Levelling may be defined as the art of determining the relative height or elevants of points or objects on the earth’s surface.
Instruments used for leveling:
ü Levelling Staff
Level: The purpose of a level is to provide a horizontal line of sight.
ü A telescope to provide line of sight
ü A level tube to make the line of sight horizontal
ü A leveling head to bring the bubble in its centre of run
ü A tripod to support the instrument
The telescope is rigidly fixed with the support and therefore, can neither be rotated about the longitudinal axis, nor can it be removed from its support. A long bubble tube is attached to the top of the telescope. The leveling head generally consists of two parallel plates with either three foot screws of four foot screws. The upper plate is known as tribrach and the lower plate is known as trivet which can be screwed on a tripod
A leveling Staff is a straight rectangular rod having graduations, the foot of the staff representing zero reading. There are various type of graduated staff available and only one type is described here.
Folding type of 4m Levelling Staff:
It consists of two wooden pieces each of 2m of length with the joint assembly. The thickness and width of staff are respectively 18m and 75mm. The folding joint is of the detachable type with locking device at the back.
Hence the stad can be made into two parts for easy handling. When two pieces are locked together, the two pieces form a straight rigid leveling staff of length equal to 4m.
Principle of leveling
When the level is set up correctly and leveled the line of collimation will be horizontal telescope is rotated about is its vertical axis it will revolve in a horizontal plane known as the plane of collimation and therefore, all staffs readings taken with the will be the vertical measurements made downwards from this plane.
To find by how much amount the line of sight is above the bench mark and To ascertain by how much amount the next point is below or above the
line sight. Height of instrument= Elevation of B.M + Back sight Elevation of pt B = Height of instrument – Foresight
reduction level Height of instrument method
In this method, the height of instrument is calculated for ach setting of instruct, and then the elevation or reduced level of the turning point is calculated with respect to the height of instrument
Rise and fall method:
In rise and fall method the difference of level between consequence point is found comparing staff readings on the two points of same setting of the instrument