Contours vs Breaklines: What Each One Does and When You Need Both
Both are lines on a topographic drawing and both carry elevations, so they are easy to confuse. They play opposite roles. A contour is an output: the line where a surface meets a level plane. A breakline is an input: a line where the ground changes slope abruptly, which the surface has to follow. Get the order wrong and the contours inherit every rounded edge the surface was built with.
What a contour line is
A contour joins points of equal elevation — the trace of a horizontal plane through the terrain surface. Drawn at a fixed vertical interval, contours crowd together on steep ground and spread apart on flat ground, which is how a map shows relief. Every fifth contour is usually drawn thicker and labelled as an index contour, so elevations can be counted without reading every label.
Contours are derived. They are only as good as the surface they were traced from, and they add no information of their own.
What a breakline is
The USGS lidar specification defines a breakline as a linear feature that describes a change in the smoothness or continuity of a surface: the crest and toe of a slope face, the edge of a road or a bench, the bank of a channel, the top of a wall. Breaklines are 3D polylines, with an elevation at every vertex.
In a triangulated surface they are enforced as triangle edges, so no triangle crosses them. That is their whole purpose: the surface bends exactly on the line instead of somewhere between two sample points.
Why a surface without breaklines rounds off edges
Build a surface from points alone and the triangles near a crest connect a point on the bench to a point on the face. The edge is cut off, and the surface sits below the true crest along its whole length. A grid does the same at its own resolution. Volumes computed from that surface are wrong along every edge, and contours traced from it bend smoothly where the ground actually breaks.
The USGS makes the same point about water: without breaklines along the banks, triangulation runs across water bodies and leaves artifacts that pass straight into the DEM and the contours — which is why lidar DEMs are hydro-flattened with breaklines.
Contours cannot replace breaklines — or the reverse
Old maps tempt people to rebuild a surface from its contours. It works poorly: between two contours the surface is guesswork, and a crest that fell between them is simply lost. Breaklines, on the other hand, are sparse by design; they describe edges, not the shape of the ground between them. A good surface needs both kinds of input — points for the general shape, breaklines for the edges — and the contours are then drawn from it.
| Aspect | Contour | Breakline |
|---|---|---|
| What it is | Line of equal elevation | Line along an abrupt change of slope |
| Role in the surface | Output — traced from it | Input — the surface must follow it |
| Elevation | One value along the whole line | Changes from vertex to vertex |
| How it is made | Generated from a DEM or TIN at an interval | Extracted from slope breaks or digitised |
| Typical layers | Index and intermediate contours | Crest, toe, road edge, ditch, bank |
| Common mistake | Rebuilding a surface from contours alone | Leaving them out and rounding every edge |
How each is produced from drone and lidar data
Contours are generated from the DEM or from a design surface at the interval you choose, with index lines and labels, and exported as editable lines. Breaklines used to be digitised by hand from stereo models or orthophotos; from a dense DEM they can now be extracted.
STREAM's Extract Crest/Toe finds every slope face steeper than a threshold — 60 percent by default, written into the result — and adds the crest and toe of each face as 3D lines on separate layers, keeping ramps apart from benches. The lines are ordinary polylines: check them, correct a vertex where needed, and use them.
What to deliver
Deliver both, on separate layers. Contours go to DXF with the index contours on their own layer; breaklines go to DXF as 3D polylines, named by type — crest, toe, road edge, ditch. The surface built from points and breaklines goes out as a TIN, in LandXML or DXF 3DFACE, so the receiving software does not have to rebuild it and round the edges off again.
Frequently asked questions
What is the difference between contours and breaklines?
A contour is a line of equal elevation traced from a surface — an output. A breakline is a 3D line along an abrupt change of slope that the surface must follow — an input. Contours show relief; breaklines keep the surface's edges sharp.
Do I need breaklines if I have a dense DEM?
For a TIN or any simplified surface, yes: without them the triangles cut across crests and toes and every edge is rounded off. A dense DEM holds the edges at its own resolution, and breaklines extracted from it carry them into the lighter surface.
Can I build a surface from contour lines?
You can, but poorly: between two contours the surface is guesswork, and any edge that fell between them is lost. Points plus breaklines make a better surface, and the contours are then drawn from it.
What are crest and toe lines?
The breaklines at the top and bottom of a slope face. On a bench, the crest is the upper break where the flat bench meets the face, and the toe is the lower break where the face meets the bench below.
How do I extract breaklines automatically?
From a DEM, by finding where the slope changes abruptly. STREAM's Extract Crest/Toe scans the selection — or the whole survey — for slope faces above a threshold (60 percent by default) and writes their crest and toe lines as editable 3D polylines on separate layers.
Sources
Extract crest and toe breaklines automatically
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