Factors affecting contour interval

The contour interval is the constant vertical distance between two successive contours. It is chosen from the following factors.

1. Purpose of the map: a small CI is used for detailed engineering design (dam, building site); a large CI for a general reconnaissance map or a preliminary route location.
2. Scale of the map: a large-scale map (1:500) has a small CI (0.25 m to 0.5 m); a small-scale map has a large CI (5 m to 20 m). In general, the CI is increased as the scale becomes smaller, so that the contours do not crowd together.
3. Nature of the ground: flat ground needs a small CI (0.25 to 1 m) to show the relief; steep or hilly ground needs a large CI (5 to 10 m or more) or the contours crowd together.
4. Time and money available: a small CI needs more field points and more time, so cost increases.
5. Accuracy required: if the work needs high accuracy (earthwork volume) a small CI is chosen.
6. Size of the area: for a large area the CI is large; for a small area the CI is small.

The contour interval should be kept constant for one map so that the slope can be seen from the spacing of the contours.