Page 75 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.25

The instrument is set up over the center stake, leveled carefully, and turned so that the hair
lines up with a rod or pole held vertically over one of the end stakes. The instrument is turned
exactly 180°, or, if it is a transit, flipped over vertically. The hair should now line up with a ver-
tical stick on the other stake.
If it does not, the crosshair should be moved one-quarter of the way toward the stick by means
of adjustment screws on the sides of the crosshair frame. These work in the same manner as the
upper and lower screws.
After the one-quarter adjustment, the telescope is turned until hair and stick coincide. A 180°
angle is again measured off and the rod on the first stake sighted. The hair is adjusted to move
toward it one-quarter of any distance, then centered on it by moving the scope.
Additional half-circle turns, and adjustments, are made until the hair will coincide with both
sticks, 180° apart.

LASER

The word “laser” stands for “light amplification by stimulated emission of radiation” (Fig. 2.22).
The type originally used in construction was a tube filled with a mixture of helium and neon gases,
stimulated by electric current.
Stimulation causes the gas atoms to emit energy in the form of light. This process produces light
with only one frequency, with such intensity that it emerges as a continuous, coherent, narrow
beam of red light. Its sides remain almost parallel for considerable distances, instead of diverging
and widening as ordinary light.
In recent times the most common lasers used in construction are diodes. A diode is a two-electrode
device having an anode and a cathode with marked unidirectional characteristics. This type of
laser is an electron-wave tube, which derives its characteristics from the interaction of electrons
which are in a beam of initially uniform charged density. The electron wave is focused in a
narrow beam of high intensity. It may be in the visible light or infrared (IR) light range. Most
commonly used in construction are red or, with certain filters, green which is more intense than
the red light.

FIGURE 2.22 Meaning of “laser.”