Four inch Everest theodolite, c 1851-1900.
© Science Museum / Science & Society Picture Library
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. The theodolite's origin is unknown but it has been traced back to 16th century English mathematician Leonard Digges. The theodolite has changed over time. In its current form it is a telescope mounted to swivel both horizontally and vertically. The theodolite is used to find angles in road building, tunnel alignment, and other civil-engineering work. This example was made by Troughton and Simms to the design of George Everest.