# What Is The Basic Principle Of Levelling

Leveling is the art of determining the relative heights or elevation of points or objects on the earth's surface. It is the science of measurement in the vertical plane.

There are two steps in leveling.
The first step is to find the elevation or RL of the plane of collimation or H.I of the level by taking a backsight on a benchmark.

In the above picture staff reading taken on the benchmark is called backsight reading. If the RL of the benchmark =100, and staff reading= 1.320
then the plane of collimation or height of instrument=100+1.320=101.320.or                                                                                                               =RL of B.M+B.S

The second step is to find the elevation or R.L of any other point by taking a reading on the staff held at that point.
If the foresight reading of the second point of the leveling  staff=2.250
then RL of the second point=101.320-2.250=99.070.
It is necessary to bear in mind that after every backsight, however many intermediate sights there may be a foresight.

Leveling should always commence from a permanent benchmark.

Principle of Levelling

1. Simple Leveling:- Simple Leveling is required to find the difference in elevations between two points. Both points should be visible from a single position of the level.

In the above figure, A and B are two points and O is the level instrument set up station which is approximately midway between A and B.

If the staff reading of A= 2.240 and RL of A = 100 .
Then Height of the instrument of O= 100+2.240=102.240.

Keep the level central direct the telescope to point B and read the staff reading on B.Staff should be in the verticle position.
If the staff reading of B=3.235
Then RL of B=102.240-3.235=99.005.

2.Differential Levelling:- This method is also used to find the elevation between two points. It is suitable if the points are far apart or if the difference of elevation between points is great or if there are obstacles intervening.

In the above figure
When instrument set up of O1
a= The back sight-reading of staff station of A
c1= The fore sight-reading of staff station of C.

When instrument set up of O2

c2= The back sight-reading of staff station of C.

d1= The fore sight-reading of staff station of D.

When instrument set up of O3

d2= The back sight-reading of staff station of D.
b= The back sight-reading of staff station of B.

CD is called a change point.
The difference of level between A and B= a-c1
The difference of level between C and D= c2-d1

The difference of level between A and B is equal to the algebraic sum of these differences, and it is equal to the sum of back sight and the sum of foresight.