Tripod
The tripod is the base or foundation that
supports the survey instrument and keeps it stable
during observations. A tripod consists of a head
to which the instrument is attached, three wooden
or metal legs that are hinged at the head, and
pointed metal shoes on each leg to be pressed or
anchored into the ground to achieve a firm setup.
The leg hinge is adjusted so that the leg will just
begin to fall slowly when it is raised to an angle
of about 450. The tripod head may have screw
threads on which the instrument is mounted
directly, a screw projecting upward through the
plate, or a hole or slot through which a special
bolt is inserted to attach to the instrument.
Two types of tripods are furnished to
surveyors: the fixed-leg tripod and the extension-
leg tripod. The fixed-leg type is also called a
STILT-LEG or RIGID tripod, and the extension-
leg tripod is also called a JACK-LEG tripod. Both
types are shown in figure 11-37. Each fixed leg
may consist of two lengths of wood as a unit or
a single length of wood split at the top, attached
to a hinged tripod head fitting and to a metal shoe.
At points along the length, perpendicular brace
pieces are sometimes added to give greater
stability. The extension tripod leg is made of two
sections that slide longitudinally. On rough
ground, the legs are adjusted to different lengths
to establish a horizontal tripod head or to set the
instrument at the most comfortable working
height for the observer. A leg may be shortened
and set as shown in the extreme right view of
Figure 11-37.-Types of tripods.
The fixed legs must be swung in or out in
varying amounts to level the head. Instrument
height is not easily controlled, and the observer
must learn the correct spread of the legs to get
the desired height.
WIDE-FRAME tripods, like those shown in
figure 11-38, have greater torsional stability and
tend to vibrate less in the wind.
You should grip the surveying instrument
firmly to avoid dropping it while you are
mounting it on the tripod. Hold the transit by the
right standard (opposite the vertical circle) while
you are attaching it. The engineer’s level should
be held at the center of the telescope, while
theodolites and precise levels should be gripped
near the base of the instrument. The instruments
should be screwed down to a firm bearing but not
so tightly that they will bind or the screw threads
will strip.
In setting up the tripod, you should be sure
to place the legs so that you achieve a stable setup.
On level terrain, you can achieve this by having
each leg form an angle of about 600 with the
ground surface.
Loosen the restraining strap from around the
three legs, and secure it around one leg. An
effective way to set the tripod down is to grip it
with two of the legs close to the body while you
stand over the point where the setup is required.
By using one hand, you push the third leg out
away from the body until it is about 50° to 60°
with a horizontal. Lower the tripod until the third
leg is on the ground. Place one hand on each of
the first two legs, and spread them while taking
a short backward step, using the third leg as a
Figure 11-38.-Wide-frame tripods.
11-38
figure 11-37.
