Se c t ion I ll. OPERAT I ON OF EQU I PM EN T
bubble of the circular level is centered. Then
1 2 . Ge ne ra l
level the target with reference to the target
a. The instructions in this section are pub-
level (para 13e).
lished for the information and guidance of the
personnel responsible for the operation of the
1 5 . Surve ying T a rge t Se t Ope ra t ion
surveying target set.
a. Setting Up Tripod. For angular measure-
b. The operator must know how to perform
ments, the instruments must be set up over a
every operation of which the surveying target
point of the terrain which has been determined
set is capable. This section gives instruction
previously. The plug-in sleeve of the plumb line
on centering the optical plummet, centering
is plugged into the central fixing screw from
below apex mark, checking the optical plummet
below and secured by a quarter turn to the
and checking the target. Since nearly every job
right. The tripod is then set up so that the
presents a different problem, the operator may
plumb-bob, which hangs from the middle of
have to vary given procedures to fit the indi-
the central opening, points to the station point
vidual job.
from within inch, and the tripod's plate is
approximately horizontal. To achieve this, the
1 3 . Ce nt e ring Wit h Opt ic a l Plum m e t
leg, whose turning axis at the tripod plate is
a. The eyepiece of the optical plummet (fig.
inclined most, must be displaced laterally. Any
3) must be turned until its plumbing mark
other method will affect the centering. The leg
appears in focus. At the same time the station
points are set firmly into the soil, with care
mark on the ground will be seen, over which
being taken to maintain the centering within
the instrument must be centered.
to 1 inch. Now the plumb-bob may be re-
moved if the final centering is to be done
b. Loosen the fixing screw of the tripod
optically. Otherwise in calm weather the plumb-
slightly to allow the tribrach to be shifted until
bob may be used on its own.
the ground mark appears in the center of the
plumbing mark.
b. Securing Base Plate of Tribrach. The
tribrach itself is located on a star shaped base
c. During the shifting, the bubble in the cir-
plate, where the three leveling screws (fig. 4)
cular level must be maintained exactly in the
are kept fast by a rotatable spring plate. The
center.
spring plate is normally held in position from
d. Tighten the fixing screw.
below by means of a lock screw. Withdrawing
e. Center the target level. Then turn the
this screw allows the spring plate to be rotated
target through 90 and center the level bubble
a few degrees. The base plate is thus released
with the third leveling screw. Repeat this pro-
and can be separated from the tribrach. For
cedure until the bubble remains central
normal use, the spring plate should always be
throughout a 360 rotation of the target. The
secured to prevent the tribrach from falling
bubble should now be centered in any position.
out.
If not, adjust the circular level.
c. Fixing and Leveling the Tribrach. The
tribrach is fixed to the tripod with the central
1 4 . Ce nt e ring Be low a pe x M a rk
fixing screw. The circular level bubble is cen-
tered by turning the leveling screws.
In galleries, the station points are often
marked by hooks cemented in the roof. In this
d. Fixing the Target. `Turn the lock plate
case, fix a plumb line on the hook, so that the
level (fig. 4) towards the right. Insert the feet
point of the plumb will be just at the height of
of the target into the corresponding holes of
the tribrach and turn the lever firmly towards
the upper center mark (fig. 12) of the target.
the left until it snaps over the clutch and holds
Shift the target on the tripod until its mark
the target fast.
is vertically below the plumb point when the
AGO 10139A
