Description and History of Site:-
North West marker of the baseline is located about 30 metres north of the Perimeter Road with access by footpath. Major-General William Roy (1726-1790) conceived the idea of surveying Britain by triangulation. A half buried cannon replaced the original wooden marker in 1791. The cannon was removed 1944 but later returned to its original position. The baseline runs from here to the SE datum in Hampton, 27406.19 feet away (see entry for General Roy's cannon). Commemorative plaque to the north in Nene Road.
The baseline across Hounslow Heath was the initial English reference line for the Anglo-French Survey of 1784-1790. This aimed to determine the relative locations of the observatories in London and Paris. Having created and accurately measured the length of the baseline, the survey proceeded towards the Greenwich and the coast using triangulation. This was the first accurate survey using triangulation. The baseline was re-measured for the Ordnance Survey of Britain by Captain Mudge in 1791.

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Key Words :- Ordinance Survey datum

Viewing the Site :- In public area but close to airport security fence so best approached on foot

Address :- North Perimeter Road, Heathrow, London, Greater London
Grid Ref :- TQ 07714 76789
Co-ordinates :- Lat 51.479812 , Long -0.45029
Local Authority :- Hillingdon London Borough Council
Pre 1974 County :- Greater London Council
Site Status :- Listed - Grade II
Historic England List No - 1119717,
Site Condition :- Site conserved and open to the public
Site Dates :- 1784 -
Contributor :- GLIAS Database - 2 June 2018

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