THE ORDNANCE SURVEY, TRIANGULATION PILLARS AND BENCH MARKS

The origins of the Ordnance Survey may be said to go back to a government survey of Scotland (unpublished), undertaken for military purposes between 1747 and 1755 after the Jacobite uprising of 1745. The next identifiable step occurred in 1783–4 when the Royal Societies of London and Paris arranged a triangulation system to confirm the relative positions of the astronomical observatories in the two cities. (Triangulation is the method of determining the location of a point by forming triangles from other known points). For this purpose the Royal Society in London purchased a very accurate three-foot theodolite from Jesse Ramsden, who was one of the finest makers of scientific instruments in the eighteenth century. The English part of the operation was placed under the direction of Major-General William Roy (1726–1790) and was completed in 1790. The following year the Duke of Richmond, the Master-General of the Ordnance, arranged the purchase of a very similar three-foot theodolite by Ramsden so that surveys could be continued independently of the Royal Society if need be. The expenditure was authorised by the Board of Ordnance on 21st June 1791 and this is accepted as the ‘foundation date’ of the Ordnance Survey. Using both theodolites the Board of Ordnance carried out a survey of much of England over the following twenty years. The first ‘Ordnance Survey’ map to be published was of the County of Kent on four sheets at the scale of 1:63,360 (one inch to the mile) in 1801. The Principal Triangulation of Great Britain, the first really accurate trigonometric survey of the country, was eventually completed in 1853.

The development of the Ordnance Survey as the UK’s national map agency is too extensive to be discussed here and those desirous of learning more are directed to the publications of the Charles Close Society for the Study of Ordnance Survey Maps, see https://charlesclosesociety.org/ or to the books listed below. Here it is sufficient to record that in 1990, after nearly two hundred years as a government department, the Ordnance Survey became an Executive Agency with its own budgetary responsibilities. Since 2015 it has operated as Ordnance Survey Ltd, a government-owned company. It is currently (2020) accountable to the Secretary of State for Business, Energy & Industrial Strategy.

Until recent times the Ordnance Survey’s production of accurate maps and plans relied largely upon triangulation and levelling using a variety of precision instruments. By the 1930s the Principal Triangulation required revision to enable a new series of large-scale surveys to be undertaken to meet the requirement that the future 1:2500 plans be recast on National rather than County sheet lines. Work on the new triangulation began in 1935. The triangulation stations were marked by a new design of standard pillar marker that would be strong enough to withstand damage from a variety of causes including building work or disturbance by archaeologists. The pillar was in the shape of a truncated concrete pyramid, 4 feet high and 2 feet square at the base. It incorporated a brass ‘spider’ with three arms 120 degrees apart upon which the instrument was mounted and automatically centred. Vertically below were two mark bolts, one at surface level and the other below ground, both set in separate and mutually isolated concrete blocks. A brass flush bracket (see below) affixed to the side provided a bench mark for heighting by spirit levelling. Eventually about 6500 such pillars were erected, sited on the tops of mountains and in other prominent locations throughout the country. These are the ‘trig pillars’ so familiar to hill-walkers.

MILITARY BOUNDARY MARKERS

The use of inscribed boundary stones to indicate the limits of military sites goes back to the eighteenth century. A useful brief account of their history can be found here: https://frontlineulster.co.uk/wd-boundary-stones/ . The earliest were marked B O (Board of Ordnance) but this became W D after the Board was disbanded in 1855 and replaced by the War Department. As an official name this was of short duration, becoming the War Office in 1857. It appears to have been convenient to perpetuate the name War Department for various applications, in particular for the supply of railway locomotives and rolling stock for military purposes both at home and overseas, and also for the designation of military property generally. Thus the Army’s boundary stones continued to be marked W D. The corresponding stones for the Admiralty were marked by a fouled anchor and those for the Air Ministry by A M.

Boundary stones continued to be marked with the broad arrow and W D or the other services’ equivalents until 1964 when the Army, the Royal Navy and the Royal Air Force were placed under unified control in the Ministry of Defence. From that time onwards new boundary stones were marked with M O D and the broad arrow. At an unknown date the design of the arrow also changed from the distinctive three tapered strokes meeting at the top to a simplified symbol recalling those used on direction signs in hospitals or supermarkets. The reasons for changing from a long-standing familiar and recognizable mark to something bland and characterless appear at best opaque.