Line engraving of details of the first great theodolite (three-foot horizontal circle). The theodolite was constructed by Jesse Ramsden (1735-1800) between 1784 & 1787 for General William Roy of the Royal Engineers for the Principal Triangulation survey to link the observatories of Greenwich and Paris. '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' Vol 80, 1790.
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