MATHEMATICAL 3-ARM PROTRACTOR

W. B. Ruhnow

Peer Reviewed

Abstract:	The mechanical 3-arm protractor is used to answer (graphically) the question: “Given three positions (landmarks) and two angles, what is present position?“. A marine sextant is typically used to obtain the angles while a 3-arm protractor is used to determine present position from a chart, However, while the sextant is fairly accurate, the 3-arm protractor is awkward to use and sacrifices much of the accuracy gained by careful sextant work. Fortunately, the function that the 3 arm protractor performs graphically can be implemented on a pocket calculator once this function is described mathematically. Thus, savings of time, accuracy, and money may be realized along with the elimination of plotting hardware. The math is presented here along with an example for flat earth calculations. An iterative approach is also presented for spherical earth computations.
Published in:	NAVIGATION: Journal of the Institute of Navigation, Volume 31, Number 1
Pages:	38 - 46
Cite this article:	Ruhnow, W. B., "MATHEMATICAL 3-ARM PROTRACTOR", NAVIGATION: Journal of The Institute of Navigation, Vol. 31, No. 1, Spring 1984, pp. 38-46.
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