Meridians of the Royal Observatories of Greenwich and Paris, 1799.

UNITED KINGDOM - NOVEMBER 20: Engraving showing a plan of the triangles connecting the meridians of the Royal Observatories of Greenwich and Paris. Other locations used for the survey include Windsor and Dover Castles, and various churches. Places such as Clapham Common, Streatham, Hampstead, Fulham and Battersea are now part of Greater London. 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)