The Principal Triangulation of Britain was the first high-precision triangulation survey of the whole of Great Britain and Ireland, carried out between 1791 and 1853 under the auspices of the Board of Ordnance. The aim of the survey was to establish precise geographical coordinates of almost 300 significant landmarks which could be used as the fixed points of local topographic surveys from which maps could be drawn. In addition there was a purely scientific aim in providing precise data for geodetic calculations such as the determination of the length of meridian arcs and the figure of the Earth. Such a survey had been proposed by William Roy (1726–1790) on his completion of the Anglo-French Survey but it was only after his death that the Board of Ordnance initiated the trigonometric survey, motivated by military considerations in a time of a threatened French invasion. Most of the work was carried out under the direction of Isaac Dalby, William Mudge and Thomas Frederick Colby, but the final synthesis and report (1858) was the work of Alexander Ross Clarke. The survey stood the test of time for a century, until the Retriangulation of Great Britain between 1935 and 1962.
History
In the aftermath of the Jacobite rising of 1745, the Duke of Cumberland (military commander against the rebels) – advised by Lieutenant-Colonel David Watson, a Deputy Quartermaster-General of the Board of Ordnance – recognised that there was a need for an accurate map of the Scottish Highlands. Watson initiated the necessary survey in 1747. Watson employed William Roy as a civilian assistant to carry out the bulk of the work. Subsequently Roy, having taken a commission in the Corps of Engineers as a "practitioner engineer", and having become a very competent surveyor, proposed (in a memorandum to the king dated 24 May 1766) a national survey which would be a plan for defence at a time when French invasions were threatened. The proposal was rejected on grounds of expense. Roy continued to lobby for a survey and his ambitions were realised to a certain extent by an unexpected development. In 1783 the French Academy of Sciences claimed that the latitude and longitude differences between the Royal Observatory, Greenwich and the Paris Observatory were incorrect, and it was proposed (to the Royal Society) that the differences could be reconciled by high precision triangulation over the intervening terrain. The Royal Society agreed and, jointly with the Board of Ordnance, they invited Roy to oversee the project.
Hounslow Heath baseline Roy's first task (1784) was to measure a baseline between Hampton Poor House (51.42633°N 0.36617°W / 51.42633; -0.36617) and King's Arbour (51.4798°N 0.4503°W / 51.4798; -0.4503) on Hounslow Heath, a distance of just over 5 miles (8 km). This was a painstaking process: three rods each of 20 ft. were supported on trestles and the ends aligned to an accuracy of a thousandth part of an inch. The first rod was then carried to the end of the third, an operation to be repeated 1,370 times. The first set of rods, from instrument maker Jesse Ramsden, was made of wood and, in the particularly wet weather of the Summer of 1784, these were found subject to warping and expansion. After delay of two weeks, Ramsden delivered a new set of rods, now made of solid glass tubing. Mounted in rigid wooden boxes with open ends, these had a sprung brass pin mounted against an ivory scale at one end, and this showed when the rods were joined exactly. The final measurement gave the length of the base as 27,404.01 feet (8,352.74 m).
For the subsequent triangulation, Roy ordered a new theodolite from Jesse Ramsden. This Ramsden theodolite, delivered in 1787, for the first time divided angular scales accurately to within a second of arc. The theodolite was the largest ever constructed but, despite its massive size, it was carried from London to the Channel coast and employed on hills, steeples and a moveable tower. At each location the angles to other vertices of the triangulation mesh were measured many times, often at night time using newly devised lights. Finally the angle data was used to calculate the sides of the triangles by using spherical trigonometry. The final results were inconclusive, for triangulation was inferior to the precision of astronomical measurements, but the survey paved the way for all future work in terms of high precision measurements of length and angle, together with the techniques of calculating on an ellipsoidal surface. In his final report, published posthumously, Roy once again pressed for the extension of the survey to the rest of Britain. Charles Lennox, 3rd Duke of Richmond, as Master of the Board of Ordnance from 1782, viewed Roy's work with great interest. At the same time he was acutely aware that Britain, lacking a national survey, was falling behind the standards of many other European countries. Moreover, the renewed threat of French invasion made him alarmed at the lack of accurate maps, particularly of the southern counties. Consequently, in 1791, he put into action Roy's plan for the extension of the survey. The catalyst was the sudden availability of a new improved Ramsden theodolite which had been intended for the East India Company. The purchase of this instrument on 21 June 1791 by the Board is taken as the inauguration of the Ordnance Survey. The very next day Richmond appointed Isaac Dalby as its first employee, with a brief to extend Roy's survey. In the following month Richmond appointed two officers of the Royal Artillery, Major Edward Williams and Lieutenant William Mudge, as directors.
Re-measurement of the Hounslow baseline
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