Roy Grove was a surveyer who measured very accurately the baseline for the Ordannce Survey. This cannon and monument marks the Hampton end of the first baseline of the triangulation of Great Britain. General William Roy was just 3 inches off.
The baseline for this triangulation went from Hampton Poor House and King's Arbour on Hounslow Heath, a distance of just over 27,400 feet. The triangulation was completed in 1787.
His actions lead to the creation of the Ordnance Survey in 1791. He was dead already one year at the moment.