Direct and Indirect contouring
1) Direct contouring:
In this method the actual contour is pegged out on the ground and its planimetric position located.Abacksight is taken to an appropriate BM and the HPC of the instrument is obtained, say 34.800 m AOD. A staff reading of 0.800 m would then place the foot of the staff at the 34 m contour level. The staff is then moved throughout the terrain area, with its position pegged at every 0.800 m reading.
In this way the 34 m contour is located. Similarly a staff reading of 1.800 m gives the 33 m contour and so on. The planimetric position of the contour needs to be located using an appropriate survey technique. This method, although quite accurate, is tedious and uneconomical and could never be used over a large area. It is ideal, however, in certain construction projects that require excavation to a specific single contour line.
(2) Indirect contouring:
This technique requires establishing a grid of intersecting evenly spaced lines over the site.Atheodolite and steel tape may be used to set out the boundary of the grid. The grid spacing will depend upon the roughness of the ground and the purpose for which the data are required. All the points of intersection throughout the grid may be pegged or shown by means of paint from a spray canister. Alternatively ranging rods at the grid intervals around the periphery would permit the staff holder, with the aid of an optical square, to align himself with appropriate pairs and thus fix each grid intersection point, for example, alignment with rods B-B and 2-2 fixes point B2 (Figure ).
Alternatively assistants at ranging rods B and 2 could help to line up the staff holder. When the RLs of all the intersection points are obtained, the contours are located by linear interpolation between the levels, on the assumption of a uniform ground slope between each pair of points. The interpolation may be done arithmetically using a pocket calculator, or graphically. Consider grid points B2 and B3 with reduced levels of 30.20mand 34.60mrespectively and a horizontal grid interval of 20m(Figure ). The height difference between B2 and B3 is 4.40mand the 31mcontour is 0.80 m above B2. The horizontal distance of the 31 m contour from B2 = x1
(20/4.40) = 4.545 m = K
and x1 = K × 0.80 m = 3.64 m
Similarly for the 32 m contour:
x2 = K × 1.80 m = 8.18 m
and so on, where (20/4.40) is a constant K, multiplied each time by the difference in height from the reduced level of B2 to the required contour value. For the graphical interpolation, a sheet of transparent paper (Figure ) with equally spaced horizontal lines is used. The paper is placed over the two points and rotated until B2 obtains a value of 30.20 m and B3 a value of 34.60 m. Any appropriate scale can be used for the line separation. As shown, the 31, 32, 33 and 34 m contour positions can now be pricked through onto the plan.
Grid layout for contouring
Graphical contour plotting