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The maximum height change that can be measured is dependent on the length
of the rod used and the height of the instrument (HI). If the HI is 1.8 meters
and the next point you want to record is 2.00 meters above your elevation,

you will not be able to see it because you will be looking .2 meters below the
bottom of the rod. The distance between sights should not exceed 70 meters

to maintain reading accuracy.

There are three basic terms used in differential levelling that help keep things
straight when recording notes in your field book. The terms of importance are

back sight, foresight, and turning point. A back sight is a point with a known
elevation, e.g. a benchmark. A back sight may also be a point in which an
elevation is assumed so that relative vertical differences can be established.

For example, a surveyor can assume an elevation on a control point in the
absence of a known elevation. A foresight is a point with no known

coordinates. This means that the foresight elevation can be established only
after a vertical difference is calculated between the back sight (known

elevation) and the foresight (unknown elevation). A turning point is a point
that initially begins as a foresight with an unknown elevation.

After an elevation has been established by determining the vertical difference,

the foresight has a known elevation and can therefore be used as a back sight
as you move along a transect. In review, after the foresight has an established

elevation, it can then be used as a back sight point whereby a known elevation
can be transferred to other foresight points with unknown elevations. Before
we do the levelling or determine the elevation of the surface we have to do

the adjustment.

There are two types of level adjustments: (a) Temporary adjustments And (b)
Permanent adjustments. The temporary adjustments are those, which have

to be done at each set-up of the level. They are necessary adjustments to take
readings. They are setting up the level, levelling up and focussing the eye-

piece and object glass to eliminate parallax. While the permanent.

2 | Soil Engineering Practical Workbook