�General View of the Instrument�, 1799.

UNITED KINGDOM - NOVEMBER 20: Engraving. 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). 'The Primary Triangulation of Great Britain' was a network of measured triangles across the country, to underpin the first general survey of the country. The network was checked in a number of places by setting up a baseline, several miles long, and comparing measured and calculated dimensions. The first line was used on Hounslow Heath, London, in 1784. 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)