The
retriangulation of Great Britain was a
triangulationIn trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...
project which involved erecting concrete pillars (
trig pointA trigonometrical station / point, triangulation pillar, trig beacon or trig point is a fixed surveying station for the geodetic surveying and other surveying projects on nearby areas...
s) on prominent hilltops throughout
Great BritainGreat Britain is an island lying to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island. With a population of about 59.6 million people, it is the third most populated island on Earth. Great Britain is surrounded by over 1000 smaller...
. The aim was to replace the original triangulation of Britain, known as the
Principal TriangulationThe Principal Triangulation of Britain was a triangulation project carried out between 1783 and about 1853 at the instigation of the Director of the Ordnance Survey General William Roy ....
, 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 SurveyOrdnance Survey is an executive agency of the United Kingdom government. It is the national mapping agency for Great Britain, and one of the world's largest producers of maps...
, Major-General
Malcolm MacLeodMajor-General Malcolm Neynoe MacLeod was Director General of the Ordnance Survey from 1935 to 1943.In 1935 he started the retriangulation of Great Britain, an immense task which involved erecting concrete triangulation pillars on prominent hilltops throughout Britain...
. 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 were carefully specified in advance to attempt to produce the most accurate measurements possible given the then-current 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 weathers. 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 which would extend the reference frame of the primary mesh over shorter distances.
The results of the retriangulation were then used to create the
British national grid reference systemThe British national grid reference system is a system of geographic grid references commonly used in Great Britain, different from using latitude and longitude....
which would be the basis of the
Ordnance SurveyOrdnance Survey is an executive agency of the United Kingdom government. It is the national mapping agency for Great Britain, and one of the world's largest producers of maps...
's new maps. The retriangulation generated a co-ordinate system which is still used today, and which allows accurate plotting of the entire country, achieving a relative accuracy from twenty metres over the scale of the whole country, down to less than a metre over distances of a few tens of kilometres (the more local the area covered by the map, the smaller the possible relative distortions). It represented a triumph of the available technology at the time.
However, the triangulation method of surveying has now been rendered obsolete by satellite-based GPS measurements, which can obtain a precision of 15 m from end-to-end, with re-measurements taking hours rather than years. As a result of this, the trig point network is no longer actively maintained, except for a few trig points that have been reused as part of the Ordnance Survey's National GPS Network.
Further reading
- Ordnance Survey 1967. The history of the retriangulation of Great Britain 1935-1962. HMSO, London.
External links