Slope measures how steep the ground is at each cell of a digital elevation model, usually expressed in degrees or percent, and it is one of the most useful derivatives of elevation data. Builders use it to find flat land, hikers to judge trail difficulty, hydrologists to predict runoff speed, and planners to flag landslide-prone hillsides. A slope map turns an abstract elevation raster, which mostly looks like a grey blur, into an immediately readable picture of the landscape's shape.

The free elevation data most beginners start with is SRTM at 30 metres or ALOS at 30 metres, both downloadable without cost, and QGIS reads them directly as GeoTIFF files. Before analysis, reproject the DEM into a projected coordinate system with metres as units, such as the local UTM zone, because slope calculated in degrees of latitude and longitude gives meaningless results. This single preparation step is the most common beginner error, and checking your units takes ten seconds.

Open the Processing Toolbox and search for Slope, then select your reprojected DEM, choose degrees or percent as the output unit, and run the tool. Degrees are intuitive for most readers, while percent slope suits engineering contexts, where 100 percent means a 45-degree angle. The output raster can be styled with a graduated colour ramp, gentle greens for flat ground shifting through yellows to reds for steep slopes, and a hillshade layer underneath adds depth that makes valleys and ridges pop visually.

To make the map useful, combine slope with other information: overlay roads, settlements, or land cover, and use the Raster Calculator or Reclassify tool to highlight cells above a threshold, such as slopes over 30 degrees where building is risky. Zonal statistics can then summarise the average slope within each administrative boundary or land parcel, turning a raster into numbers a report can quote. Save your workflow as a processing model if you repeat it for several areas.

Slope maps inherit the limits of their DEM: a 30-metre DEM smooths over small cliffs and terraces, so a slope map may understate the steepest ground in rugged terrain, and artefacts such as stripes or pits in the DEM create false slope patterns. Compare your map with a hillshade and a topographic basemap, and treat isolated extreme pixels with suspicion, since they often mark data errors rather than real cliffs. For site-level decisions, always ground-check with a field visit or finer data.

Used with those caveats, slope analysis is a fast, powerful first step in terrain work, and it pairs naturally with aspect, which shows slope direction, and with watershed tools that trace where water flows. Together they form the core of terrain analysis, a skill set that underpins everything from trail design to flood modelling, and all of it is available free in QGIS.

Once the basic slope map works, the next step is to combine slope with aspect and elevation into a single terrain story, for example identifying south-facing gentle slopes that suit solar panels, or north-facing steep slopes where snow lingers and trails stay icy. QGIS can generate aspect with the same tool family as slope, and overlaying the two rasters reveals patterns neither shows alone. Present the combined map with a short written interpretation, because a slope map without explanation invites misreading by non-specialists. Terrain analysis is most valuable when it answers a specific question in plain language, such as where a path is safe or where a building plot is flat enough, and that habit of answering questions, not just producing rasters, is what turns a GIS technician into a trusted analyst.