DEPARTMENT OF CIVIL ENGINEERING

​

TOPIC-DETERMINING THE DIFFERENCE OF LEVEL BETWEEN TWO POINTS

​

SEMESTER-4^TH

BY-MRS. MANDALINI MOHANTA

AY:2021-2022

​

Leveling: is a branch of surveying, the object of which is to find or establish the elevation of a given point with respect to the given or assumed Datum (reference point).

1.422

​

​

Basic Principle of Leveling

Bs

Fs

Δh = Bs - Fs

Benchmark (BM): A relatively permanent object, natural

or artificial, having a marked point whose elevation

above or below a reference datum is known or assumed.

Back sight (BS): It is the sight taken on the level staff, of a known elevation with the intention to obtain the elevation of plane of collimation. It is called PLUS sight because it is added to elevation of that point to get height of instrument or plane of collimation.

Intermediate sights(IS): These are the sight taken after back sight and before sighting the final point. These are called MINUS sights. These are subtracted from plane of collimation to find the reduced level of different points.

​

​

​

​

Fore sight (FS): The last reading taken from the instrument..

Change point (CP) or turning point (TP): The point at which both BS and FS are taken.

Reduced level (RL): The elevations of the points with respect to assumed datum.

Station: A point where the levelling staff is kept.

Height of instrument: It is the elevation of the plane of sight with respect to assumed datum. It is also known as plane of collimation.

There are two methods for obtaining the elevations at different points:

• Height of instrument (or plane of collimation) method
• Rise and fall method

Height of Instrument method

The basic equations are

Height of instrument for the first setting= RL of BM + BS(at BM)

Subtract the IS and FS from HI to get RL of intermediate stations and change points.

Checking:

∑BS - ∑FS = Last RL – First RL.

This is –ve (fall) and +ve for ( rise)

When: the difference in the elevation of two nearby points is required then simple levelling is performed.

Assume the elevation of BM Rock is known to be 820.00 ft.

The BS at BM Rock is 8.42ft.

So HI = (820 + 8.42)ft.

Now the FS on “X” is 1.2ft.

So the RL at “X” = HI – FS =

828.42ft

Note that the RL of the instrument station will never comes in the calculation.

Differential levelling

Performed when the final point is very far from the final point.

We have to find RL at B.

It is given that RL at A is 100m and BS at A

is 2.45m

So, HI at L1=(100+2.45)=102.45m

FS at CP1=2.14m

RL at CP1=(102.45-2.14)=100.31m

Now BS at CP1=1.43m

HI at L2=(100.31+1.43)=101.74m

FS at CP2=2.18m

RL at CP2=(101.74-2.18)=99.56m

BS at CP2=1.38m

HI at L3=(99.56+1.38)m=100.94m

FS at B=1.54m

RL at B= (100.94-1.54)=99.4m (ans)

Note that ∑BS- ∑FS=last RL – first RL

H.W:

The following staff reading observed by level, the instrument having been moved after the second, fifth and eight reading, all reading staff were as follows

0.675, 1.230, 0.750, 2.565, 2.225, 1.985, 1.835, 3.220, 3.115 and 2.875

The first staff reading was taken the staff held on B.M=100.00m .

Enter the reading in level list and find R.L of all points and check the results.