The alidade is an ancient measuring instrument that determines the positional characteristics of distant objects using line of sight. Its applications are varied and include surveying, navigation, and tracking celestial bodies. Its basic principles of geometry remain in modern usage, with lasers and telescoping alidades producing accurate measurements. Military targeting systems still use tools that have remained unchanged since ancient times.
An alidade is an ancient measuring instrument that uses line of sight to determine the positional characteristics of an object that is remote from the observer. The application of the information obtained from this device has been, and continues to be, as varied as human ingenuity. In its most basic form, it is simply a straight edge that can be rotated along a scale of any known or defined choice. One end of the ruler represents the observer; the other end is moved to see the object. The scale is read as a measure of their relationship.
The first alidade designs addressed the two essential characteristics of its use. To improve its visual accuracy, the straight edge is mounted flat and the point-to-point scopes are constructed as perpendicular lugs on each end. Oscilloscope designs included pointers, keyholes, and fine wires within the keyhole. To improve its measurement accuracy, the ruler is constructed, typically also at the ends, with pointers to point to precise scales. The instrument’s name comes 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.
An alidade works on the most basic principles of geometry. Two points define a straight line; thus, when pointed at a distant object, the straight edge of the resulting ruler also represents the true position of that object. Two straight lines bisect to form a measurable and reproducible angle. One of the first applications of these principles was to mount an alidade on the straight edge of a protractor that measures angles from zero to 180 degrees. To this day, this is a basic surveying tool used by cartographers and construction engineers to accurately measure grade and terrain contours.
Mounted on a portable measuring plane, the alidade can become a navigation tool. The magnetic field compass is a modern example, although its design is counterintuitive. The pointing mechanism of most recreational compass designs is its platform, usually engraved with a rough visual pointer; the measurement plane is a free-spinning ring calibrated to match magnetic north. An older navigational tool is the maritime sextant, although its design is counterintuitive. In effect, the stationary Pole Star, visually aligned through an alidade, is the observer while the ship at sea is the distant moving object to be measured.
One of the oldest, most enduring, influential, and most sophisticated tools of alidade was the astrolabe. It was built with several layers of concentric circles of independent measurements. Not only could it be used for tasks such as triangulation, land surveying, and calculating geographic latitude, it was also used to track the movement of celestial bodies with remarkable accuracy. Today it is described as the forerunner of the watch and as an analog computer.
While digital computers and precision motors have been integrated, the same basic functioning of the alidade remains in modern usage. Planes can be placed along any axis, allowing for the registration of objects in three-dimensional space. The line of sight and measurements are located with the aid of lasers. Telescoping alidades produce a corresponding magnitude of accurate measurements. Military warship electronic targeting systems use tools that are functionally unchanged from ancient Bronze Age civilizations.
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