Levelling is defined as “an art of determining the relative height of different points on, above or below the surface”.
Principle of Levelling:
The principle of levelling is to obtain horizontal line of sight with respect to which vertical distances of the points above or below this line of sight are found.
The objective of Levelling:
- To Find the elevation of given point with respect to some assumed reference line called datum.
- To establish point at required elevation respect to datum.
Terms used in Levelling:
It is the surface parallel to the mean spheroidal surface of the earth.
Line lying on level surface.
Horizontal plane through a point is a plane tangential to level surface.
It is a straight line tangential to level line.
“It is an arbitrary level surface from which elevation of points may be referred”. In India mean sea level is considered as datum of zero elevation it is situated at Karachi.
Mean Sea Level
It is the average height of sea for all stages of tides it is derived by averaging the hourly tide height over a period of 19 years.
Elevation or Reduced level
It is height or depth of any point above or below any datum. It is denoted as R.L.
Bench Mark (B.M.)
It is a fixed reference point of known elevation with respect to datum.
Line of collimation
It is a line joining the intersection of cross hairs of diaphragm to the optical centre of object glass and its continuation. It is also known as line of sight.
Height of instrument
It is the elevation of line of collimation with respect to datum.
It is a staff reading taken at a known elevation. It is the first staff reading taken after setup of instrument.
Fore sight( F.S.)
It is the last staff reading taken denoting the shifting of the instrument.
It is staff reading taken on a point whose elevation is to be determined. All staff reading between B.S. and F.S. are Intermediate sight.
Change Point (T.P)
It is a point on which both fore and back sight are taken.
Instruments for levelling:
The following instruments are essentially required for levelling
- Levelling Staff
Types of Level:
- Dumpy level
- Tilting level
- Wye level
- Automatic level
Levelling staffs are scales on which these distances are measured.
Levelling staffs are of two types
- Self Reading staff
- Target staff