A digital elevation model, a DEM, is the foundation of terrain mapping: slope, hillshade, watershed and viewshed analysis all start from it. Three free global DEMs dominate practical work, and they are not interchangeable. SRTM, ALOS and the Copernicus DEM each have different strengths, different artefacts, and different best uses. Picking the right one is half the job.

The Shuttle Radar Topography Mission flew in 2000 and mapped most of the world between 60 degrees north and 56 degrees south at 30 metre resolution. SRTM's great virtue is maturity: it has been used, tested and documented for over two decades, its voids are well known, and almost every GIS tutorial assumes it. For regional analysis and teaching, it remains the safe default.

Its weaknesses are age and coverage. The data is a snapshot from 2000, so landscapes changed since then, new reservoirs, mines and cities, will not appear. It also stops at high latitudes, so northern Scandinavia, Canada and Russia need another source. And radar shadows in steep mountains leave voids that must be filled before analysis.

Before any analysis, visualise the DEM as a hillshade and look at it with your own eyes. Voids show up as flat holes or spikes, striping artefacts appear as regular patterns, and forest canopy effects show as suspiciously bumpy terrain over wooded areas. A five-minute visual check catches problems that statistics miss, because your eyes are excellent at spotting patterns that do not belong in a landscape.

Also compare the DEM against a second source wherever the result matters. If SRTM and Copernicus agree, your slope or watershed result is probably robust. Where they disagree, usually in steep terrain or dense forest, treat the result as uncertain and say so in your report. A DEM is a measurement with error bars, not a perfect copy of the ground, and honest uncertainty beats false precision every time.