What is Levelling in Surveying? Explained with Video

Levelling is a branch of surveying that deals with the measurement of vertical distance of a point or position with respect to a known surface. 

The  vertical distance measured is called as elevation and the known surface is called as datum. 

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Objectives of Levelling

The main objectives of levelling are:

1. To find the elevations of given points in site with respect a given datum or assumed datum. This is used for works that need to be designed.
2. To establish a point at a given location at site, with respect to a datum or assumed datum. This is used for setting out engineering works.

Important Definitions in Levelling

As shown in the figure below, the spheroidal surface is the surface of the earth. All the definitions are stated here with respect to this spheroidal surface.

Level Surface and Level Line in Surveying

A surface that is parallel to the spheroidal surface is called as the level surface. A line lying on a level surface is called as level line. If you consider any point on a level line, it will be perpendicular to the plumb bob line at that point.

Horizontal Plane and horizontal line in Surveying

A plane that is tangential to the level surface at a point is called as horizontal plane. While a line that is tangential to the level line at a point is called as horizontal line.

Datum in Surveying

A datum is an reference surface to which points are referred. Around the world, the mean sea level is considered datum. Hence, elevations are measured this much above or below the seal level. As shown in figure below, point A is 'h' distance above mean sea level and B is h'' distance below the mean seal level.

Mean Sea Level in Surveying

Average height of sea for all stages of the tides. The mean sea level for a particular place is the average of the hourly tide heights over a long period of 19 years.

Bench Mark in Surveying

Benchmark is a permanent point of reference, whose elevation with respect to a datum is known.

Also Read:

 Surveying-Definition, Principle and Classification

2.       Principles of Surveying

3.      Linear measurement in surveying

4.      Reconnaissance Survey and Index Sketching