Civil Engineering Reference
In-Depth Information
lean forward in order to look through the telescope. In order to keep balance, there
is a temptation to grab one of the tripod legs. Doing so will almost certainly move
the instrument.
On warm pavement or on thawing ground, movement of the instrument may
be relatively slow. In such cases better accuracy can be achieved by measuring
the angles as quickly as possible. Just before measuring any angle, the instru-
ment level should be checked and the optical plummet checked to insure that the
instrument is still over the point. Never assume that zero is still set on a back-
sight. Always check zero before measuring an important angle. In any situation
where there is a possibility that the instrument might move, it will be necessary to
turn multiple angles or sets. In this way, an inaccurate angle will be noticed imme-
diately because the precision will be poor. Without multiple angles, a bad angle
might not be noticed until the crew is back in the office and it is discovered that
the traverse will not close. This usually means another trip back to the swamp.
Tracking total stations help solve some of the problems encountered on soft-
ground setups because the instrument person doesn't need to look through the tel-
escope. She can stand a short distance away while the total station automatically
locks on to the prisms and measures the angle. Some total station manufacturers
claim that the ability of their total station to lock onto a target is superior to that of
the human eye. This claim may be true or not true, but I would not want to be the
one sitting in a witness chair during a trial concerning an incorrectly located lot
corner, trying to explain to a jury why I did not look through the telescope to see if
the total station was actually pointing at the target.
If many locations must be made during a setup, it is wise to occasionally go
back and check zero on the backsight, particularly if there is any reason to sus-
pect that the instrument may have moved. It is disappointing to find that, when
checking the backsight after 50 locations have been made, the backsight it is sub-
stantially off. In such cases there is usually no way to tell which locations are
good and which are bad. In other words, it cannot be known exactly when the
instrument moved. When this happens the only choice is to take all of the loca-
tions again. This sad state of affairs is easily avoided by frequently checking the
backsight whenever an opportunity presents itself, for example when the rod per-
son will need some time in walking to another place before resuming locations.
Checking the backsight should only take a few seconds so frequently doing so will
not interrupt the flow of the work very much. It should be standard practice for
the instrument person to always check the backsight as a final step just before the
instrument is taken off the tripod when the locations are complete. Whenever the
instrument person believes that the instrument may have moved he or she must
immediately report it to the party chief, who will then decide what action to take.
It should be standard practice for the instrument person to always check the
backsight as a final step just before the instrument is taken off the tripod
when the locations are complete.
