How to export contour lines to DXF for AutoCAD and other CAD software

Contour lines are the starting point of most site drawings. This guide takes you from a place on the map to a DXF that opens in your CAD program with every contour at its real height, and explains the few settings that decide whether the result is useful.

The workflow in TOPOLUMA

1. Choose the area. In the studio, search an address, paste coordinates as lat, lon, or draw a rectangle. Keep the area close to what you will draw: a tighter area keeps the file small and the preview fast. Areas go up to 150 km².
2. Set the contours. Open Layers → Contour lines. Pick the interval (0.25 m to 100 m), how often an index line appears (every 2nd, 4th, 5th or 10th line), the smoothing and whether index lines carry elevation labels. The studio suggests an interval for the site's relief.
3. Generate the preview and check the contours on the map. Turn on the layers you also want in the drawing: buildings, roads, water, trees.
4. Choose the coordinates. In Export → Coordinates, pick Local · SW (origin at the south-west corner), Local · centre, or the site's UTM zone. See below for which to use.
5. Export. Tick DXF 2D and download. Several formats at once arrive as a ZIP with a README that states the coordinate system and the source data.

What is in the file

• DXF R12 (AC1009), the most widely readable DXF version: AutoCAD, BricsCAD, ArchiCAD, Revit, Vectorworks, Rhino, SketchUp Pro and QGIS all open it.
• Units are metres. If your template is in millimetres, scale by 1000 on insert or set the insert units to metres.
• Every contour is a polyline at its elevation (the Z of the polyline), so commands that read heights work straight away. Civil 3D, for example, can build a TIN surface from the contour polylines.
• Named layers: CONTOUR_MINOR, CONTOUR_INDEX, BUILDINGS, ROADS_MAJOR and so on, so you can restyle a whole category at once.

Local coordinates or UTM?

CAD programs lose precision far from the origin: with real-world coordinates in the hundreds of thousands of metres, snapping and display can start to jitter. That is why the default is a local origin at the south-west corner of the area, which keeps every number small. Use Local · centre if you prefer to draw around the middle of the site.

Choose UTM when the drawing must line up with other georeferenced material: a survey, cadastral data or GIS layers in the same UTM zone. The export states the zone and its EPSG code, so you can check that it matches the other files.

Interval and smoothing

The interval sets how much the drawing says, not how accurate it is. A 0.5 m interval over terrain data with a 30 m grid gives smooth, plausible lines, but they are interpolated between samples 30 m apart. Choose the interval for the scale of your drawing (see choosing a contour interval) and read the accuracy from the data source: about 30 m in most of the world, down to about 1 m where a national model is open (data sources and accuracy).

Smoothing is applied to the terrain before contouring, which removes the stair-step look a coarse grid would otherwise give. Use light smoothing for drawings, more for presentation diagrams, and none if you want to see the raw shape of the data.

Before you rely on the drawing

• Check a few spot heights against a known point, such as a benchmark or a survey of the plot.
• In cities and forests, global models such as Copernicus GLO-30 measure the surface of roofs and canopies in places, so ground levels can read several metres high.
• For design development and permits, survey the plot. Exported contours are context, not a survey.