A cannon in Roy Grove marks the Hampton end of the baseline measured in 1784 by General William Roy in preparation of the Anglo-French Survey (1784–1790) to measure the relative situation of Greenwich Observatory and Paris Observatory. This high precision survey was the forerunner of the Principal Triangulation of Great Britain which commenced in 1791, one year after Roy's death. In the report of the operation Roy gives the locations of the ends of the baseline as Hampton Poor-house and King's Arbour.