What is levelling in surveying?

MEANING OF LEVELLING, LEVELLING STAFF, TEMPORARY ADJUSTMENT IN LEVELLING AND TYPES IN LEVELLING

Levelling is a branch of surveying, which involves in determining the elevation of a given point with respect to a datum (mean sea level) and to establish a point at the given elevation.

Levelling is mainly used in setting up the road, railway or drainage alignment also to calculate the capacity of the reservoir.

After this article, you will be able to understand what is the levelling, terms used in levelling, temporary adjustments in setting up the instrument and types of levelling.

I recommend knowing some of the terms used in levelling.

Benchmark: It is a relatively permanent point of reference whose elevation with respect to a datum is known. It is used as a starting point for levelling.

Datum: It is any surface to which elevation is referred. The mean sea level is taken as a convenient datum.

Elevation:  Elevation of a point on the surface of the earth is the vertical distance above the datum.

To carry out levelling we need levelling instruments i.e., Level and a Levelling staff.

A level is to provide a horizontal line of sight.

A level consist of four parts

1.    A telescope

2.    Spirit level

3.    Levelling head

4.    Tripod

A telescope is to provide a line of sight.

A Sprit level or levelling tube is to make a line of sight horizontal.

 A levelling head used to bring the bubble to centre.

A Tripod is to support the instrument.

levelling staff

A levelling staff is a rectangular straight rod with graduations mainly in meters with 0 reading at the foot.

The purpose of levelling staff is to determine the amount of which station is above the line of sight.

TWO STEPS IN LEVELLING:

To find how much amount the line of sight Is above the benchmark.

To find, by how much the amount of the next point is below the line of sight.

TEMPORARY ADJUSTMENT IN A LEVEL

Setting up the level, levelling up and elimination (removing) of parallax are the temporary adjustment to be made at every time the instrument set up at the new station.

Setting up the level includes fixing the instrument to a tripod.  levelling instrument approximately using adjustable tripod legs.

 Levelling up is carried by the foot screws and with reference plates levels. The bubble is centred with these foot screws.

Focusing is important to remove parallax. It is a condition when the image formed at the objective is not in the same plane as the crosshairs.

Parallax is removed both by focusing eyepiece or the objective, both have a different procedure.

Now we shall discuss some terms that are commonly used in direct levelling.

Station: This is a point where a levelling staff is held for taking observations with a level and temporary adjustment is carried at a station.

 Backsight (BS): It is a sight taken on a staff held at a point of known elevation. It is first sight to level staff.

BS is also called as ‘+’ (plus) sight.

Backsight is necessary to know the height of the instrument.

 Foresight: It is the sight taken on staff at a point of unknown elevation.

It is the last sight taken on staff before shifting the instrument to the new station.

It is also known as ‘ –‘ (minus) sight.

Height of instrument: It is the elevation of the line of sight with respect to the assumed datum.

Turning point (change point): It is the point where we change the position of the level and have two readings BS and FS.

 Reduced level: Reduced level of a point is its height relative to the datum or It is a difference of Height of instrument and foresight.

TYPES IN DIRECT LEVELLING  

There are many types of direct levelling some of them are given below

Simple levelling:

The operation of levelling is to determine the elevation of two points which are nearby keeping the instrument approximately middle of that point.

Differential levelling:

The operation of levelling to determine the elevation of points at some distance apart is called differential levelling.

It is done when the distance between two points is large.

Profile levelling:

It is a process of determining the elevation of points at a short measured interval along a fixed line.

It may be a single line or may be composed of sections of the straight line.

Reciprocal levelling:

When there are obstacles like a river between two points and if there is no place to level, at this situation this special method reciprocal levelling is used.