Civil Engineering Reference
In-Depth Information
Remember that once the legs are spread apart, the tribrach will be lower than
when the legs are together so this must be taken into account when extending the
legs. If the tripod is set up on soft soil, stepping on the legs will cause them to sink
in several inches so this must also be taken into consideration. For beginners, a
good place to start is to have the top of the tripod at about chin height when the
legs are together. The goal is to end up with the total station telescope roughly
at eye level so the target can be comfortably sighted. Consideration must also be
given to the types of measurements that will be taken which might affect the opti-
mum height. For example, if it will be necessary to sight a point on the ground
just a few feet away from the instrument, the telescope eyepiece may end up being
quite high necessitating that the instrument person stand on their toes or on a box
in order to see through the telescope. If there is a hill between the instrument and
the target, it may be necessary to set the instrument higher than normal in order to
see over the crest of the hill. If more than one person will be using the instrument
and one person has a tall stature while the other is short, the instrument will need
to be at a height which will accommodate both persons.
If the tripod is to be set up on the side of a hill, two legs should be downhill and
one leg uphill. When measuring angles, it is easier to stand between two of the legs
rather than having to straddle a leg. When many locations must be made in many dif-
ferent directions, straddling a leg may be unavoidable but if only one or two angles
need to be measured, the leg orientations can be taken into consideration. When it
will be necessary to use a tape to measure from the point over which the tripod is set,
you must be careful to insure that one of the legs is not on the line to be taped; other-
wise it will be necessary to move the tripod in order to make the measurement. The
basic premise here is to simply think ahead and try to envision what will need to be
accomplished during the setup and position the tripod to the best advantage.
Once the tripod legs have been extended, the tripod can be placed over the
point. The legs are then spread apart similar to the tripod shown in Fig.
5.8
. If
the legs are too close together, the tripod will not be as steady as if the legs have
a substantial spread. Once the legs are spread apart the tripod can be moved over
the point as follows. Stand between two of the legs and grab the third leg, which
is now the furthest leg from you, with one hand. Move in close to the tripod so the
two closest legs are pressing against the body. The method used in lifting a tripod
is shown in Fig.
5.8
. This procedure should be practiced until the tripod can be
lifted off the ground without changing the leg spread. Once lifted off the ground,
the tripod can be set over the point. Positioning the tripod over the point is usually
done visually, however a plum bob can also be used. Some tripods have a hook
for attaching a plum bob string as shown in Fig.
5.6
. Using a plumb bob takes a
little longer but it allows the tripod to be easily located over the point, rather than
visually guessing at the location. If a plumb bob is used, it is customary to tie the
string using a slip knot so that the plumb bob can be raised and lowered easily. For
most easy setups it is faster to position the tripod by eye than to use a plumb bob.
For difficult setups, however, a plumb bob or laser plummet can be handy. With
the tripod over the point, the top plate of the tribrach is now leveled by lifting each
leg slightly off the ground and pivoting it in or out so that the bullseye level in the
