'General View of the Instrument', 1799.
© Science Museum / Science & Society Picture Library
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.