Principal Triangulation of Great Britain
In 1782, General Roy 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. 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.
The Principal Triangulation was subsequently superseded by the Retriangulation of Great Britain some 150 years later.
Baseline measurement 
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 achieved an 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 were more than 180 degrees in a triangle) were all allowed for.
See also 
- Major General William Roy biography, Royal Engineers Museum
- Michael Knowles, THE FRENCH CONNEXION – BETWEEN ENGLISH AND FRENCH MAP SURVEYS, Proceedings of the BRLSI Volume 7 2003
- Lieut.-Colonel T. Pilkington White, The Ordnance Survey Of The United Kingdom, Scottish Mountaineering Club Journal Volume 2 Number 5
Further reading 
- Rachel Hewitt (2010), Map of a Nation, London: Granta Books. ISBN 978-1-84708-254-1
- PDF file including history and map of the Irish part and its links to Britain
- Information and Maps on many aspects of Triangulation (& Levelling) in Great Britain
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