Triangulation Instrument

A triangulation instrument is a form of surveying instrument often recommended for range-finding by gunners as well as for land surveying. It was alleged to be a convenient way to apply the principle of triangulation, and works by forming a triangle with the arms of the instrument similar to the triangle on the ground. One arm can be considered equivalent to the base-line on the ground, while a second arm, with sights, pivots at one end, and a third, also with sights, pivots on a mount that slides on the base-line arm. Each arm is marked with the same linear scale. The distance between the pivoted arms is set to a scaled equivalent of the base-line on the ground. Sightings on the target are taken with either pivoted arm at respective ends of the base-line. The lengths of either arm to their point of intersection give the equivalent distances to the target at the known scale.

A basic design of this type was by Jost B?rgi; one with semicircular degree scales for each of the pivoted arms was by Philippe Danfrie; another where the arm on the movable mount is replaced by independent sights sliding on the other two arms was by Leonhard Zubler. The designs are described in books published in the second half of the 16th or in the early 17th century. The instruments are very rare and were probably little used.

Jim Bennett
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