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 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 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 (456784 m E, 80150 m N) north to Great Whernside (400202 E, 473904 N) the new lat-lons were adjusted to stay within a metre of the old ones. Once the new lat-lons of those eleven points were finally 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 presumably out of the question.
The results of the retriangulation were used to create the British national grid reference system which would be 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 twenty metres over the scale of the whole country, down to less than a metre over distances of a few tens of kilometres[1] (the more local the area covered by the map, the smaller the possible 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 a metre or better from end-to-end, with re-measurements taking hours rather than years. As a result 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.