Surveying Instruments - Practical Boundary Surveying - page 60

terrain. A tripod can be seen in Fig. 5.4 . The lower sections of the legs slide in
and out of the upper sections. The upper sections have clamps which can be used
to hold the legs at the desired length. Although it takes a few minutes to set up a
tripod, it is very steady and rigid, so accurate and precise measurements can be
made. When a total station or prism is set up over a point, it is crucial that it is per-
fectly level. For an instrument, horizontal angles must be measured along a plane
that is perfectly horizontal. The most efficient way to level an instrument is to have
the instrument attach to a tripod fixture which can be leveled. This fixture is called
a Tribrach . The top plate of the tripod is a machined flat surface to which a tri-
brach can be attached.
A tribrach can be seen in Fig. 5.5 . Tribrach dimensions are standardized so
the instruments and prisms from different manufacturers can be attached to the
tribrach interchangeably. There is a screw under the top of the tripod, shown in
Fig. 5.6 , which attaches the tribrach to the tripod. By loosening this screw, it is
possible to slide the tribrach an inch or so in any direction over the top of the
tripod. The long slot allowing adjustment can be seen just above the large black
attachment screw. This adjustment allows the tribrach to be precisely centered over
the point. The adjustability means that the tripod itself need not be perfectly cen-
tered over the point.
The tribrach is leveled using the three leveling screws until the bullseye level is
centered. The bullseye level can be seen in Fig. 5.5 . There is a small optical tele-
scope on the tribrach (left side of the image) called an Optical Plummet . The opti-
cal plummet contains a magnifying lens with cross hairs and a right angle prism.
The right angle prism is located exactly in the center of the tribrach so when the
surveyor looks through the optical plummet she can see the point on the ground.
Fig. 5.5 A tribrach used to attach total station or prism to tripod