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, but theodolites have been adapted for other specialized purposes in fields like
meteorology and
rocket launch technology. A modern theodolite consists of a telescope mounted movably 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.
The transit 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 arc-seconds. In the middle of the 20th century, transits came to be known as 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 transits still find use as a lightweight tool for construction sites. Some transits do not measure vertical angles.
The builder's level 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.
Both axes of a theodolite are equipped with graduated circles that can be read out through magnifying lenses. The vertical circle (the one associated with the horizontal axis) should read 90° or 100 grad when the sight axis is horizontal (or 270°, 300 grad, when the instrument is in its second position, "turned over" or "plunged"). If not, we call half of the difference with 300 grad index error.
