Beaman arc—A specially graduated arc attached to the vertical circle of an alidade or transit to simplify computing elevation difference for inclined stadia sights. The arc, designed by W. M. Beaman of the U. S. Geological Survey, is so graduated that each division on the arc is equal to 100 (1/2 sin 2A), where A is the vertical angle. In use, the index is set exactly on a graduation and the difference in elevation along an inclined line of sight is obtained by multiplying the stadia intercept by the number of Beaman arc divisions.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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