Retriangulation of Great Britain
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.
[5] 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.
[3] 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 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.
[3] The corresponding local scale factor must be employed to convert a site measured plane length to a projection distance, and vice versa.
[3][11] 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.
[3][7] The triangulation was still incomplete at the outbreak of World War II, with five of the seven blocks completed, and two main baselines (one between Whitehorse Hill and Liddington Castle, and the second in Lossiemouth) measured.
A member of the survey team suffered a dislocated shoulder when he was attacked by Arctic skuas, whose nesting had been inadvertently disturbed by his work.
In addition, a connection with France was made across the Strait of Dover in collaboration with the Institut national de l'information géographique et forestière.
The Kippure to South Barrule (Isle of Man) ray, 95 miles long and obscured by smog from Dublin, was eventually abandoned.
[6] On 3 September 1952, work began to observe the longest ray in the entire retriangulation, measuring 98 miles (158 km) between the Preseli mountains (Wales) and Ballycreen in County Wicklow.
[3] The Ordnance Survey Ireland team then moved to the Hill of Tara and Forth Mountain in Wexford, but deteriorating weather conditions meant that the work could not be completed until 8 October 1952.
[6] From July to September 1953, the US Air Force used HIRAN to survey a link between three geodetic stations in Norway and three on the Scottish mainland and Shetland islands.
[6] The operation was largely successful, but the Ordnance Survey considered that the results were not of a geodetic standard necessary for primary triangulation, and a 12 metres (39 ft) discrepancy existed in the measurements between Norwegian stations.
[3] Concurrently with the retriangulation programme, a procedure was put in place for overhauling and updating 1:2500 Ordnance Survey maps in dense urban areas.
The first stage required the old maps to be updated to eliminate distortions in size and shape, aligning them with the new projections and control from the retriangulation process.
Two solutions emerged: a complete resurvey, or fixing and incorporating additional control in a way that restored the overhaul accuracy standard at a significantly lower cost.
