect: compute terrain steepness and direction. Used in ecology, agriculture, ski-route planning.

Viewshed analysis: compute which areas are visible from a given location. Used in telecommunications-tower placement, scenic-view analysis, and warfare.

Volume calculations: compute earthwork volumes (cut/fill) for construction. Used in road engineering, mining, dam construction.

3D visualization: generate terrain perspectives, flyovers, virtual reality terrain. Game engines use DEMs as base layers.

Aviation obstacle detection: ensure flight paths clear terrain. Requires DSM (planes hit trees too).

Flood modeling: route flood water across terrain. Critical for emergency management; the Netherlands AHN exists specifically for this.

Archaeology: lidar can reveal subtle ground features beneath vegetation — many ancient sites have been discovered through lidar-derived DTMs.

Common misconceptions

“Higher resolution means higher accuracy.” No — they're independent. A 1 m DEM resampled from SRTM has SRTM's accuracy. Always pair resolution with reported accuracy.

“SRTM is current.” SRTM was acquired in February 2000. Terrain has changed in many places since then (new construction, deforestation, earthquakes). For applications needing recent data, use Copernicus DEM (~2014) or national lidar.

“DEMs are the same as topographic maps.” Topographic maps are derived from DEMs (or other elevation sources) through contour generation. The DEM is the data; the map is one visualization.

“All global DEMs cover the whole world.” SRTM is 60°N to 56°S (no high latitudes). ASTER GDEM goes to 83°N/S. Copernicus DEM is global. Antarctic and Arctic coastlines need specific high-latitude products.

“DEMs include bathymetry.” Most “DEMs” are land-only. For continental shelves and ocean floors, you need bathymetric products like GEBCO or NOAA Coastal DEM. Some products like ETOPO combine both, but treat them as distinct datasets.

“Lidar replaces all other DEMs.” Where available, lidar is the highest-accuracy source — but coverage is limited (mostly developed countries, partial in others). For global or remote-area work, satellite-derived DEMs remain essential.

“DEM elevation = MSL.” Usually it's orthometric height above the geoid, not MSL above a specific tide gauge. The difference is small but matters for high-precision work. See /learn/mean-sea-level-explained.

“A 30 m DEM gives 30 m accuracy.” No. 30 m is the grid spacing; accuracy is separate. SRTM has 30 m spacing but ±16 m vertical accuracy. Don't conflate.

“DEMs and DSMs are the same.” They're different:

• DSM: top of surface, including trees and buildings (what SRTM and ASTER measure).
• DTM: bare ground (post-processed from lidar point cloud, or from DSM by filtering).

“Free DEMs are inferior.” Modern free DEMs (SRTM, ASTER GDEM, Copernicus DEM, AW3D30) are excellent — Copernicus DEM in particular rivals commercial products. The free options are sufficient for most applications; commercial DEMs are needed only for the very highest precision or specific licensing requirements.