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What Is an Alidade?

Meg Higa
Meg Higa

An alidade is a measuring instrument of ancient origin that employs line-of-sight to determine the positional characteristics of a remote object in relation to the observer. Application of the information obtained by this device has been, and continues to be, as varied as human ingenuity. At its most basic, it is simply a straight edge which can be rotated along a scale of any known or defined choosing. One end of the straight edge represents the observer; the other end is moved to sight the object. The scale is read as a measurement of their relationship.

Early designs of alidades addressed the two essential characteristics of its use. To improve its visual accuracy, the straight edge is mounted flat, and point-to-point view scopes are built as perpendicular vanes on each end. Scope designs included pointers, keyholes, and fine wires within the keyhole. To improve its measurement accuracy, the straight edge is constructed, typically also at the ends, with pointers for aiming at precise scales. The name of the instrument is derived from the Arabic word for “ruler,” and is believed to have been in use in the region as early as the Bronze Age of human civilization.

Woman holding a book
Woman holding a book

An alidade works on the most basic principles of geometry. Two points define a straight line; therefore, when pointed at a distant object, the resulting ruler’s straight edge also represents that object’s true position. Two lines bisect to form a measurable, and reproducible, angle. One of the earliest applications of these principles was to mount an alidade to the straight line edge of a protractor that measures angles from zero to 180 degrees. To this day, this is a basic surveying instrument used by cartographers and construction engineers to precisely measure the grade and contours of land.

Mounted on a portable measurement plane, the alidade can become an instrument of navigation. The magnetic field compass is a modern example, though its design is counterintuitive. The sighting mechanism of most recreational compass models is its platform, usually etched with a crude visual pointer; the measurement plane is a freely rotating ring that is calibrated to match magnetic north. A more ancient instrument of navigation is the maritime sextant, though its design also is counterintuitive. In effect, the stationary North Star, aligned visually through an alidade, is the observer while the ship at sea is the distant moving object being measured.

One of the oldest, most enduring, influential and sophisticated alidade instrument was the astrolabe. It was constructed with several layers of concentric circles of independent measurements. Not only could it be used for tasks such as triangulation, land survey, and geographic latitude calculation, it was used to chart the movement of celestial bodies with remarkable accuracy. It is described today as the precursor to the time clock and as an analog computer.

While digital computers and precision motors have been integrated, the same basic functioning of the alidade remains in modern use. Planes can be positioned along any axis, enabling the recording of objects in three dimensional space. Line-of-sight and measurements are pinpointed with the aid of lasers. Telescopic alidades produce a corresponding magnitude of accurate measurements. The electronic targeting systems of military warships employ instruments that are functionally unchanged from ancient bronze age civilizations.

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      Woman holding a book