Monument at North Western End of General Roys Survey Base

This is a cast-iron cannon monument, installed in 1791 to mark the north-western end of a baseline surveyed by General Roy in 1784 across Hounslow Heath. The baseline stretched from King's Arbour to Hampton Poor House and is considered the origin of the Ordnance Survey. Initially, General Roy had marked the line using wooden posts. In 1791, two condemned cannons were selected from Greenwich to serve as markers. The cannon is approximately nine feet long, with its lower six feet buried in the ground. A plaque affixed in 1926 commemorates the 200th anniversary of the birth of Major General William Roy (1726-1790), who conceived the triangulation of Great Britain and laid the foundation for the Ordnance Survey. The plaque details how the baseline was measured in 1784 under Roy’s supervision for determining the relative positions of the Greenwich and Paris Observatories, with the work facilitated by the support of King George III, who inspected the project on August 21, 1784. The base was re-measured in 1791 by Captain Mudge, initiating the principal triangulation of Great Britain. The plaque provides measurements of the base: 27404.01 feet as measured by Roy, 27404.24 feet as measured by Mudge, and 27406.19 feet as determined by Clarke in 1858 using the Ordnance Survey Standard 01. A corresponding monument marking the south-east end of General Roy's baseline is listed at Roy Grove, Hampton.