Direct Contouring, method of Contour Surveying.

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Direct Contouring Surveying :- It consists in finding vertical and horizontal controls of the points which lie on the selected contour line.
For vertical control levelling instrument is commonly used. A level is set on a commanding position in the area after taking fly levels from the nearby bench mark. The plane of collimation/height of instrument is found and the required staff reading for a contour line is calculated.
Procedure of Contouring :-

1. B.M (RL) + B.S = H.I

2. H.I - F.S = R.L (of that point where staff is held)

Where, B.M = Bench mark
B.S = Back site reading
H.I = Height of Instrument
F.S = Fore site reading
R.L = Reduced level

Fig. 1

Example :- Temporary B.M = 7.34 M

B.M (R.L) + B.S = H.I, 

However, H.I = 7.34 + 1.241= 8.581 M

And,  H.I - F.S = R.L
    R.L¹ = 8.581 - 0.625 = 7.956 M
    R.L² = 8.581- 0.581 = 8.0 M
    R.L³ = 8.581 - 0.543 = 8.038 M ... Etc

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