1 Describe
the methods of inverted staff reading & reciprocal levelling
In levelling there are
other levelling method which is inverted staff reading and reciprocal
levelling. Inverted staff it may be necessary to determine the reduced levels
of points such as the soffit of a bridge, underpass or canopy. Generally, these
points will be above the line of collimation. To obtain the reduced levels of
such points, the staff is held upside down in an inverted position with its
base on the elevated points. When booking an inverted it is entered in the
levelling table with a minus sign, the calculation proceeding in the normal
way, taking this sign into account. An inverted staff position must not be used
as a change point since there is often difficulty in keeping the staff vertical
and in keeping its base in the same position for more than one reading. An
inverted staff position must be used as a change point because it is often
difficult to keep the staff vertical and to keep iys base in the same position
for more than one reading.
True differences in
height are obtained by ensuring that back sight and fort sight lengths are
equal when levelling. This eliminates the effect of any collimation error that
may be present in the level used and also eliminates the effects of curvature and
refraction. There are certain cases, however, when it may be not be possible to
take readings with equal sight lengths as, for instance, when a line of levels
has to be taken over a wide gap such as river. In these cases, the technique of
reciprocal levelling can be adopted.
Figure down shows two
point A and B on opposite sides of a wide river. The line of collimation has
been assumed to be elevated above the horizontal plane. This may not be the
case but does not affect the calculations. To obtain the true difference in
level between A and B a level is placed at I1 about 5m from A, and a staff is
held vertically at A and B . Staff readings are taken at A (a1) and B (b1). The
level is next taken to position I2 where readings a2 and b2 are recorded. Since
the observations are taken over the same sighting distances with the same
level, the effects of the collimation error will be the same for both cases.
When repciprocal levelling with one level, the two sets of observations must
follow each other as soon as possible so that refraction effects are the same
and are therefore eliminated. Where this not possible, two levels have to be
used simultaneously. It must be realised
that the levels should have the same collimation error or the true
height different will not be obtained.
2
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