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 for this triangulation was established in 1784 between Hampton Poor House and King's Arbour on Hounslow Heath, a distance of just over 27,400 feet.[1][2] The triangulation was completed in 1787.
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. In 1794 a seven mile long baseline of verification was measured on Salisbury Plain.[3]
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 (1:10,560) map of Ireland was necessary for accurate land taxing.
The Principal Triangulation was subsequently superseded by the Retriangulation of Great Britain some 150 years later.
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The original base-line across Hounslow Heath was measured with deal rods and iron bars to be 27,404 ft. When it was re-measured with 1,370 placements of glass tubes as 27,406 ft. This was corrected for temperature and mean sea-level, and the value and achieved accuracy of 1 inch in 27,404 ft is 3 parts per million.
During subsequent triangulation errors due to atmospheric diffraction, deflection of plumbobs, temperature, the spherical nature of the earth (meaning there was more than 180 degrees in a triangle) were all allowed for.