Double Rodding Double rodding is a form of differential leveling
in which a continuous check is maintained on
the accuracy of the leveling procedure. It is called
double rodding because it can be done most conveniently
by two rodmen. However, it is possible
to carry out the procedure using only one rodman.
In double rodding, you determine the HI at each
setup point by backlights taken on two different
TPs. If no mistake or large error has been
made, the result will be two HIs that differ slightly
from each other. Elevations computed this way
will also differ slightly. In each case, the average
is taken as the elevation.
Figure 1421 shows doublerodded level
notes for a run from one BM to
another by way of three
Figure 1421.Sample field notes
from doublerodded levels.
intermediate TPs. In each case, a "higher" TP (as
TP1)
and a "lower" TP (as TP~~) was used, resulting
in two different HIs for each. Computed by
way of the higher HIs, the elevation of BM2 came
to 851.98 ft. Computed by way of the lower HIs,
it came to 852.00 ft. The mean (average) of 851.99
ft was taken as the correct elevation.
INDIRECT LEVELING
Indirect methods of leveling encompass both
trigonometric and barometric leveling. TRIGONOMETRIC
LEVELING uses vertical angles and a
horizontal distance to compute the
difference in elevation, BAROMETRIC LEVELING
uses the difference in atmospheric pressures
that are observed by a barometer or an altimeter
to determine the elevation differences. Indirect
methods of leveling will be discussed at the
EA2 level.
