Contour interval, index contour, contouring methods

Contour interval

Contour interval (CI) is the constant vertical distance between two consecutive contours, for example 1 m or 5 m. It is the same over one map. Small CI is used for flat ground and large-scale maps; large CI for hilly ground and small-scale maps.

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.

Direct method

In the direct method the contours are located and marked directly on the ground, and the points are then surveyed and plotted. It is accurate but slow and costly, so it is used for small areas and large-scale maps.

Steps

1. Establish control (traverse stations) and a benchmark in the area.
2. Set up a level at a convenient point, take a back-sight on the BM and find the height of instrument: $HI = RL_{BM} + BS$.
3. For a contour of RL $h$, the staff reading required is $r = HI - h$. A staff man moves up or down the slope until the staff reading equals $r$; that point lies on the contour. Mark it with a peg.
4. Find several points of the same contour, then move to the next contour (staff reading changed by the CI).
5. When the level is shifted, a new HI is found by sighting a fixed point (change point).
6. The pegs are then located by the plane table, theodolite (tacheometer), chain or compass from the control stations and plotted on the sheet. The points of equal RL are joined by a smooth curve.

For steep ground a clinometer or a hand level may be used with a ranging rod.

Indirect method

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.