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. 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.
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. At the outbreak of the war, the Ordnance Survey regional offices in
Bristol,
Tunbridge Wells,
London, and
Edinburgh were reduced to a
care and maintenance basis, with only occasional activity connected to wartime survey projects. This was the situation until 1944, when an increase in staff levels was made by men returning from war service. Shortly after World War II, the US Air Force had carried out a readjustment of all the triangulations of continental Europe to produce a
geodetic datum known as
ED50, a single system on the
Universal Transverse Mercator coordinate system. The North Atlantic Tie initiative aimed to create a
geodetic link between North America and Europe, by measuring a
trilateration network, and permitting the positioning of European triangulation stations relative to the
North American Datum. 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. This marked the initial phase of a larger project which connected surveys of Norway, Iceland, and
Greenland to
Canada. The network linking Scotland to Norway comprised fifteen measured lines: three among the Norwegian stations, three among the Scottish and Shetlandic stations, and nine lines across the
North Sea. The most inaccurate of the rejected survey missions deviated from the accepted measure by 0.0055 miles (29 feet), and the average disparity between a rejected measure and the mean of the accepted measures was 0.0013 miles (6 feet). The final results and assessment were computed from observation of ground survey positions, including stations in both Iceland and the
Faroe Islands. 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 discrepancy existed in the measurements between Norwegian stations. == The Cotswolds adjustment ==