Editing Heathrow (hamlet) (section)

===General Roy's western baseline===
{{main|Anglo-French Survey (1784–1790)}}
In 1784 General [[William Roy]] chose the orchard of King's Arbour to be one end of first base line of the [[Anglo-French Survey (1784–1790)]] trigonometrical survey for the first triangles of a triangulation grid reaching across the [[English Channel]].  He chose Hounslow Heath for his lines as it was near-flat, near barracks and about 15 miles from the [[Royal Observatory, Greenwich|Royal Observatory]].  The east/south end was the [[Poor House]] in [[Hampton, Middlesex|Hampton]]. The ends were originally marked by vertical wooden pipes (which could support flagstaffs), but in the resurvey of 1791 they were found to be rotting and were replaced by upright [[cannon]] heads which are still to be seen.<ref>For full information and references see [[Anglo-French Survey (1784–1790)]]</ref>

The marker and landmarks on the Bath Road enables visitors and historians to picture features on old maps when visiting today's airport, without the use of grid references.<ref>[[:File:Heathrow Before World War II Map.jpg#Road names]]</ref>