Four datasets dominate free elevation work. SRTM, from a 2000 Space Shuttle mission, covers most land between 60 degrees north and 56 degrees south at 30-metre resolution and remains the most widely used DEM in teaching and practice. ALOS World 3D, from Japan's optical stereo mission, also offers 30 metres globally with generally better accuracy in forests and mountains, though it has occasional artefacts in cloudy tropical regions. Copernicus DEM, derived from the TanDEM-X radar mission, provides 30 metres globally and 90 metres in its most open tier, with excellent consistency, while NASADEM is a reprocessed SRTM with improved methods.

Resolution is only part of the story. Vertical accuracy, typically quoted as a few metres for these products, varies with terrain: flat open ground is accurate, while steep slopes and dense forests degrade every optical and radar DEM, because the sensor measures canopy tops or struggles with radar shadow. Always read the product's accuracy statement and treat elevation values as estimates with error bars, not exact heights.

For continental or national analysis, SRTM or Copernicus at 30 metres is usually sufficient and keeps file sizes manageable. For watershed delineation or slope analysis in hilly terrain, ALOS or Copernicus often gives cleaner drainage networks, because their processing handles voids and artefacts better than the original SRTM. For coastal or flood work, check whether your area has a higher-resolution national LiDAR dataset, since even the best 30-metre DEM cannot resolve levees, ditches, and small embankments that decide where water goes.

Download routes differ: SRTM and NASADEM come from USGS EarthExplorer or the OpenTopography portal, ALOS from JAXA's portal after free registration, and Copernicus DEM from the Copernicus Data Space or the AWS open-data mirror. All arrive as GeoTIFF tiles that QGIS opens directly, and tiling means you usually download several files for a large study area and mosaic them before analysis.

Three mistakes recur in beginner DEM work. First, analysing a DEM in geographic coordinates, degrees rather than metres, which corrupts slope and distance calculations; reproject to a local projected system first. Second, ignoring voids, the holes where the original sensor recorded no data, which appear as extreme values and poison statistics; fill them with QGIS's fill-no-data tool before analysis. Third, mixing DEMs of different resolutions or datums in one project without noting it, which creates artificial steps at tile boundaries.

With those cautions, free DEMs support serious work, from hiking-route planning to regional flood screening, and comparing two independent DEMs over your area is a quick sanity check on any surprising result. If both datasets agree, trust the pattern; if they disagree sharply, investigate before publishing a map that others will act on.

Before committing to a DEM for a large project, run a quick comparison over a small test area: download SRTM and Copernicus tiles for the same place, subtract one from the other in the Raster Calculator, and inspect where they differ most. Large differences usually cluster on steep slopes, forest canopies, or water bodies, and that pattern tells you which dataset to trust for your terrain type. Document the test in your project notes, since a reviewer will reasonably ask why you chose one DEM over another, and an empirical comparison is a far stronger answer than brand loyalty. This small experiment takes under an hour and prevents the far costlier mistake of discovering a DEM problem after analysis is complete.

Finally, remember that the best DEM is the one whose limitations you understand. Read the product guide for whichever dataset you choose, note its stated accuracy and known artefacts for your terrain, and mention those limits in your report. A map built on a DEM you understand, limits included, is far more trustworthy than a map built on a higher-resolution product whose errors you never investigated.