Civil Engineering Reference
In-Depth Information
off a stone creating enough shock and vibration to damage an instrument or throw
it out of calibration. Although some survey crews have been known to leave a total
station on the tripod while setting up, there is really no justifiable reason to do so,
as it only takes 5 or 10 seconds to move the instrument from its carrying case to the
tribrach. Setting up a tripod on level ground should only take a few minutes, and an
expert can do it in a minute or two. If setups are averaging longer than 4 or 5 min,
there is probably something wrong with the procedure being used.
When a tripod is moved from one setup to the next, or when using the tripod
for the first time on the job site, it is customary to turn all three tribrach leveling
screws to their center position so that the top and bottom plates of the tribrach are
parallel. The tribrach attachment screw should also be loosened and the tribrach
centered on the tripod top plate. This procedure only takes a few seconds and it
insures that when the next setup is started, there will be full adjustment available.
Tribrach screws should also be centered at the end of the work day when the tri-
brach is removed from the tripod and the equipment put away in the survey vehicle.
The basic steps in setting up a tripod are as follows:
1. Adjust the legs to the correct length so that the tripod will be at the proper height;
2. Roughly position the tripod over the point with the legs at the correct spread
and height;
3. Move the tripod and/or swing the legs in or out so the tribrach is roughly level
and approximately over the point;
4. Step on the legs to firmly embed them into the ground. This will probably
move the tribrach off level and away from the point;
5. Re-adjust the leg lengths, if necessary, to bring the tribrach back to level and
over the point;
6. Adjust the tribrach leveling screws to precisely level tribrach and,
7. Slide the tribrach over the tripod top plate until it is perfectly centered over the
point.
Setting up a tripod is an iterative process. Each adjustment will affect another adjust-
ment so it is necessary to repeat the adjustments incrementally until the desired result
has been obtained. The adjustments are initially course, progressing to very fine and
precise adjustments as the tribrach becomes level and centered over the point.
The first step is to start off with all three legs extended to the correct length.
The easiest way to insure that all three legs are the same length is to keep the three
legs together when adjusting their lengths. A simple method is to use one hand
to hold the tripod off the ground at the desired height, loosen one leg clamp and
carefully let the leg slide down to the ground. Don't just let it drop as this could
damage it or dull the point. Tighten the clamp and allow the one leg to support
the tripod. The remaining two clamps can then be loosened and the legs allowed
to extend to the ground. All three legs will now have the same extension length.
Another way to extend the legs is to start with the collapsed tripod standing on the
ground, loosen all three clamps and lift the top section to the correct height. With
the clamps loose, gravity will cause the lower sections to stay on the ground while
the top section is lifted. All three clamps can then be tightened.
