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. Roy's scientific legacy Roy's use of scientific advancements and accurate mathematical formulae paved the way for modern geodesic surveying. His tenure and his work are the dividing line between older, approximate mappings and newer, highly accurate ones in Britain. He is cited repeatedly in early nineteenth-century mathematics textbooks for his use of spherical trigonometry in surveying. Early twentieth-century technical books on modern surveying and geodesy include Roy's work as the historical starting point for the modern profession. Perhaps his greatest legacy of all was the work of the Ordnance Survey which commenced in 1791, one year after his death, by extending the basic Anglo-French