Theodolite

A theodolite (/θiːˈɒdəlaɪt/) is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are mainly used for surveying applications, and have been adapted for specialized purposes in fields like metrology 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 target object, the angle of each of these axes can be measured with great precision, typically to seconds of arc.

Theodolites, such as the Brunton Pocket Transit commonly employed for field measurements by geologists and archaeologists, have been in continuous use since 1894.

Theodolites may be either transit or non-transit. Transit theodolites (or just 'Transits') are those in which the telescope can rotate in a complete circle in the vertical plane Non-transit theodolites are those in which the telescope can rotate only in a semicircle in the vertical plane. 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 level plane.