Retriangulation of Great Britain

The retriangulation of Great Britain was a triangulation project which involved erecting concrete pillars (trig points) on prominent hilltops throughout Great Britain. 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.

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