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Briefly, a horizontal line of sight is first established with an instrument called a
level. The level is securely mounted on a stand called a tripod, and the line of sight
is made horizontal. Then the surveyor looks through the telescopic sight towards a
graduated level rod, which is held vertically at a specific location or point on the
ground. A reading is observed on the rod where it appears to be intercepted by the
horizontal cross hair of the level; this is the vertical distance from the point on the
ground up to the line of sight of the instrument.
Generally, if the elevation of point A is already known or assumed, then the rod
reading on a point of known elevation is termed as a back sight reading (plus sight,
because it must be added to the known elevation of point A to determine the
elevation of the line of sight).
For example, suppose the elevation of point A is 100.00 m (above MSL), and
the rod reading is 1.00m. It is clear that the elevation of the line of sight is
100.00+1.00 = 101.00m. The elevation of the horizontal line of sight through the
level is called the height of instrument (HI).

Fig.5.1 Differential leveling to measure vertical distance and elevation. (a) Step 1: take a
backsight rod reading on point A (b) Step 2: rotate the telescope toward point B and take
foresight rod reading

Suppose we must determine the elevation of point B. The instrument person
turns the telescope so that it faces point B, and reads the rod now held vertically on
that point.
The operation of reading a vertical rod held alternately on two nearby points is
the essence of differential leveling. The difference between the two rod readings is,
in effect, the vertical distance between the two points.

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