Leveling is the
process of determining differences in the elevation of points, i.e., the vertical distance
between them. The instruments employed are the wye level and a graduated rod which
is held vertically and is read to feet and tenths. Leveling is used for determining
the exact elevation (vertical distance) of definite points, called bench marks, above the
datum (reference plane); these bench marks are used for establishing grades for roads,
drainage, buildings, and for other construction work. Leveling is also employed in
obtaining data to depict the shape the of the earth's surface by means of lines of equal
elevation (called contour lines).
Unlike the transit, the level
does not have to be set up over a definite point; it is set up at any place from which the
rod may readily be seen and read. Therefore no chain with hook is provided for
attaching a plumb-bob.
The wye level is so
named because the telescope is held in Y-shaped supports which are attached to the ends of
a horizontal bar. The bar is parallel to the telescope. At the middle of the
bar, and attached perpendicular to it on the under side, is a vertical conical axis.
This axis fits into a conical socket in the leveling head, which acts as a bearing in
which the axis can turn. The leveling head is similar to the leveling head of a
transit. When in use, like the transit, the level is placed on a tripod. The
instrument is so constructed that the telescope may be changed end-for-end in the Y's and
thus be readily reversed for adjusting. Attached and parallel to the telescope is a
sensitive spirit level for leveling the instrument. When the level is in adjustment
the axis of the bubble tube and the line of sight are parallel and both are perpendicular
to the vertical axis.