Transit (surveying)

A theodolite /θiːˈɒdəlaɪt/ is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are used mainly for surveying applications, and have been adapted for specialized purposes such as meteorology and rocket launch.

A modern theodolite consists of a movable telescope mounted within two perpendicular axes: the horizontal or trunnion axis and the zenith axis. A theodolite measures vertical angles as angles between the zenith, forwards or plunged—typically approximately 90 and 270 degrees. When the telescope is pointed at a target object, the angle of each of these axes can be measured with great precision, typically to milliradian or seconds of arc.

A theodolite may be either transit or non-transit. In a transit theodolite, the telescope can be inverted in the vertical plane, whereas the rotation in the same plane is restricted to a semi-circle in a non-transit theodolite. Some types of transit theodolites do not allow the measurement of vertical angles.

The builder's level is sometimes mistaken for a transit theodolite, but it measures neither horizontal nor vertical angles. It uses a spirit level to set a telescope level to define a line of sight along a horizontal plane.

A theodolite is mounted on its tripod head by means of a forced centering plate or tribrach containing four thumbscrews, or in modern theodolites, three for rapid leveling. Before use, a theodolite must be precisely placed vertically above the point to be measured using a plumb bob, optical plummet or laser plummet. The instrument is then set level using leveling footscrews and circular and more precise tubular spirit bubbles.

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