diamond geezer

 Thursday, April 08, 2010

I bought a book called 'Map Addict' last week. It's not a self-help guide for surveyors, it's a celebration of British cartography, and it's rather wonderful. Its author Mike Parker has recently been delivering a ten-part related series on Radio 4, a fraction of which remains on iPlayer for your delectation and delight. And here's a top-notch historical story from the London suburbs which I'd never have been aware of otherwise. You'll never guess where the Ordnance Survey started...

In the late 18th century there were no accurate maps of the UK. The man who changed this was Major-General William Roy, a Scottish engineer who realised the military importance of knowing precisely where things were. He set out to establish the relative positions of the Greenwich and Paris observatories, and decided that triangulation was the way to go. Establish a baseline somewhere, measure the angle of a third point from both ends, and a network of interlocking triangles would eventually pinpoint the location of everywhere on the grid.

For his initial baseline Roy needed to pick two endpoints about five miles apart with a large flat expanse inbetween. He found the ideal location to the west of London, on Hounslow Heath. This was a highwayman's haunt covering 25 square miles of undeveloped scrubland, and its lack of contours made it ideal for taking extended measurements. This contourlessness also made it ideal, two centuries later, for the construction of London's largest airport. Only a rump of Hounslow Heath remains today after the majority disappeared beneath the all-consuming monster of Heathrow. But one thing that wasn't wiped out was Roy's northwestern endpoint, which it's still possible to visit without disturbing national security.

plaque at Lewisham stationAlong the northern edge of Heathrow Airport, between the A4 and runway 09L/27R, runs the Northern Perimeter Road. It's a major road artery, definitely not designed for pedestrians, but provides those who venture here with a grandstand view of enormous aircraft roaring into the skies alongside. Head to the section immediately above the road tunnel that emerges from the middle of Terminal Island, and hunt for a small ornamental garden in the corner of a Business car park. In any economically-driven world this car park would extend right up to the road junction, but instead it's been held back to reveal an upright cannon rammed into the ground. General Roy's baseline began here, now also marked by a plaque and by a rather delightful hedge. You'd not deduce it from the ground but, viewed from above, the shrubs form the shape of a finger, or maybe a cannon, pointing southeast towards the other end of the line. Even in this hinterland of hotels and long-term parking, one tiny scrap of historical magic lives on. [photo]

From here at Heath Row, Roy's men set about painstakingly measuring the distance to Endpoint Two using rods and iron bars. They came up with a distance of 27404.01 feet, which turned out later to be within two inches of the correct length. Not GPS-perfect, but damned impressive for 1784 all the same.

memorial cannon in Roy Grove, HamptonIt's easy to nip to the other end of Roy's baseline by catching the 285 bus. This service runs approximately along his invisible corridor - round Hatton Cross, through Feltham and past Hanworth Sainsburys. Alight at the far end of Uxbridge Road, just before the bus reaches Hampton Hill, and the second memorial cannon is close by. It used to be set in parkland, but a chunk of residential infill has left the monument adrift up a council cul-de-sac. It's not in Cannon Close, though that's nearby, but up at the end of adjacent Roy Grove. A wide gap has been left inbetween the houses at number 7 and number 9, and here the commemorative ironwork emerges from the grass. I'm not sure what the neighbours think of sharing their suburban backwater with a nationally-significant memorial, but I bet it makes cutting the lawn tricky. [photo]

From Heathrow and Hampton, General Roy's triangulation spread out first across London and then the entire country. Most importantly, the mapping of Britain completed from this baseline gave birth in 1791 to the Ordnance Survey. Originally established for military reconnaissance, this august body now maps the nation with pinpoint digital accuracy. But they've not forgotten the Scottish engineer whose initial measurements kicked off their work. OS maps of Heathrow Airport still mark William's survey point with the unusual legend "Cannon West end of General Roy's Base (site of)". If you're a UK map addict, it's the only place to start.

OpenStreetMap showing the precise track of Roy's baseline
pre-airport OS map showing General Roy's baseline at Heathrow
More from Tlatet; More from Twickenham Museum
Map Addict: A Tale of Obsession, Fudge and the Ordnance Survey

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