Cannon fodder

This post is a modest entry for the Your Nearest Site carnival....
In a Hampton cul-de-sac, a couple of streets from my house, an upturned cannon sits in the ground surrounded by a patch of grass in the middle of a small council estate.

It is a interesting relic of General William Roy's pioneering late C18th triangulation work which laid the basis for the Ordnance Survey.

The photo is courtesy the Twickenham Museum website, which tells the story of how Roy, a Royal Engineer, used triangulation to measure the distance between the Paris and Greenwich observatories. Until the advent of GPS, triangulation was the only means to measure distances over water.

The other end Roy's baseline, 5 miles to the north, is marked by another cannon in a slightly more noisy location on the perimeter road around Heathrow Airport.

To quote the Royal Engineers Museum site, "This line was measured in the summer of 1784, three times over, by means of cased glass tubing, seasoned deal rods and a steel chain. The discrepancy between these three methods was less than 3 inches".