Plan of the base measurement on Hounslow Heath, Summer 1784.

Plate taken from the summer, 1784 edition of 'Philosophical Transactions', a journal of the physical sciences and engineering. '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 William Roy of the Royal Engineers.