Aquaculture Engineering

Unit 1- Surveying

Find out the elevation of points using different methods

Steps in levelling
There are two steps in levelling

• To find by how much amount the line of sight is above the bench mark.
• To ascertain by how much amount the next point is below or above the line of sight.

A level is set up approximately mid way between the benchmark and the point of elevation of which is to be ascertained by direct levelling. A back sight is taken on a rod held at the bench mark.

H.I. = Elevation of the Benchmark + Back sight
Turning the telescope to bring into view the rod held on the point “B” a foresight is taken.
ELEVATION = H.I – Fore sight
Eg: Bench mark = 210.852 m.
Back sight = 2.324 m
Fore sight = 1.836 m
H.I = Elevation of B.M + B.S
= 210.852+2.324=213.176m.
Elevation of the point “B” = H.I – Fore sight
= 213.176-1.836=211.340 m.

It is to be noted that if a back sight is taken on a bench mark located on the roof of a tunnel or on the ceiling of a room with the instrument at a lower elevation the back sight must be subtracted from the elevation to get the height of the instrument. Similarly if the foresight is taken on a point higher than the instrument, the fore sight must be added to the height of the instrument to get the elevation of the point.

Differential levelling
Levelling is to determine the elevation of points at some distance apart is called “differential levelling”. When two points are at such a distance from each other that they cannot both be within range of level at the same time the difference in elevation is not found by single setting but the distance between the points is divided in two stages by turning points on which the staff is held and the difference of elevation of each of succeeding pair of such turning points is found by separate setting up of the level.

A and B are the two points. The distance AB has been divided into three parts by choosing two additional points on staff readings has been taken. Points 1 and 2 are turning points.

Reduced level of point A = 240.000m. The height of the first setting of the instrument is therefore 240.000+2.024 = 242.024m. If the following Fore sight is 1.420. The R.L of TP1=242.024-1.420=240.604 m. The back sight for the second set up of instrument is 1.986. The H.I for second set up is = 240.604+1.986=242.590 m. By similar process of calculations R.L. of TP2 = 240.490 and R.L of B = 241.202.