Generate Smooth Contour Lines from a Drone DTM in QGIS

Raw contour lines generated from a drone DTM usually look jagged and triangulated — a consequence of the mesh structure in photogrammetry outputs. This script smooths the raster first, then smooths the lines, to produce clean cartographic-quality contours ready for client maps.

Here’s the real output on a forest estate survey — thick, labelled major contours every 25 m, thin minor contours every 5 m, draped over the orthomosaic:

What it does

The generate_contours.py script runs a three-stage process:

1. Gaussian smoothing — applies a Gaussian filter to the DTM raster to remove the triangular mesh artefact
2. Contour generation — runs QGIS’s built-in contour algorithm at your chosen interval
3. Chaikin’s algorithm — smooths the resulting contour lines by iterative corner-cutting, removing the stepped appearance

Major contours (every Nth line) are styled thicker and labelled with their elevation in metres. The output is a single GeoJSON file added to your project.

What you need

• QGIS 3.28 or newer
• numpy, scipy, osgeo.gdal (bundled with QGIS)
• A DTM raster with “dtm” in the layer name, checked in the Layers panel
• The generate_contours.py script from the Terrain Analysis Pack

The DTM this script reads, as coloured hillshade — the ridge and valley lines the contours above trace out are already visible here:

Steps

1. Load your DTM raster into QGIS. The layer name must contain “dtm”.
2. Check (tick) the DTM in the Layers panel.
3. Open Plugins → Python Console → Show Editor.
4. Paste the script. At the bottom, find the configurable parameters:
   • contour_interval — contour spacing in metres (default: 5.0)
   • smooth_sigma — Gaussian smoothing strength in pixels (default: 3.0; recommended range 2–5, increase for very noisy DTMs)
   • major_interval — every Nth contour is drawn thick and labelled (default: 5, so major contours every contour_interval × major_interval — 25 m at the defaults)
   • line_smooth_iterations — Chaikin corner-cutting passes on the contour lines (default: 3; 0 disables line smoothing)
   • line_smooth_offset — Chaikin smoothing offset, 0–0.5 (default: 0.25)
5. Click Run.

The output {dtm_name}_contours_{contour_interval}m.geojson is saved in the DTM directory and added to your project with major/minor styling applied.

Choosing the right interval

Terrain type	Suggested interval
Flat to gently sloping	0.5–1 m
Undulating plantation terrain	1–2 m
Hilly or steep terrain	5–10 m
Mountain survey	10–25 m

Too small an interval on flat terrain produces dense, overlapping contours. Too large an interval on steep terrain misses detail. Start with 2 m and adjust.

Related scripts

• generate_contour_level_polygon.py — answers a different question from the same DTM: not “what does the terrain look like,” but “what area is at or below a specific elevation” (useful for flood/fill extent rather than cartographic contour lines)
• calculate_lake_volume.py — computes water storage volume for a lake or pond polygon on the same DTM

Get the script

generate_contours.py is part of the Terrain Analysis Pack — five terrain analysis scripts for QGIS at $35 USD.