The Retriangulation of Great Britain was a triangulation project carried out between 1935 and 1962 that sought to improve the accuracy of maps of Great Britain. Data gathered from the retriangulation replaced data gathered during the Principal Triangulation of Great Britain, which had been performed between 1783 and 1851. The work was designed to form a complete new survey control network for the whole country, and to unify the mapping of the United Kingdom from local county projections into a single national datum projection and reference system. Its completion led to the establishment of the OSGB36 datum and Ordnance Survey National Grid in use today.
History and overview
The retriangulation was begun in 1935 by the Director General of the Ordnance Survey, Major-General Malcolm MacLeod. It was directed by the cartographer and mathematician Martin Hotine, head of the Trigonometrical and Levelling Division (TLD). The work was halted by the outbreak of World War II in 1939, by which time the primary triangulation network covered all of England and Wales, but only as far as the Moray Firth in Scotland. Secondary triangulation had commenced in 1945, and after the end of the war, the retriangulation work was focused on secondary and lower-order survey work, to expedite the completion of new large-scale surveys. The wartime priorities of the TLD were focused on survey work in connection with the war effort, such as airfield and military construction, survey and computations for anti-aircraft and coastal battery positions, and survey of radiolocation sites. One-third of the Ordnance Survey staff were called up during the war, and the headquarters in Southampton was bombed and badly damaged. Staff were relocated to the Home Counties, where they produced 1:25,000 scale maps of France, Italy, Germany and most of the rest of Europe in preparation for invasion. Primary triangulation observations were not resumed until 1949, and completed in 1952. A problem during the Principal Triangulation was that the exact locations of surveying stations were not always rediscoverable, relying on buried markers and unreliable local knowledge. To overcome this, a network of permanent surveying stations was built, most familiarly the concrete triangulation pillars (about 6,500 of them) found on many British Isles hill and mountain tops, but there were many other kinds of surveying stations used. To minimise differences between the 1783–1851 survey and the retriangulation, eleven Principal Triangulation stations, ranging from Dunnose on the Isle of Wight to Great Whernside in Yorkshire, were chosen and pillars erected on them to act as the core framework from which all other measurements were made. The main work of the Retriangulation was finished in 1962, creating the Ordnance Survey National Grid. This system continued to be used, and measurements refined by ground-based surveying, into the 1980s, after which satellite use took over. Electronic measuring devices were introduced towards the end of the Retriangulation, but at that time were not proven reliable enough to replace traditional surveying.
The Primary Retriangulation and survey field work
One of the first steps in the retriangulation was the adoption of a new projection for the mapping, with the existing Cassini projection replaced by the Transverse Mercator. This was preferred by the Ordnance Survey because the use of the Cassini projection would have resulted in angular distortion of almost four minutes of arc in the survey.
Planning and work on the new projection Neither the solid form of the Earth, nor the geoid (an equipotential surface of Earth's gravity field, comparable to the imaginary surface that mean sea level would follow in the absence of winds, tides and currents), can be fully defined by simple formulae. The spheroid is the nearest mathematical model, but as no one spheroid fits worldwide, a number have to be used. The Airy spheroid provides a good fit in the region of the British Isles, and the Transverse Mercator Projection of this spheroid was therefore adopted by the Ordnance Survey as the basis of the national co-ordinate system. No projection can be true to scale across its entirety. In the Transverse Mercator, the scale at any given point increases in correlation with its east or west distance from the central meridian. The scale along the north-south line that contains the point remains consistent. The true origin of the projection lies at latitude 49° N, longitude 2° W. A false origin positioned roughly 170 kilometres west of The Lizard was established to ensure all national grid coordinates remained positive, as the whole country is further east and further north than that point. In this system, the central meridian is 400 km east. The scale on the central meridian should be correct, or 1. However, to ensure that scale error is imperceptible on the national mapping at the eastern and western boundaries, a scale reduction of 1:2500 was applied. This provides a local scale factor of 0.9996 at the central meridian. The scale continually increases with distance from the central meridian, east and west, reaching 1 at 580 km east and 220 km west. It continues to rise, reaching 1.0005 at the eastern and western extremes. The corresponding local scale factor must be employed to convert a site measured plane length to a projection distance, and vice versa. As the spheroid is set at mean sea level, any surveyed length must be reduced to mean sea level before applying the local scale factor.
Commencement of retriangulation fieldwork The primary triangulation work commenced with the division of survey work into blocks. The size of these blocks was governed by the largest number of survey observations which could be computed in a simultaneous least-squares adjustment. Reconnaissance of survey stations was commenced in 1935, using Tavistock theodolites to confirm the inter-visibility of stations. Survey of the triangulation commenced in April 1936, with observations made during the hours of darkness to electric beacon lamps manufactured by Cooke, Troughton & Simms. In flat areas of the country, such as East Anglia, Bilby towers designed by the United States Coast and Geodetic Survey were used.
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