Telescope used by General Roy, Hounslow Heath, Middlesex, 1784.
© Science Museum / Science & Society Picture Library
Telescope used by General William Roy during the time he carried out his ground-breaking survey of Britain from 17for measuring base line to link Paris and Greenwich, with box. Collar engraved with inscription R S 53. The principal triangulation of Britain was a surveying project undertaken between 1787 and 1790 by General William Roy, to link the observatories of Greenwich and Paris, using theodolites made by Jese Ramsden (1735-1800). 'The Primary Triangulation of Great Britain' was a network of measured triangles acros 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 by General William Roy of the Royal Engineers.