Alidade vs. Theodolite — What's the Difference?

An alidade (/ˈæ.lɪ.deɪd/) (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site using a plane table drawing of intersecting lines in the direction of the object from two or more points or to measure the angle and horizontal distance to the object from some reference point's polar measurement.

A theodolite is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but they are also used extensively for building and infrastructure construction, and some specialized applications such as meteorology and rocket launching.It consists of a moveable telescope mounted so it can rotate around horizontal and vertical axes and provide angular readouts.

An indicator or a sighting apparatus on a plane table, used in angular measurement.

An optical instrument consisting of a small mounted telescope rotatable in horizontal and vertical planes, used to measure angles in surveying.

A topographic surveying and mapping instrument used for determining directions, consisting of a telescope and attached parts.

A surveying instrument, consisting of a small mounted telescope, used to measure horizontal and vertical angles.

A sighting device used for measuring angles.

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 portion of a graduated instrument, as a quadrant or astrolabe, carrying the sights or telescope, and showing the degrees cut off on the arc of the instrument

A surveying instrument for measuring horizontal and vertical angles, consisting of a small telescope mounted on a tripod

Surveying instrument consisting of the upper movable part of a theodolite including the telescope and its attachments

Surveying instrument used with a plane table for drawing lines of sight on a distant object and for measuring angles