Elevation data, often called a digital elevation model or DEM, is the foundation for flood maps, hiking routes, watershed analysis, and 3D terrain visualisation. Fortunately, several high-quality DEMs are available free, covering most of the world at resolutions suitable for many projects. Choosing the right source depends on resolution, accuracy, coverage, and whether you need bare ground or surface elevation.

The most widely used free DEMs include SRTM, which covers most land between 60 degrees north and 56 degrees south at 30-metre resolution, and ALOS World 3D, which offers similar coverage with 30-metre data derived from optical stereo imagery. NASADEM is an improved reprocessing of SRTM data, while Copernicus DEM, derived from TanDEM-X radar, offers 30-metre and 90-metre versions with consistent global coverage and good accuracy.

For the United States, the USGS 3DEP program provides higher-resolution data, including 10-metre and, in many areas, 1-metre or 3-metre lidar-derived DEMs, all free through The National Map. In Europe, many national mapping agencies publish lidar DEMs through open data portals, often at 1-metre or 2-metre resolution. Always check the licence and resolution for your specific area, because availability varies by country and region.

Resolution is not the same as accuracy. A 30-metre DEM samples elevation every 30 metres, but its vertical accuracy may be several metres, depending on terrain and sensor. Steep mountains, dense forests, and urban areas are harder to measure accurately than open, flat land. Check the dataset documentation for vertical accuracy, and be cautious about using a coarse DEM for fine-scale engineering or flood decisions that need metre-level precision.

Voids, or missing data, are another issue. SRTM has gaps in steep terrain and water bodies, while optical DEMs can have errors in cloudy or shadowed areas. Most providers offer filled versions, but filling is an interpolation, not a measurement, so note where voids were filled if accuracy matters for your project. For critical work, compare two independent DEM sources and look for agreement.

Download a DEM as a GeoTIFF, open it in QGIS, and create a hillshade to visualise terrain, contours for a map, and slope for analysis. Combine it with other layers, such as rivers, roads, and land cover, to answer practical questions: where does water flow, which areas are steep, or which route avoids a climb? Document the source, resolution, and date, so others can reproduce your work.

Free elevation data has democratised terrain analysis. What once required expensive surveys is now available to anyone with a computer, and with careful source selection, it supports credible, useful mapping for education, planning, and research.

Free DEMs are excellent for regional analysis, education, and planning, but they may not suffice for engineering design, precise flood modelling, or construction, where metre-level accuracy and current data matter. In those cases, a commissioned survey, lidar acquisition, or a commercial high-resolution DEM may be necessary, and the cost is justified by the risk of using coarse data for a critical decision.

A practical rule is to match data resolution to decision scale: use free global DEMs for regional and national studies, national lidar for local planning, and surveyed data for site design. That tiered approach keeps costs down while ensuring accuracy where it matters most.

Choose your DEM with the same care you would choose any dataset: check its source, resolution, accuracy, and date, and your terrain analysis will stand on solid ground.

With a little care in choosing and documenting your source, free elevation data can support maps and analyses that are both credible and genuinely useful.