Levelling | Principle, Purpose & Terms used levelling


The art of determining relative altitudes of points on the surface of the earth of beneath the surface of earth is called 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 main purpose of levelling in surveying are:

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


  1. DATUM:- Datum plane is an arbitrarily assumed level surface or line with reference to which level of other line or surface are calculated.
  2. REDUCED LEVEL (RL):- Height or depth of a point above or below the assumed datum is called Reduced level.
  3. BENCH MARK (BM):- B.M. is a fixed reference point of known elevation. It may be of the following types. 
    • GTS Bench mark ( Geodetic Triangulation Survey):- These Bench marks are established by national agency like Survey of India. They are established with highest precision. Their position and elevation above MSL is given in a special catalogue known as GTS Maps ( 100 km. interval).  
    • Permanent Bench Mark:- They are fixed points of reference establish with reference to GTS Bench mark (10 km. interval). 
    • Arbitrary Bench mark:- These are reference points whose elevations are arbitrarily assumed. In most of Engineering projects, the difference in elevation is more important than their reduced levels with reference to MSL as given in a special catalogue known as GTS Maps ( 100 Km. interval).
  4. Mean Sea Level (M.S.L.):- M.S.L. is obtained by making hourly observations of the tides at any place over a period of 19 years. MSL adopted by Survey of India is now Bombay which was Karachi earlier. e) Level Surface : The surface which is parallel to the mean sphereoidal surface of the earth is known as level surface.
  5. Line of Collimation:- It is the line joining the intersection of the cross hair and the optical center of the objective and its extensions, it is also called line of sight or collimation. 
  6. Height of Instrument (HI):- The elevation of the line of sight with respect to assumed datum is known as HI.
  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. Fore sight 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 height of Instrument. 
  9. Change Point (C.P.):- The point on which both the foresight and back sight are taken during the operation of levelling is called change 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.

It may be noted that for one setting of a level, there will be only one back sight and one foresight but there can be any number of intermediate sights.

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