Surveying Instruments - Practical Boundary Surveying - page 71

Fig. 5.10 Prism offset
a tripod. A person simply walks along and places the pole on the feature to be
located. The person on the total station then makes the measurement and records
the location in the data collector. Surveyors often call this Taking a Shot . Because
the prism pole cannot be held perfectly steady it is not considered to be as precise
as a tripod mounted prism, but it is much faster, lighter and easier to carry. Notice
in Fig. 5.11 that a circular array of small prisms allows a measurement to be taken
from any direction. The person holding the pole need not be concerned whether or
not the prism is aimed exactly at the total station. Not all prism assemblies have
multiple prisms. Sometimes a single prism is mounted on a prism pole. In such
cases the person holding the pole must be careful to aim the prism at the total sta-
tion. Bipods are sometimes attached to prism poles. A bipod has two adjustable
legs and a clamp which allows it to be attached to any prism pole. Similar to a
tripod, the bipod legs can be pressed into the ground to provide a rigid support
for the prism pole. Setting up a prism pole and bipod is slower than taking a shot
with just the pole, but it is faster than setting up a conventional tripod. Bipods
also weigh a lot less than tripods so they are easier to carry. A bipod can also be
quickly removed from the prism pole when it is no longer required.
Because the prism pole cannot be held perfectly steady it is not considered
to be as precise as a tripod mounted prism, but it is much faster, lighter and
easier to carry.