Three-armed compass

This measuring instrument has a circle calibrated from 0º to 180º and from 180º to 0º, featuring 2 moving rulers with pressure screws, verniers, and a file. The rulers can be used to form two horizontal angles using 3 points on the ground which, when transferred to the plotter, can determine the position. The instrument is used to create nautical charts, or to allow a navigator or hydrographer to establish their position on a map using the measurement (provided by the graphometer or sextant) of the two angles formed by 3 known points on the land.

It was made by the English scientific instrument makers Heath & Co.