Ramsden's 18 inch geodetic theodolite, late 18th century.

The theodolite, a surveying instrument used for measuring horizontal and vertical angles with a rotating telescope, measures an angle sufficiently accurately for the effect of the curvature of the earth's surface to be taken into account. This example, built by English instrument maker Jese Ramsden (1735-1800), was used on the Principal Triangulation of Great Britain and Ireland from 1792 to the early 19th century.