The Twickenham Museum
For children

A Cannon at Hampton
William Roy and the start of the Ordnance Survey

The Hampton cannon in Roy Grove

Buried cannonIn Roy Grove, Hampton the barrel of a cannon, about 1 metre high is buried upright. Another cannon is buried over five miles away near the tunnel entrance to Heathrow Airport. What are these for? And who put them there?Making accurate mapsBefore the 18th century, making accurate maps over long distances had not been possible. A new method known as triangulation was invented by William Roy. This involved measuring a series or network of triangles to survey and map out a territory.

The plaque by the Hampton cannon

The first triangulationThe first survey, using this new method, was carried out by William Roy in 1784. He needed to find out the accurate positions of the Greenwich and Paris Observatories.

The cannons were buried as fixed marks. The distance between the two cannons was measured accurately using measuring rods. Then, by using this as the original base line he was able to survey and map a series of triangles and find the relative positions of Greenwich and Paris.

The cannon is a reminder of the important connection between Hampton and Ordnance Survey mapping.Major General William Roy FRSMajor-General William Roy (1726-90), after whom Roy Grove is named, was a Royal Engineer and geodesist: a person who makes scientific study of the earth's shape and size.

The tablet next to the cannon in Roy's Grove says "He conceived the idea of carrying out the triangulation of the country and of constructing a complete and accurate map and thereby laid the foundations of the Ordnance Survey".

After Roy's death in 1790 his work was gradually extended outward from the south-east of England and by 1823 covered much of the country.

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