Ordnance Survey observatory perched on the cross on top of St Paul’s Cathedral

The Ordnance Survey’s name indicates its original military purpose, which was to map Scotland in the wake of the Jacobite Rising of 1745. There was also a more general and nationwide need in light of the potential threat of invasion during the Napoleonic Wars. In 1747, Lieutenant-Colonel David Watson proposed the compilation of a map of the Highlands to help to subjugate the clans. In response, King George II charged Watson with making a military survey of the Highlands. The survey was produced at a scale of 1in to 1000 yards (1:36,000). One of Watson’s assistants was William Roy, who later had an illustrious career in the Royal Engineers, rising to the rank of General, and he was largely responsible for the British share of the work in determining the relative positions of the French and British royal observatories. This work was the starting point of the Principal Triangulation of Great Britain (1783-1853), and led to the creation of the Ordnance Survey itself.

From the 1840s, the Ordnance Survey concentrated on the Great Britain ‘County Series’, modelled on an earlier Ireland survey of the 1830s. A start was made on mapping the whole country, county by county, at six inches to the mile (1:10,560).

The ‘Illustrated London News’ of 24 June 1848 reported on the Ordnance Survey’s survey of London being made by the Royal Engineers, and the construction of an observatory for that purpose on top of St Paul’s Cathedral, which can be seen in our pictures ST201 and ST202:

‘In order to conduct the Survey, it is necessary to select high buildings, as principal observing stations; whence “are taken angles with theodolites from the most commanding objects to the other remarkable objects, such as church spires, or towers, cupolas, factory chimneys, &c, for the purpose of ascertaining their relative bearings and distances.”

‘St. Paul’s Cathedral, by its position and commanding height, is, probably, the best station in or about London. Accordingly, there has been erected a scaffold from what is termed the Golden Gallery to the summit of the Cross. Upon this scaffold is a stage, 10 feet square, which supports an observatory, within which is placed upon a table a theodolite 18 inches in diameter of the circle. A railing, roughly but securely put up, surrounds the stage. The observatory is a hexagon figure, 3 feet in diameter, and about 11 feet high, with panelled sides and canvas roof.

… ‘The ascent is by the inside of the tower or lantern to the circular openings; then passing to the outside to the foot-ladders, which are set at the north-east corner, parallel to the north-east principal post, inside the scaffold. The whole of the materials have been drawn up from the floor by a permanent windlass (erected in the tower) to the Golden Gallery; and thence passed to the outside horizontally, through an aperture 32 inches wide. The greatest care has been taken to make the stage secure, and fit for the important use for which it has been constructed. It is about five tons weight; and, deducting the time for stoppages (during the performance of Divine Service in the Cathedral), as well as taking into account all the attendant difficulties, the raising has been by no means slow. The whole has been executed under the superintendence of Corporal Beaton, by direction of Captain Yolland, of the Royal Engineers.

‘The probable number of points to be observed is about 900; and the number of observations may be 1500 or 1800.

‘We subjoin a few additional details, explanatory of the Survey, from the Builder: “In every extensive survey, conducted with a due regard to scientific accuracy, it is a matter of the first importance to determine with unerring correctness the relative distances and bearings of the principal or most conspicuous objects within the country or district to be surveyed. To effect this, the first operation is to measure a ‘base line’—that is, a straight or right line between two points, varying in length according to the extent of the survey and the facilities afforded by the nature of the ground for the measurement … The base line being measured, the next step is to connect its extremities by means of angles taken with theodolites or other angular instruments, with all the conspicuous objects visible; the relative distances and bearings of which with each other, and with the base, become thus determinable by means of certain well-known trigonometrical formulae. By selecting the most suitable of these observed objects for new observing stations, the calculated distances become, in their turn, base lines for expanding the operations of the ‘ triangulation’, until the whole country, by a connected line of observing stations, becomes united or interlaced by a perfect net-work of triangles … The London survey will be connected by its triangulation with the general survey of the country, and in its levelling with the one uniform datum plane to which the altitudes of the Ordnance six-inch map are referred.”’

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