The Principal Triangulation of Britain was a triangulation project carried out between 1783 and about 1853 at the instigation of the Director of the Ordnance Survey General William Roy (1726-1790).

In 1782, General Roy had commissioned the building of the Ramsden theodolite from leading instrument maker Jesse Ramsden. The Ramsden theodolite for the first time divided angular scales accurately to within a second of arc. General Roy and his team used it to accurately triangulate the distance between the London and Paris observatories.

The baseline derived during that work, together with the new theodolite, served as the basis for the planning and execution of the subsequent work on the Principal Triangulation. Around 1791, shortly after his death, Roy's team began the field work, using the specially built Ramsden theodolite. The base line was on Salisbury Plain.

Eventually the triangulation extended to cover the whole of the British Isles, after it was decided in 1824 that a 6-inch-to-the-mile map of Ireland was necessary for accurate land taxing.

The Principal Triangulation was subsequently superceded by the Retriangulation of Great Britain some 150 years later.

External links

• PDF file including history and map of the Irish part and its links to Britain