What is Levelling? | Principle | Uses | Terms Used In Levelling

What Is Levelling?

The technique of determining the relative altitude of a point on the earth's surface below the earth's surface is called LEVELLING.

Principle Of Levelling

The principle of levelling is to obtain a horizontal line of sight at which the vertical distance of a point above or below this line of sight is found.

 

The Purpose Of Levelling 

The main purpose of balancing in the survey is:

·       Find the heights of the given points in relation to the given data.

·       Establishing points at given heights or at different heights in relation to given or considered data.

 

TERMS USED IN LEVELLING

1.   DATUM: Data plane refers to the arbitrary position of a level surface or other line or surface that calculates any size.

 

2.   REDUCED LEVEL (RL):  The height or depth of a point above or below the considered data is called the reduced level.

 

3. BENCH MARK (BM):- B.M. Is the fixed reference point of known height. It can be of the following types.

a. GTS Benchmark (Geodetic Triangulation Survey): This benchmark is set by state agencies such as the Survey of India. They are set to the highest precision. The location and altitude above the MSL are listed in a special catalogue called GTS Maps (100 km apart).

b. Permanent Benchmark: This is a fixed reference point set by referring to the GTS benchmark (10km intervals).

c. Arbitrary Benchmark: This is the reference point where the altitude is assumed to be random. For most engineering projects, the elevation difference is more significant than the reduced level with respect to the MSL provided in a special catalogue known as GTS Maps (100 Km. Interval).

4. Mean Sea Level (M.S.L.): Mean sea level is an average level of the surface of one or more of Earth's bodies of water from which heights such as elevation may be measured.

 

5.   Line of Collimation: Line joining the intersection of the cross-hairs to the optical centre of the objective and its continuation. It is also known as Line of sight. Line of sight: is defined as the intersection of the crosshairs and the optical centre of the objective lens.

 

6. Height of Instrument (HI): The height of the line of sight with the considered data is called the HI. - The elevation of the line of. The sight of the telescope. Starting point.

 

7.   Back sight : (B.S.): The first sight taken on a levelling staff held at a point of known elevation. B.S. enables the surveyor to obtain HI +sight i.e. Height of Instrument or line of sight.

 

8.  Fore Sight (F.S.):  It is the last staff reading taken from a setting of the level. It is also termed as minus sight. Foresight is the sight taken on a levelling staff held at a point of unknown elevation to ascertain the amount by which the point is above or below the line of sight. This is also called minus sight as the foresight reading is always subtracted from the height of the Instrument. 

 

9.  Change Point (C.P.): The point at which both foresight and rear view are taken during the levelling process is called the change or shift point.

 

10.Intermediate Sight (IS): The foresight taken on a levelling staff held at a point between two turning points, to determine the elevation of that point, is known as intermediate sight.

 

Note:  one setting of a level, there will be only one backsight and one foresight but there can be any number of intermediate sights.

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