Page 56 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.6 THE WORK

The telescope should now be swung through the other three-quarters of the circle. If the adjusting
screws are tight and the tripod has not been disturbed, the bubble should not move. If it does move,
and the table cannot be leveled so that it will not move when swung, the spirit level is probably
out of adjustment.

Releveling. During use, the spirit level should be checked occasionally and the instrument
releveled if necessary. The tripod may settle into the ground, particularly if on some unstable base
such as ice, wet clay, or oil road top. Jars from focusing the telescope, or the wind, or other causes,
may disturb it. Sometimes it is necessary to put small boards under the legs to avoid settling.

Centering. Exact placement of the instrument is usually not important in setting grades, but it
is essential in most other work. It involves locating the vertical axis on which the telescope swings
directly over a marker, often a nail in a 2 2 stake.
The standard procedure is to hang a pointed, balanced weight called a plumb bob from the center
of the instrument by a light chain and/or string. The tripod is maneuvered until the point of the
bob is just over the center of the nail head.
This location may need readjustment after leveling the instrument, and such relocation calls
for releveling.
The tripod head in Fig. 2.6B allows horizontal shifting a distance of 2 inches in any direction,
without disturbing its level. This device saves time and trouble.
The optical plummet, Fig. 2.7, replaces the plumb bob with a low-magnification telescope in
the rotating base, which, by means of a prism, looks down the vertical axis. A small bull’s-eye
and cross line make possible exact alignment with a marker.
Transit Theodolite. A transit has a vertical swivel and support yoke mounted on the turntable,
which permits the telescope to be tilted up and down on a vertical axis in addition to its horizontal

FIGURE 2.6B Shiftable tripod head.