Various types of Levelling Instruments and Important Definitions | Surveying
- Dumpy level
- Modern Tilting level
- Automatic level
The levelling instruments essentially consist of the following:
A levelling head with three foot screws which enables to bring the bubble at its centre.
Telescope that provides line of sight to bisect distinct objects.
A bubble tube to make the line of sight horizontal either mounted on top or side of the telescope.
A tripod for supporting the levelling instrument.
Important Definitions in Levelling
Line of Collimation or line of sight
The line joining the point of intersection of the cross wires of the diaphragm to the optical centre of the objective and its imaginary continuation.
The vertical distance measured above or below the mean sea level or benchmark is called as reduced level.
It is a permanent reference point whose elevations or reduced levels are known.
All the levelling operations start from benchmark.
The reading taken by levelling instrument on a levelling staff held on a point whose elevation is known.
It is very first reading taken on the benchmark after setting up the instrument.
Fore sight reading
The reading taken on the point whose elevation is to be found out.
It is the last reading before shifting the instrument.
Any other staff reading taken on a point of unknown elevation from the same setup of the instrument.
All sights which are taken between back-sight or fore-sight or intermediate sight.
It is a point on which fore-sights and back-sight are taken.