Theodolite

Theodolite is a portable instrument used in land-surveying for measuring horizontal and vertical angles. A telescope is fitted on bearings, so that it can rotate about a horizontal axis at right angles to its optical axis, and a graduated circle is provided for observing the amount of such turning; the uprights which support the telescope are fixed to a circular horizontal plate capable of being rotated on a vertical axis. This plate is also graduated on its edge, and verniers (q.v.) are provided for each circle. The whole is fixed to a tripod stand, and screws and levels are provided to enable the honzontal disc to set truly horizontal. A magnetic compass is also fitted to the horizontal plate. To use the instrument it is levelled, an object is sighted in the telescope, and the reading on the cucle is noted; if now another object is sighted and a fresh reading taken, the difference between the two readings gives the angle between lines joining these two objects with the centre or the instrument.