An instrument used, especially in trigonometrical surveying, for the accurate measurement of horizontal angles, and also usually of vertical angles. It is variously constructed.
The theodolite consists principally of a telescope, with cross wires in the focus of its object glass, clamped in Y’s attached to a frame that is mounted so as to turn both on vertical and horizontal axes, the former carrying a vernier plate on a horizontal graduated plate or circle for azimuthal angles, and the latter a vertical graduated arc or semicircle for altitudes. The whole is furnished with levels and adjusting screws and mounted on a tripod.
Origin: Probably a corruption of the alidade. See Alidade.