The first Ramsden three-foot theodolite, 1790.
© Science Museum / Science & Society Picture Library
Engraved plate after Milne showing a plan view of the theodolite used in the survey to connect the Paris and Greenwich Observatories. 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 by General Roy. Illustration from the 'Philosophical Transactions of the Royal Society', 1790.