Contour gradient, index contour, interval factors

Contour gradient

Contour gradient is the line of constant slope (uniform gradient) that cuts the contours at the same horizontal equivalent. It is laid on the ground to fix the route of a road, railway, canal or pipe line. The gradient = CI / HE = 1 in (HE/CI).

Example: for a road of 1 in 20 gradient and CI = 1 m, the horizontal equivalent is 20 m, so a divider opened to 20 m (at map scale) is stepped from contour to contour to trace the route.

Index contour

Index contour is every fifth contour (for example every 5th line when CI = 2 m, giving 10 m, 20 m, 30 m ...) drawn as a thicker line and labelled with its RL, so that the map can be read quickly. The other contours are called intermediate (minor) contours.

Factors determining contour interval

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.