A sighting device used to locate points in the field from which the survey drawing is plotted. The index of an astrolabe, quadrant or other graduated instrument, carrying the sights or telescope, and showing the degrees cut off on the arc of the instrument.

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

a pointer which can be rotated about the centre of the back of an astrolabe, and its angular position can be found on the degree scale

A rule equipped with simple or telescopic sites and used for determination of direction (as part of an astrolabe).

Instrument, or part of an instrument, for determining direction, either horizontal or vertical. In its simplest form, a peepsight or telescope mounted on a straightedge and used for plotting directions graphically. In such instruments as transits and theodolites, the alidade is the part containing the telescope and its attachments.

That part of a surveying instrument which consists of a sighting device and index, with accessories for reading and recording data. A telescopic alidade is part of a theodolite or surveyor's transit, and when mounted on a straightedge, in conjunction with a plane-table, is used in topographic surveying. (See theodolite and transit).

A sighting apparatus used by lookouts to determine the horizontal bearing and sometimes the vertical angle of a fire.

straight rule with sights. An alidade could be used for surveying with a plane table but it also appears in a different form as part of an astrolabe.

Upper part of a transit or a telescopic instrument used to take bearings.