Plan whereby the distance between the Observatories of Paris and Greenwich was determined. 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 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. Illustration from the Philosophical Transactions of the Royal Society, 1790.
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