In 1935, the new Director General of the Ordnance Survey, Major-General Malcolm MacLeod, started the retriangulation of Great Britain, an immense task which involved erecting concrete triangulation 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.
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 6000 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 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 accurate plotting of the entire country, varying in accuracy from 20 metres to up to 1 metre (depending on the scale of the map). 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 mm 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 As a result of this, the trig point network is no longer actively maintained, except for a few trig points that have been reused as part of the Ordnance Survey's National GPS Network.

See also

Geodesy
OSGB36
Reference frame
Triangulation