Methods of contouring and uses of contour maps

Methods of contouring

Contouring is done by two methods: the direct method and the indirect method.

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.

Uses of contour maps

1. To find the elevation of any point and the slope between two points from the map (without going to the field).
2. To draw the longitudinal section and cross-sections of a road, railway, canal or pipe line on paper.
3. To fix the best alignment of a road, railway or canal with a given gradient (contour gradient).
4. To find the intervisibility of two points (to decide if a signal station can be seen from another).
5. To calculate the capacity of a reservoir, the catchment area and the volume of earthwork (area between contours).
6. To select suitable sites for a dam, building, bridge, airport or a water tank, and to mark the drainage lines (watershed).
7. To find the quantity of cutting and filling for levelling a site and to plan irrigation and drainage layout.
8. To study the general topography (hills, valleys, ridges) of a large area, and for military purposes and town planning.