Levelling

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Levelling 
                 Levelling is a brench of surveying the object of which is
a) to find the elevation of given points with respect to a given or assumed datum
b) to establish points at a given elevation or at different elevations with respect to a given or assumed datum.

There are three different methods of determination of levels
1) Barometric levelling
2) Trigonometric levelling
3) Spirit levelling

Instrument used in levelling
a) A telescope to provide line of sight
b) A level tube to make the line of sight horizontal
c) A levelling head to bring the bubble in its center of run
d) A tripod to support the instrument

Following types of level are geny used
a) Dummy level
b) Wye ( or Y level )
c) Reversible level
d) Tilting level

Error in levelling

 A) instrument errors
1) Errors due to imperfect adjustment
2) Errors due to sluggish bubble
3) Error due to movement of objective slide
4) Rod not of standard length
5) Errors due to defective joints

B) Natural error
1) Earth curvature
2) Atmospheric refraction
3) Variation in temperature
4) Settlement of tripod or turning point
5) Wind vibrations


C) Personal errors
1) Mistake in manipulation
2) Rod handling
3) Mistake in reading
4) Error in sighting
5) Mistake in recoding

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