Page 57 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.7

FIGURE 2.7 Optical plummet.

rotation. The angle from the horizontal is shown on a vertical scale with a vernier. When it is used
as a level, the reading on that scale should be zero.
A level transit like the one shown in Fig. 2.4 can be tilted in the vertical plane 45 degrees,
either up or down. Full transits have an extended support yoke that permits the telescope to turn
in a full vertical circle within it. This makes it possible to make a back sight (180° turn) without
changing the setting of the horizontal circle.
The latest transit theodolite is computerized for easy, quicker setup and readings. Figure 2.8
shows one of these electronic theodolites for routine layout and measurements on a building site.
The compact instrument offers user guidance through the measuring processes and programs.
There are only seven keys used for control of the straight forward and fast operation. On the press
of a key, both the computed and measured data appear in a clear four-line display. In the measur-
ing programs, the user guidance is supported by a graphical display, making the solution of the
problem easier to follow and accept or correct.

SELF-LEVELING LEVEL

The self-leveling or automatic level uses a three-screw leveling base with a circular level vial, for
approximate leveling of the instrument. Fine leveling is done by a gravity-controlled device (pen-
dulum or compensator) inside the telescope. See Figs. 2.9A, 2.9B, and 2.9C.

LEVELING ROD

The surveying instrument’s best companion piece is the leveling or target rod. This is a measur-
ing stick, marked in feet, tenths of feet, and hundredths of feet; or in feet, inches, and eighths of
inches. It may be 8 to 15 feet long, and usually is in two or three pieces which slide on each other,
or, occasionally, are hinged or pegged together. The sliding type must be fully closed or fully open
to be accurate.
Long rods are very desirable in hilly country.
Spaces may be marked by fine lines similar to those on a ruler, in which case it is called a New
York rod. A Philadelphia rod uses the division lines as units of measurement in themselves. See