three-arm protractor

three-arm protractor

[′thrē ¦ärm prȯ′trak·tər]
(navigation)
An instrument consisting essentially of a circle graduated in degrees, to which is attached one fixed arm and two arms pivoted at the center and provided with clamps so that they can be set at any angle to the fixed arm, within the limits of the instrument; used for finding a ship's position when the angles between three fixed and known points are measured. Also known as station pointer.