A theodolite is an instrument for measuring both
horizontal and vertical angles, as used in triangulation networks. It
is a key tool in surveying and engineering work, particularly on
inaccessible ground, but theodolites have been adapted for other
specialized purposes in fields like meteorology and rocket launch technology.
A modern theodolite consists of a movable telescope mounted within two
perpendicular axes—the horizontal or trunnion axis, and the vertical
axis. When the telescope is pointed at a desired object, the angle of
each of these axes can be measured with great precision, typically on
the scale of arcseconds.
refers to a specialized type of theodolite that was developed in the
early 19th century. It featured a telescope that could "flop over"
("transit the scope") to allow easy back-sighting and doubling of
angles for error reduction. Some transit instruments were capable of
reading angles directly to thirty arcseconds. In the middle of the 20th
century, "transit" came to refer to a simple form of theodolite with
less precision, lacking features such as scale magnification and
mechanical meters. The importance of transits is waning since compact,
accurate electronic theodolites have become widespread tools, but the
transit still finds use as a lightweight tool on construction sites.
Some transits do not measure vertical angles.
is often mistaken for a transit but is actually a type of inclinometer.
It measures neither horizontal nor vertical angles. It simply combines
a spirit level and telescope to allow the user to visually establish a
line of sight along a level plane.