Terrain derivatives

A terrain raster becomes much more readable when you derive surfaces that emphasise shape. Each derivative answers a different question.

Hillshade

Hillshade simulates illumination from a chosen azimuth and elevation. It is excellent for reading scarps, terraces, channels and earthworks but creates artificial light/shadow. Never interpret darkness as vegetation or material type.

Use several illumination directions or multidirectional hillshade when a single light angle hides linear features.

Slope

Slope measures rate of height change. It is useful for papakāinga screening, erosion, access, restoration and landform interpretation. State whether slope is degrees or percent.

Aspect

Aspect is the direction a slope faces. Flat areas can have undefined/unstable aspect. It can help with exposure, vegetation and site-context questions.

Curvature and roughness

Curvature highlights convex/concave surface form; roughness measures local surface variation. Both depend strongly on cell size and neighbourhood settings, so they are best used comparatively rather than as self-explanatory truth layers.

Contours

Contours are derived lines. Creating 0.1 m contours from a 1 m DEM does not create 10 cm accuracy. Choose an interval that matches data quality and map scale.

Flow direction and accumulation

Hydrological terrain tools infer where water would move across a DEM. They can identify drainage paths and catchments, but culverts, bridges, stormwater pipes, channel burning and hydro-flattening can materially alter results.

Profiles and viewsheds

Profiles show elevation along a line. Viewsheds estimate line of sight from observer positions. DSM-based viewsheds include vegetation/buildings differently from bare-earth terrain viewsheds.

For QGIS steps see LiDAR in QGIS.

Last reviewed: 26 August 2026