alidade—Part of a surveying instrument consisting of a sighting device, with an index, and reading or recording accessories. The alidade of a theodolite or surveyor’s transit is the upper part of the instrument: the telescope, micrometer microscopes, or verniers, and accessories, mounted on what is termed the “upper motion” of the instrument, and used in observing a direction or angle on the graduated circle, which is mounted on the “lower motion.” The alidade used in topographic surveying consists of a straightedge ruler carrying a telescope or other sighting device, and is used to record a direction on the plane-table sheet, which corresponds to the graduated circle of the theodolite. If a telescope is used, the instrument is often termed a telescopic alidade. The movable arm of a sextant is an alidade. See also alidade, peepsight; alidade, pendulum; eccentricity of alidade.

alidade, peepsight—A type of alidade consisting of a peepsight mounted on a straightedge.

alidade, pendulum—A telescopic alidade in which a pendulum device replaces the conventional bubble for establishing a horizontal reference line from which vertical angles may be measured.

alidade, photo—See photoalidade.

alidade, telescopic—See alidade.

–

Source: NSPS “Definitions of Surveying and Associated Terms“, used with permission.

Part of LearnCST’s exam text bundle.