Retriangulation of Great Britain
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The retriangulation of Great Britain was a triangulation project which involved erecting concrete pillars (trig points) on prominent hilltops throughout Great Britain, then computing their relative positions to the best possible accuracy and using this information to create a new basis for Britain's mapping system. The aim was to replace the original triangulation of Britain, known as the Principal Triangulation, which had been performed between 1783 and 1853, with a more modern and accurate triangulation.
It was commenced in 1935 by the new Director General of the Ordnance Survey, Major-General Malcolm MacLeod. The effort was directed by the cartographer and mathematician Martin Hotine, head of the Trigonometrical and Levelling Division, who planned the operation in a manner similar to a military campaign. Every detail of the operation and measurements was carefully specified in advance to attempt to produce the most accurate measurements possible, given available technology.
Erecting new trig points and making measurements frequently required materials and instruments to be carried on foot, up hills and mountains and to isolated islands, in all kinds of weather. The network of trig points was built and measured between 1936 and 1962, starting with a set of several hundred primary trig points, most of which were placed on high hills so as to be able to link to one another across long distances. In addition, a larger set of roughly six thousand secondary trig points were added to allow the construction of a finer mesh that extended the reference frame of the primary mesh over shorter distances.
The calculations were constrained; it was hoped to minimise the shifts from the coordinates based on the old triangulation. At eleven primary trig points from Dunnose on the Isle of Wight (456784 m E, 080150 m N) north to Great Whernside in Yorkshire (400202 E, 473904 N) the new lat-lons were adjusted to stay within a metre of the old ones. Once the new Latitude and Longitude of those eleven points were fixed the calculated location of every other point in the triangulation was based on them. By the time the retriangulation was completed, electronic distance-measuring devices had come into use which could have greatly reduced the overall error (it now seems Great Britain is 20+ metres shorter than OSGB36 implies) but starting over from scratch was out of the question.
The results of the retriangulation were used to create the British national grid reference system which became the basis of the Ordnance Survey's new maps. The retriangulation generated a co-ordinate system which is still used today, and which allows plotting of the entire country with a relative accuracy from 20 metres (66 ft) over the scale of the whole country, down to less than 1 metre (3 ft 3 in) over distances of a few tens of kilometres (the more local the area covered by the map, the smaller the possible distortions).
The triangulation method of surveying has been rendered redundant for many surveying applications by more convenient satellite-based GPS measurements, which can obtain an accuracy of 3.5 metres (11 ft)  for best cases which deteriorates due to multipath, irregularities in troposphere propagation delay and electromagnetic interference. As a result, much of the trig point network is no longer actively maintained due to financial reasons, except for a few trig points that have been reused as part of the Ordnance Survey's National GPS Network.
Notes and references
- GPS and Positioning FAQ, Q4, Ordnance Survey, accessed 2007-11-25
- FAA GPS Performance Analysis Report
- Happy birthday to the Trig Pillar – 75 years young today – Ordnance Survey Blog
- Ordnance Survey (1967). The history of the retriangulation of Great Britain 1935-1962 (PDF). London: Her Majesty's Stationery Office. Diagrams.
- Owen, Tim; Pilbeam, Elaine (1992). Ordnance Survey, map makers to Britain since 1791. Southampton: Ordnance Survey (HMSO). ISBN 9780319002490. OCLC 28220563. Freely available online at the Ordnance Survew, Owen and Pilbeam
- Seymour, W. A., ed. (1980). A History of the Ordnance Survey. Folkestone, England: Dawson. ISBN 0-7129-0979-6. OCLC 654935343. Freely available online at the Ordnance Survey, Seymour
- Trigpointing UK
- Daily Telegraph article about the retriangulation of Great Britain
- National GPS Network
- Information and Maps on many aspects of Trigangulation (& Levelling) in Great Britain
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