Locations used for surveying, 1799.

UNITED KINGDOM - NOVEMBER 20: Engraving showing plans of buildings where a portable transit instrument was placed for triangulation. These include: Greenwich Royal Observatory, Hanger Hill Tower, Severndroog Castle at Shooter's Hill, the steeples of Swingfield Church, Lydd Church, Tenterden Church, Goudhurst Church, Frant Church, and the keep of Dover Castle. These were �stations in a series of triangles�. The Principal Triangulation of Britain was carried out by General William Roy of the Royal Engineers between 1787 and 1789, using theodolites designed and built by Jesse Ramsden (1735-1800). Illustration from the first volume of �An account of the operations carried on for accomplishing a trigonometrical survey of England and Wales� by William Mudge (1762-1820), published in London in 1799. (Photo by SSPL/Getty Images)