Slope and Hillshade from a DEM: A Complete Guide

A raw digital elevation model (DEM) is just a grid of elevation numbers — useful, but not something you can read at a glance. Slope and hillshade are the two most common ways to turn that grid into something a person can actually interpret: one measures steepness, the other shows terrain shape through simulated light and shadow. They answer different questions and get used for different things, so it's worth understanding both properly.

Slope — how steep is the ground, actually

Slope measures the rate of elevation change between each cell and its neighbors — in plain terms, how steep the ground is at every point in your DEM. It's calculated in one of two real units, and picking the right one matters for how the result gets used:

Degrees are the intuitive choice for most mapping and visual work — a 30° slope reads naturally. Percent slope is the standard for engineering and regulatory contexts (road grades, stormwater design, many building-code slope restrictions are written in percent) — worth checking which one a specific requirement actually asks for before you export, since 45% and 45° are very different steepnesses.

Hillshade — simulated light and shadow, not a light measurement

Hillshade doesn't measure anything physical about the terrain the way slope does — it calculates what the surface would look like if lit from a chosen sun position, using the terrain's own slope and aspect (which direction each cell faces) to decide how bright or dark each pixel appears. The result is a grayscale image that makes ridges, valleys, and subtle terrain features visually obvious in a way raw elevation values never do.

Two real parameters control it: azimuth (the compass direction the light comes from, 0–360°) and altitude (the sun's height above the horizon, 0–90°). 315° azimuth / 45° altitude — light from the northwest, moderately high — is the conventional default, and it's a genuinely sensible one: it avoids the optical illusion where valleys can appear to bulge outward and ridges appear to sink in, which happens more easily with light from the south in the northern hemisphere, since it runs against how most people unconsciously expect terrain to be lit.

Using them together

A genuinely common, real pattern: generate a hillshade first and use it as a semi-transparent base layer under a color-classified slope map (or any other thematic layer). The hillshade supplies visual terrain context — you can see where the ridges and valleys actually are — while the slope classification supplies the real data being communicated on top of it.

How to do it

1. Upload your DEM

   Any real, single-band elevation raster (GeoTIFF, IMG, or ASC) — SRTM, ASTER, a LiDAR-derived DEM, or a survey-generated one all work.

2. Choose slope units and hillshade angle

   Slope in degrees or percent; hillshade with a real sun azimuth and altitude, or the standard 315°/45° default.

3. Run and export

   Get a color-mapped PNG for quick visual interpretation, plus the underlying GeoTIFF with real per-pixel values for further analysis.

Beyond slope and hillshade

The same Raster Tools also generates Aspect and Contour lines from the same DEM, plus NDVI, NDWI, NDBI, and a False Color Composite from satellite or drone imagery — all in one place.