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1 LEVELLING

1.1 Experiment outcomes

At the end of the lab session students should be able:
 To obtain the accurate results of levelling
 To find the difference in elevation between two points.
 To develop a contour map by using grid system.

1.2 Theory

Differential levelling is the process of determining relative elevations at
various points of interest. The elevations can be tied into a global control

system (meters above mean sea level), or referenced to a local benchmark
with an assumed elevation (i.e. Elev.=100.0 m). In most cases, since the

difference in elevation between two points is what we are interested in, the
absolute elevations usually have little bearing on local decision-making.

Differential levelling is the process by which differences between relative
elevations (vertical differences) are measured. All measurement differences

are made independent of horizontal distance. Since this method of measuring
elevation differences is based on relative differences from one elevation, it is

imperative to get the instrument level before starting. If the instrument comes
out of level while you are working, it may be necessary to return to a point of
known elevation before proceeding. If the instrument is not level, you will not

get accurate values from your work. Differential levelling may require
changing locations (set-ups), depending on two things. The vertical distance

between two points.

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