Surveying - Carpenter's and Builder's Math, Plans, and Specifications - page 199

Chapter 6
Surveying
By definition, surveying means the art or science of determining the
area and configuration of portions of the surface of the Earth. The
two general divisions of surveying may be classified with respect to
the nature of the measurements taken:
Leveling
Measurement of angles (transit work)
In surveying, leveling is the operation of determining the com-
parative levels of different points of land for the purpose of laying
out a grade or building site, and so on, by sighting through a lev-
eling instrument at one point to a leveling staff at another point
(see Figure 6-1).
The Level
This instrument (see Figure 6-2) is employed to determine the differ-
ence in elevation between points. A common form is known as the
wye level , so-called because its shape resembles the letter Y. It con-
sists of a telescope mounted on two supports that from their shape
are called Ys. The crossbar supporting the telescope is attached to
a vertical spindle that allows it to be turned in a horizontal plane.
Directly beneath the telescope and attached parallel to it is a spirit
level by means of which the line of collimation of the telescope may
be rendered horizontal. The line of collimation is the line that would
connect the intersection of the crosshairs with the optical center of
the objective.
Construction of the Wye Level
In construction, a circular plate is screwed to a tripod, and to this is
attached a similar plate parallel to the first and connected with it by
a ball-and socket joint. Four screws (sometimes only three), called
foot or plate screws , hold these plates apart by resting on the lower
one and passing through the other. A vertical spindle in the center of
the plates supports a rod, bar, or beam, and is used to revolve the in-
strument. The beam is horizontal and carries at its ends two vertical
standards or supports of equal size terminated by two forks of the
general form of the letter Y. The inside of the Ys is Y-shaped, with
an open bottom to prevent an accumulation of dirt. The top of the
Ys may be closed by semicircular straps or bridles (called clips ) that
are hinged on one side and pinned on the other. The pins are tapered
to permit fastening of the telescope. It is never clamped tightly.
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