Index contour and indirect contouring

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.

Indirect method of contouring

In the indirect method only selected points (spot levels) are surveyed and plotted, and the contours are drawn later by interpolation. It is cheaper, quicker and suitable for large areas and small-scale maps.

1. Method of squares (grid method): The area is divided into squares of 5 m to 20 m side (smaller for rough ground). The corners are pegged and their levels are taken with a level. The grid is plotted, RLs are written at the corners and the contours are interpolated along the grid lines. It is suitable for small flat areas.
2. Method of cross-sections: Cross-sections are taken at right angles to a base line or a main line at fixed intervals (10 m to 30 m). Levels are taken at regular intervals and at the break points along each section. Used for route survey (roads, railways, canals).
3. Radial line method: Radial lines are set out from a central station at known angles; points on them are located by distance and their levels are taken (by a level or by tacheometry). It is suitable for a hilly area where a hill can be observed from the summit.
4. Tacheometric method: Spot heights and positions are obtained together with a theodolite and staff; used for hilly ground.

Procedure for control: the area is first controlled by a traverse and levelled BMs. Spot levels of the points (grid corners, cross-section points or radial points) are taken from these. The points are plotted to scale, their RLs are written, and the contours are drawn by interpolation, with every fifth contour as an index contour.