Anglo-French Survey (1784–1790)

The Anglo-French Survey (1784–1790) refers to the survey to measure the relative situation of Greenwich Observatory and the Paris Observatory. The English operations, executed by William Roy, consisted of the measurements of bases at Hounslow Heath (1784) and Romney Marsh (1787), the measurements of the angles of the triangles (1787–1788) and finally the calculation of all the triangles (1788-1790). The survey is very significant as the first precise survey within Britain, and the forerunner of the work of the Ordnance Survey which was founded in 1791, one year after Roy's death.

Late in life, when he was 57, Roy was granted the opportunity to establish his lasting reputation in the world of geodesy. The opening came from a completely unexpected direction. In 1783 the Comte de Cassini addressed a memoir to the Royal Society in which he expressed grave reservations of the measurements of latitude and longitude which had been undertaken at Greenwich Observatory. He suggested that the correct values might be found by combining the Paris Observatory figures with a precise trigonometric survey between the two observatories. (The French surveys had already been carried out in the course of the preparation of the .) This criticism was roundly rejected by Nevil Maskelyne who was convinced of the accuracy of the Greenwich measurements but, at the same time, he realised that Cassini's memoir provided a means of promoting government funding for a survey which would be valuable in its own right. Approval was granted and Sir Joseph Banks, president of the Royal Society, proposed that Roy should lead the project. Roy gladly accepted and set matters in motion by submitting to the Crown a grossly-underestimated budget for manpower (by far the largest element) and new precision instruments to be constructed by Jesse Ramsden. The whole project is described in Roy's three large articles in the Philosophical Transactions of the Royal Society of London in 1785, 1787 and 1790. There are shorter accounts of the project in the History of the Royal Engineers by Porter and in every history of the Ordnance Survey, particularly the book by Seymour and also that by Owen and Pilbeam.

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