| The engineer's
transit is the "universal instrument" of the surveyor. It is used for
measuring horizontal and vertical angles, for setting points in line (horizontally or
vertically), for measuring approximate distances by the stadia principle,
for leveling operations, and as a compass.
It consists of a
horizontal circular plate (lower plate) graduated around its periphery
in degrees and fractions, and is mounted at right angles on a hollow
conical vertical axis, or spindle, at its center. Another circular
plate (upper plate) fits inside of the lower plate and is mounted on a
second spindle of the first plate. Two verniers, for reading the
graduated circle at points 180° apart, are
attached to this upper plate. The upper plate also carries two
uprights (standards) which support the horizontal axis to which a
telescope is attached at right angles. On one end of the
horizontal axis is a graduated vertical arc, or circle, for measuring
vertical angles; this is read by means of a vernier fastened to
the standard. Attached to the upper plate are two spirit levels,
at right angles to each other, for leveling the circular plates.
The outer spindle rotates in a socket which is part of the leveling
head. The leveling head contains screws by means of which the
plates are leveled, and threads by which the instrument is attached to
the tripod.
The movement of
both plates, as well as the vertical motion of the telescope about its
horizontal axis, is controlled by clamps and slow-motion screws called
tangent screws. Attached to the telescope is a spirit level
for use when the transit is being used as a level. The telescopes
of all transits may be rotated through 180° (and even through 360° in
many transits) about the horizontal axis. When the telescope level
is on the underside of the telescope, it is in the direct
position; when it is above the telescope, it is in the inverted
position.
A small hook
suspended by a chain is attached to the center of the instrument and
hangs through an opening in the top of the tripod. The plumb-bob
is attached to this hook when the instrument is being set over a point.
By releasing
pressure on any two adjacent leveling screws the entire head supporting
the instrument is slightly lowered, and the head may then be shifted a
small amount on the footplate without moving the latter. This is
of assistance in making a final precise setting over the point.
If the transit is
not provided with a level tube and a vernier circle, or arc, then it is
called a Plain Transit. These transits are used where
leveling or stadia work is not required.
Charles
B. Breed, "Surveying" (John Wiley & Sons, Inc., New York, 1942), pp.
43-47.
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