A viewshed is a map of what can be seen from a viewpoint. It answers practical questions: where should a fire lookout tower go, which houses will see a new wind turbine, and what can a hiker actually see from a summit. It is one of the most useful terrain analyses in GIS, and QGIS can run it with free data and a free plugin.

Start with an elevation model. The SRTM 30-metre data, the Copernicus 30-metre DEM or a national LiDAR-derived terrain model will all work. Download the tiles for your area, merge them if needed, and make sure the DEM is in a projected coordinate system with metres as the unit, because viewshed calculations need real distances. A geographic CRS in degrees will give wrong results.

Define your observer points. These can be a single viewpoint, a set of proposed tower sites, or a line such as a hiking trail. For each point, set the observer height above ground, typically 1.7 metres for a person standing, and the target height, which is 0 for ground visibility or higher if you are checking whether a turbine tip would be visible. The visibility plugin in QGIS lets you set these values per point.

Run the analysis from the Processing Toolbox or the Visibility Analysis plugin, which is the standard tool for this work in QGIS. It produces a raster where each cell is marked visible or not visible, and you can combine multiple viewpoints to show how many observers can see each cell. Style the result with a simple two-colour ramp, visible and not visible, and overlay it on a hillshade so the terrain reads clearly.

Check the result against reality. Viewsheds are only as good as the elevation model: a 30-metre DEM will miss a building, a tree line or a small ridge that blocks a real view, and it will show visibility where a forest hides the ground. For critical decisions, use the highest-resolution DEM you can get, and remember that the analysis assumes a clear line of sight with no atmospheric effects. Validate a few points in the field before you trust the map.