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Monument at North Western End of General Roys Survey Base

A Grade II Listed Building in Hillingdon, London

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Latitude: 51.4798 / 51°28'47"N

Longitude: -0.4503 / 0°27'0"W

OS Eastings: 507714

OS Northings: 176789

OS Grid: TQ077767

Mapcode National: GBR 23.K4Z

Mapcode Global: VHFTK.42TD

Plus Code: 9C3XFGHX+WV

Entry Name: Monument at North Western End of General Roys Survey Base

Listing Date: 15 December 1997

Grade: II

Source: Historic England

Source ID: 1119717

English Heritage Legacy ID: 469215

Location: Heathrow Villages, Hillingdon, London, TW6

County: London

District: Hillingdon

Electoral Ward/Division: Heathrow Villages

Parish: Non Civil Parish

Built-Up Area: Hillingdon

Traditional County: Middlesex

Lieutenancy Area (Ceremonial County): Greater London

Church of England Parish: St Mary the Virgin Harmondsworth

Church of England Diocese: London

Tagged with: Monument Cannon

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Monument at north-western end of General Roy's survey base


Cast-iron cannon monument, installed 1791 to mark the north-western end of the baseline measured by General Roy in 1784 across Hounslow Heath, from King's Arbour to Hampton Poor House, which is now taken to be the origin of the Ordnance Survey. Roy originally marked his line by wooden posts, but in 1791 two guns were selected at Greenwich 'from amongst those which had been condemned as unfit for the public service'. The cannon is some nine feet long, the lower six feet of which are buried in the ground. Next to the cannon is a plaque which reads as follows:
'This tablet was affixed in 1926 to commemorate the 200th anniversary of the birth of Major General William Roy FRS, born 4th May 1726 - died 1st July 1790. He conceived the idea of carrying out the triangulation of this country and of constructing a complete and accurate map and thereby laid the foundation of the Ordnance Survey. This gun marks the NW terminal of the base which was measured in 1784, under the supervision of General Roy, as part of the operations for determining the relative positions of the Greenwich and Paris Observatories. This measurement was rendered possible by the munificence of HM King George III, who inspected the work on 21st August 1784. The base was measured again in 1791, by Captain Mudge, as the commencement of the principal triangulation of Great Britain. Length of base - reduced to MGL - as measured by Roy - 27404.01 feet, as measured by Mudge - 27404.24 feet, as determined by Clarke in 1858 in terms of the Ordnance Survey Standard 01 - 27406.19 feet.'
Source: Captain William Mudge and Isaac Dalby, An Account of the Operations Carried on for Accomplishing a Trigonometric Survey of England and Wales, vol. 1, 1799, p.223.

The monument at the south-east end of General Roy's baseline is listed under Roy Grove, Hampton, in the London Borough of Richmond.

Listing NGR: TQ0771476789

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