226 years ago Great Britain was accurately geodetically linked to France for the first time. William Roy, whose vision of a national map of Great Britain led to the founding of the Ordnance Survey, was put in charge of surveying a triangulation scheme originating from a measured five mile base line across Hounslow Heath and running down to the Channel Coast. Angles to common points were observed from both ends of the base line enabling the distances of the two other sides of the triangle to be calculated using trigonometry. From the new measured base lines a network of triangles was observed across South East England with positions eventually fixed at Dover and Hastings. Angles were then measured across to points on the French coast enabling the geodetic relationship between our countries to be calculated. The French had suggested the survey to link the position of the observatories in Greenwich and Paris and had triangulated points from Paris to observe back across to Britain.
This magnificent feat has been chosen as a topic for an edition of BBC Coast and this month I assisted the Coast team filming a demonstration of triangulation in Dover. I’m an Ordnance Survey surveyor based in Kent and was able to use my local knowledge to suggest locations and provide help with equipment and also act as an ‘extra’ if required!
From The Great Tower at Dover Castle, Coast presenter Mark Horton and an historian from the French national survey organisation (IGN) and Ordnance Survey, discussed the Channel Connection and demonstrated a model of the 3-foot Ramsden theodolite used for the original observations.
Then from a 400m base line I had set up in Dover Harbour between two piers Mark used his theodolite to observe angles out to Daniel from IGN who had been taken by boat to the Outer Harbour Wall (acting as France for the day!) and was occupying the lighthouse. The 1787 observations were made at night sighting to tripod mounted bowls of luminescent compounds visible over the Channel. The Coast team (and me!) would have liked to demonstrate this but time and budgets wouldn’t allow! After measuring the angles Mark applied the ‘Law of Sines’ to calculate the distance over the sea to Daniel.
I had a great day with the Coast team experiencing the making of the programme and look forward to seeing it in summer 2014.