page_title: William Roy
section_title: The Anglo-French survey
hierarchical_section_title: William Roy / Roy as surveyor and soldier / The Anglo-French survey
caption_attribution_description: English: Cannon marking the northern end of William Roy's Hounslow baseline
context_page_description: Major-General William Roy FRS FSA FRSE was a Scottish military engineer, surveyor, and antiquarian. He was an innovator who applied new scientific discoveries and newly emerging technologies to the accurate geodetic mapping of Great Britain. His masterpiece is usually referred to as Roy's Map of Scotland.
  It was Roy's advocacy and leadership that led to the creation of the Ordnance Survey in 1791, the year after his death. His technical work in the establishment of a surveying baseline won him the Copley Medal in 1785. His maps and drawings of Roman archaeological sites in Scotland were the first accurate and systematic study of the subject, and have not been improved upon even today. Roy was a fellow of the Royal Society and a member of the Society of Antiquaries of London.
context_section_description: 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 Cassini de Thury 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. 
  Sir Joseph Banks, president of the Royal Society, proposed that Roy should lead the project. Roy accepted with enthusiasm for he saw that apart from the specific measurements proposed the survey could be the first step towards the national survey that he had advocated so often. The whole project is described by Roy in three major contributions to the Philosophical Transactions of the Royal Society  in 
  1785,
  1787 and
  1790.
  There are shorter accounts of the project in the History of the Royal Engineers,
  the records of the Royal Society
  and in every history of the Ordnance Survey.
  After a preliminary survey by Roy and three other members of the Society on 16 April, they found a suitable location for the starting baseline on Hounslow Heath, between King's Arbour and Hampton Poor-house just over 5 miles to the south-east. A preliminary measurement of the line was carried out with a steel chain prepared by Jesse Ramsden. It was the intention to measure more accurately with a set of three deal rods about 20 ft. in length but their use had to be abandoned because of their susceptibility to lengthen and shorten in wet weather. The deal rods were replaced by one-inch-thick glass tubes of the same length. The final measurement gives the length of the base as 27404.7 ft. to an accuracy of about 3 inches in 5 miles (or about 1/100,000). The precision of this baseline measurement far exceeded any previous attempts, and in recognition Roy was awarded the Copley medal  by the Royal Society in 1785.
  The triangulation itself was delayed until 1787 when Ramsden supplied a new theodolite of unsurpassed accuracy: it could measure angles to within one arc second and therefore detect the curvature of the Earth by measuring the 
  spherical excess of the triangles of the survey. By the end of that year he had completed measurements at all but two of the trigonometric stations. Many of the measurements, particularly the cross channel sightings,  were taken at night using intense flares (handled by the artillery). Others required the placing of the instrument on church towers, or even on scaffolded steeples, and in their absence it was sometimes necessary to use a specially constructed portable tower some 30 feet high.
  The final report of 1790 presents figures for the distance between Paris and Greenwich as well as the precise latitude, longitude and height of the British triangulation stations. Throughout the survey Roy took every opportunity to fix the position of as many landmarks as possible and these formed the basis of the topographic surveys from which new maps could be prepared. Roy died when only three pages of his final report remained to be proofed.
caption: Cannon near Heathrow Airport Northern Perimeter Road, marking the northern end of the baseline
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