Contour Lines Explained
Contour lines are the oldest and clearest way to show terrain shape on a flat map. Once you can read them, a topographic map tells you where it is steep, where it is flat, and where water will go. Here is how they work.
Lines of equal elevation
A contour line connects points that are all at the same elevation. Walk along a contour and you neither climb nor descend; cross contours and you change height.
Where lines are close together the ground is steep; where they are far apart it is gentle. Closed loops are hills or depressions.
Interval and index contours
The contour interval is the elevation difference between neighbouring lines — for example every 1 m or every 5 m. A smaller interval shows more detail but a busier map.
Every fifth line is usually drawn heavier as an index contour and labelled with its elevation, so you can read heights quickly.
Generated from a DEM
Modern contours are generated from a DEM. Software follows the elevation surface and traces each contour at the chosen interval, then saves them as vectors you can edit and plot.
Because they come from the DEM, the contours match the real terrain and the interval can be tuned to the map scale.
Generate contours from a DEM in STREAM
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