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mounted in the police compound's concrete at the entrance to the Heathrow Airport
access tunnel. The precise end of the fundamental base is at the center of the cap
screwed over the muzzle. The opposite end is on a similar cap 5.9102 miles away
in Hampton, in the Borough of Richmond, 2 km west of Teddington on the road
called the A312. The upturned cannon is in Roy Grove, a cul-de-sac off Hanworth
Rd. named after Major General William Roy (1726-1790), the initiator of the survey
of Britain. The tablet next to the cannon in Roy's Grove states:
This tablet was affixed in 1926 to commemorate the 200 th anniversary of the birth of Major
General William Roy, F.R.S., born 4 th May 1726 - died 1 st 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 S.E.
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 H.M. 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 m.s.l. 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 o1, 27406.19 feet.
The military origins of the maps begun by Roy can be seen in the name of the
resultant publication, the Ordnance Survey . The maps were drawn in their own
system of coordinates, the Ordnance Survey Grid. Places that were mapped were
linked by a series of surveyor's triangles to Ordnance Survey Triangulation Points
strategically located on prominent high points throughout Britain. Unlike the
French surveys, the astronomy and the surveying were executed independently and
then related later. The Ordnance Survey Grid was linked to astronomical observa-
tions by Bradley (the Third Astronomer Royal) at a transit telescope at Greenwich.
It remains linked to this Greenwich Meridian today, some 5.7 meters west of the
Greenwich Meridian as defined by Airy's transit circle. The separate development
of the Ordnance Survey accounts for the rather confusing Grid references and lati-
tude and longitude systems around the edges of the British Ordnance Survey maps
of the present time, contrasting the marginal simplicity and scientific elegance of
the French maps calibrated simply in longitude and latitude.
Cassini III made several attempts to persuade British sovereigns to carry out
surveys on the French model. George III eventually agreed and between 1763 and
1784 the triangulation of England and Ireland was carried out. The surveying
instruments were made in England, and, setting aside the French quadrant design,
the talented instrument maker Jesse Ramsden developed the theodolite 21 for the
survey. A theodolite has two graduated circles at right angles for measuring altitude
and azimuth. It was invented in the sixteenth century by Leonard Digges and pub-
lished as a design by his son Thomas in 1571. It works by a telescope pivoting at
the center of each to view the targets. In 1787, Ramsden built a large theodolite with
21 Although its etymology is subject to doubt, the word has nothing to do with God (as in the word
“theology”) but, apparently the Greek word for “sight” or “view” (as used in the word “theorem”).
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