Levelling in Surveying-Objectives and methods of levelling

Levelling is a branch of surveying which deals with deals with the determination of relative height with respect to the height of different points on or above the surface of the earth.

All the measurements are taken with respect to a base point or it is known as benchmark.The Objectives and methods of levelling are described below.

Objectives of Levelling

The main purpose of levelling in surveying are:

• To find the elevations of given points with respect to a given datum
• To establish points at given elevations or different elevations with respect to the given or assumed datum.

The measurement of points is taken in the vertical plane.By this method, we can find the difference in height between the two points, which is an important work in the construction process.

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Major definitions in surveying and levelling

1.Station

A point where levelling staff kept.

2.Benchmark

It is the point of reference from readings are taken.It is relatively permanent point and it is used as the starting point for levelling or as a point upon which to close as a check.

3.Mean sea level (Datum level)

Mean sea level is the average sea level or it is the datum for measurement of elevation and altitude.It is the equipotential surface of the earth.It is another important term used in surveying.

4.Reduced level

It is the highest point from the datum line.Don’t worry, we can simply say that, if mean sea level is the reference we take to measure another point ‘A’.Then reduced level is the vertical height between the point ‘A’ and the mean sea level.

5.Back sight

It is first reading taken to any position after placing the device so that we see the maximum number of points to take elevation

6.Foresight

It is the last reading taken to transfer device.

7.Changepoint or Turn point

A point at where both foresight and backsight are taken.

8.Intermediate sight

It is the reading taken between backsight and foresight.

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Methods of levelling

1.Barometric levelling

Barometric levelling is the levelling method based on the concept that the difference between the elevation between two points is proportional to the difference in atmospheric pressure in these points.So, a barometer is used for taking the measurement of pressure at different levels.

The barometric levelling is relatively inaccurate because the pressure will vary at different elevation points during a day or hour.

2.Trigonometric levelling or Indirect levelling

Trigonometric levelling is the method of levelling in which, the elevation between points can be determined by the vertical angles and horizontal distances measured in the field.The elevation can simply found by using certain trigonometric relations.

3.Spirit levelling

It is a branch of levelling in surveying in which the vertical distance with respect to the horizontal line is used to determine the relative difference in elevation between two adjacent points.

These are the objectives and methods of levelling

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