Question

State the methods of locating contours and explain direct method.

Answer 1

Methods of locating contours :-

1) Direct method

2) Indirect method

i) By cross section ii) By Squares (Block Contouring) iii) Tachometric Method

Direct method of contouring:-

In this method of contouring, the contours of required reduced level are plotted on ground itself. The procedure of direct method of contouring is as follows.

1. Set the level instrument at the center O as shown in figure 1 and do all temporary adjustments like levelling and focusing.

2. Take the first reading on bench mark (Reduced Level i.e. R.L. 100 m) as back sight reading (Say 1.200 m), so that R.L. of instrument axis will become 101.200 m.

3. If the contour of 100 m is required to plot, then reading on staff should be 101.200 – 100 = 1.200 m. 

4. This reading of 1.200 m is searched in radial directions (say 300 around instrument station O) by looking through telescope of level instrument. Once these points are found out, then they are marked with red coloured pegs.

5. Similarly, to set 99, 98, and 97 m contour, the reading on staff should be 2.2, 3.2 and 4.2 m respectively. These all contours can be searched in same radial directions and then marked with blue, green and yellow coloured pegs respectively.

6. By joining these identical coloured pegs, we get the required contours on ground by this direct method of contouring.