Contouring methods, direct method field procedure

Methods of contouring

There are two methods:

1. Direct method: the contours are traced on the ground and then located. It is suitable for small areas and large-scale maps.
2. Indirect method: spot levels are taken at grid points, along cross-sections or along radial lines, and the contours are interpolated. It is suitable for large areas. Its types are the method of squares, the method of cross-sections and the radial line method (tacheometry is also used).

Field procedure of the direct method

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 required staff reading is $r = HI - h$. The 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 on the same contour, then move to the next contour (the staff reading changes 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 from the control stations by plane table, theodolite (tacheometer), chain or compass and plotted on the sheet. The points of equal RL are joined by a smooth curve.

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