the principal of levelling is to find difference of levels of
two points with the instrument known as level.this can be achieved
either by simple or differential levelling
SIMPLE LEVELLING
- it is the simplest operation in levelling, when it is required
to find the difference in elevation between two points,both of
which are visible from a single point of level
- the readings cane be obtained on a staff held successively upon
these points
- the precise situation of the level is immaterial, but to
elimnate the instrument error,the sight distance too the two staff
positions should be kept as nearly as possible
- the level is set on a firm ground anywhere,not necssarily in
the same vertical plane as that of two staff positions
DIFFERENTIAL LEVELLING
- determining the difference in elevation between two or more
points without any regard to alignment of the points is called
differential levelling.
- it is used when two points are at a large distance apart or
when the difference in elevation between the ponts is large or some
obstacle intervenes between the points.
- in such cases it becomes important and essential to set up a
level in several positions and simple levelling procedure is
employed in each of the successive stages. it is also called
compound or continous levelling.