Civil Engineering Reference
In-Depth Information
Fig. 5.8
Lifting tripod. Notice that top of tripod is kept level
tribrach is nearly centered. Because the bubble is extremely sensitive, it will not be
possible to perfectly center the bubble simply by moving the legs. For now, getting
it somewhere near center is good enough.
Next, look through the optical plummet. If the point cannot be seen, place your
foot under the tripod and move it around until the toe can be seen through the opti-
cal plummet. This will tell you how far away the point is from being directly under
the tribrach. Lift the tripod again, being careful not to let the legs collapse, and
move it toward the point. If the tripod started off relatively level and the legs were
not allowed to move when it was lifted, it should still be roughly level—assuming
the ground is level. If not, pivot one or more of the legs again so that the bubble
is centered. Look through the optical plummet again. Repeat this procedure until
the optical plummet shows that the tripod is very close to being over the point—
preferably within a few hundredths or so. Remember that the tribrach must be
approximately level before the optical plummet can be used. At this stage it is only
necessary to have the tripod close to the point and roughly level. Do not waste
time trying to get it perfect because once the legs are stepped on it will move.
