direct contouring

Direct contouring

in this this method the actual contour is pegged out on the ground and its planimetric position located .A back - sight 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 establish a grid of intersecting evenly spaced over the site . A theodolite and steel tape may be used to be 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 point 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 .

Alternatively assistant at ranging rods B and 2 could help to line up the staff holder . When the RLs of all the intersection point are obtained , the contours are located by linear interpolation between the levels , 

on the assumption of a uniform ground slope between each pair of point , The interpolation may be done arithmetically using a pocket calculator , or graphically .

Consider grid point B2 and B3 with reduced levels of 30,20 m and 34,60 m  respcectivaely and a horizontal grid interval of 20 m . The height difference between B2 aand B3 is 4,40 m and the 31 m contour is 0.80m above B2 . the horizontal distance of the 31 m contour from B2 = x1

where   ( 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  -

with equally spaced horizontal line is used . The paper is placed over the two points and rotated unite 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

This procedure is carried out on other liner and the equal contour point joined up to form the contours required .

An alternative way of creating the grid intersections that does not require the use of an optical square is to set out the ranging rods as in in this case it is important that the pairs of ranging rods at A , B...... 1 , 2  , etc  are set out precisely . However once set out , the staff holder can position much more easily ,