Choosing a contour interval for a site plan
How drawing scale, slope and data accuracy decide the contour interval of a site plan, with a quick way to check line spacing on paper and typical intervals for common scales.
Updated · 6 min read
The contour interval is the height between two neighbouring contours. Too coarse and the site reads as flat; too fine and the lines merge into a grey mass. Three things decide the right value: the scale of the drawing, the steepness of the ground, and the accuracy of the data.
Check the spacing on paper
On a uniform slope, the horizontal distance between contours is the interval divided by the slope. On paper, divide that by the scale:
- Slope 10%, interval 1 m → contours 10 m apart → 20 mm apart at 1:500, 2 mm at 1:5,000.
- Slope 30%, interval 1 m → 3.3 m apart → 6.7 mm at 1:500, 0.7 mm at 1:5,000.
As a working rule, lines closer than about 1 mm on paper start to merge. If the steepest part of your site falls below that, use a larger interval, or keep the interval and draw the minor lines thinner and lighter than the index lines.
Typical intervals by scale
These are common starting points in architectural and landscape practice, not a standard. Adjust them for the relief of your site.
| Drawing scale | Typical interval | Index line |
|---|---|---|
| 1:200 – 1:500 | 0.25 – 1 m | every 4th or 5th line |
| 1:1,000 | 1 m | every 5 m |
| 1:2,500 | 2 – 2.5 m | every 10 m |
| 1:5,000 | 5 m | every 25 m |
| 1:10,000 | 10 m | every 50 m |
Pair the index line with the interval so that index lines fall on round heights: every 4th line for a 0.25 m or 2.5 m interval, every 5th for 1 m, 5 m or 10 m.
Do not out-draw the data
An interval finer than the accuracy of the elevation data produces lines that look precise but are interpolated. With a 30 m global model, 0.5 m contours describe the general shape of a slope well, but cannot show a 1 m terrace. Where a national lidar model is available, fine intervals become meaningful. TOPOLUMA lets you choose 0.25 m to 100 m and suggests a value from the relief of your area; the data page explains which model is used where.
When contours are not enough
Contours show height; a slope map shows steepness directly, which is often what a design decision needs: where a path stays under 5%, where a building needs a stepped section, where the ground is too steep to build on. TOPOLUMA's analysis diagrams include elevation, slope and aspect for the same area, and terrain analysis describes them in more detail.