Terms used in Leveling

Level surface : It is the surface parallel to the mean spheroidal surface of the earth.

Level line : Line lying on level surface.

Horizontal plane : Horizontal plane through a point is a plane tangential to level surface.

Horizontal line : It is a straight line tangential to level line.

Datum : 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 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.

Benchmark : A benchmark is a point of reference by which something can be measured. Benchmark is the point laid above or below the datum line with a known elevation. Bench mark is considered as check for the other level points.

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.

Back sight : 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.

Intermediate sight (I.S.) : 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 : It is a point on which both fore and back sight are taken.