Monument at South-Eastern End of General Roy's Survey Base

This is a cast-iron cannon monument installed in 1791. It marks the south-eastern end of the baseline measured by General Roy in 1784 across Hounslow Heath, running from King's Arbour to Hampton Poor House. This baseline is considered the origin of the Ordnance Survey. Initially, Roy had marked the line with wooden posts. In 1791, two condemned cannons were selected for the purpose from Woolwich. The cannon is approximately nine feet long, with the lower six feet buried in the ground. A plaque near the cannon commemorates the 200th anniversary of Major General William Roy's birth and explains his role in establishing the triangulation of Great Britain, which led to the Ordnance Survey. It further details that the measurement was made to determine the relative positions of the Greenwich and Paris Observatories, funded by King George III. The baseline was subsequently re-measured in 1791 by Captain Mudge, and again in 1858 by Clarke, with recorded lengths varying slightly from Roy's original measurement: Roy’s measurement was 27404.01 feet, Mudge’s 27404.24 feet, and Clarke’s 27406.19 feet. The monument at the north-western end of the baseline is located on Northern Perimeter Road, Heathrow Airport, in the London Borough of Hillingdon.