TRIANGULATION IN THE 16th CENTURY

Cartophilen. One who greatly admires and enjoys maps.

The next time you find yourself lost on a lonely hillside with only a map and a compass between you and missing the next episode of ‘Celebrity Big Brother’ or standing in the rain checking your A-Z to find the nearest tube station consider this – virtually every map in this country owes its existence to 2 military engineers, 10 men from the 12th Regiment of Foot – and a straight line paced out on Hounslow Heath.

It all started with the Jacobite Rebellion of 1745 when the “Young Pretender”, Charles Edward Stuart tried to kick George 2nd off the throne and replace him with “The Old Pretender” James Francis Edward Stuart. It ended in tears and defeat for the Jacobite cause in 1746 at Culloden in Scotland, the last battle fought on British soil.

The British Army faced many difficulties during the campaign – a determined enemy and hostile terrain being just two of them, but the most significant was the lack of reliable maps. The charts they did have depended more upon the cartographers’ imagination than any accurate study. The troops could never be sure what lay over the next hill, how far it was and what bearing they should follow to find it.

In 1747 Lieut-General Watson, a military engineer, decided to rectify the situation with an scientific map-making survey around Fort Augustus in the central Highlands. He was assisted by Quartermaster-General William Roy. By 1755 they had extended their survey south into the Scottish Lowlands at which point the work was interrupted by the Seven War Years War.

In 1783 a memoir by the French astronomer M Cassini was sent to the Secretary of State Charles Fox. In it Cassini proposed an accurate survey of the land between the observatories in Greenwich and Paris. This would determine their precise position and relationship to each other and so assist the astronomers in their observations. The memoir was passed to Sir Joseph Banks, who appointed William Roy, by now was a full-blown General, to look after the English side of the survey.

To achieve the degree of accuracy that the astronomers required General Roy used a technique called triangulation. This starts with an absolutely straight and precisely measured base line – the longer the better – laid across flat ground. The surveyors then move to either end of the base line where they take bearings on a distant object -a church steeple, for example. Using trigonometry and the angles between the object and the base line they are then able to calculate the exact length of the other sides of the triangle formed. This initial triangle then forms base lines for other triangles until the whole country is plotted. The concrete “trig points” that you sometimes see on high ground mark the corners of these survey triangles. There is one in Richmond Park.

TRIG POINT IN RICHMOND PARK

The place where General Roy decided to lay his initial base line was Hounslow Heath “because of its vicinity to the capital and the Royal Observatory at Greenwich, its great extent, and the extraordinary levelness of its surface, without local obstruction whatever to render measurements difficult.”

On 16th April 1784 General Roy and Sir Joseph Banks conducted a preliminary examination of the site. They decided that the baseline should run south-eastwards from Kings Arbour at Heathrow, to Hampton Court Poor House, at the side of Bushey Park. The distance was about 5 miles. The task of clearing and levelling the path of the baseline was undertaken by a sergeant, a corporal and 10 men of the 12th Regiment of Foot based at Windsor. Work started on 26th May 1784 and was completed in early July.

To measure the length of his baseline General Roy first used a steel chain but finding that it expanded and contracted according to the prevailing temperature he tried wooden rods made of New England pine cut from a ship’s mast. These too were not accurate enough and so he finished up using glass tubes laid on wooden cradles. Eight years later when the baseline was measured again it was found to be just 2¾ inches out (7 cms) over a distance of 5 miles (8 kms)

HOUNSLOW HEATH

By 1787 General Roy had extended his triangles to the Kentish coast, and in October of that year the triangular connection with the French surveying stations was completed. After Roy’s death in 1790 the Board of Ordnance (a precursor of the Ministry of Defence) established the Ordnance Survey with a commission to extend Roy’s triangulation outward from the south-east of England. By 1823 they had plotted much of the country. The modern Ordnance Survey still includes in its logo the arrowhead mark of the 1790’s Board of Ordnance.

Today the Hounslow baseline, upon which all subsequent Ordnance survey maps are based, is now covered with houses and the world’s busiest international airport but its ends are still marked. In Roy Grove, Hampton there is the barrel of a cannon projecting from the ground, alongside a memorial tablet celebrating the life and work of General William Roy. The other end, five miles away, is marked by a second cannon on the Northern Perimeter Road, near where it passes over the tunnel entrance to Heathrow Airport.

THE END OF THE BASELINE

HAMPTON HEATHROW
HAMPTON HEATHROW

The Ordnance Survey says a lot about us as a nation. The detail, accuracy and availability of its maps suggest that we have nothing to hide, that this land is accessible to all. The careful plotting of footpaths, bridleways, rights-of-way and half forgotten tracks serves as an invitation to go out and explore. Look at the map and we can all see where we stand and how we fit into the wider scheme of things. Most other countries – and even the most developed – do not have maps like this freely available and they are poorer for it, geographically and psychologically. Freedom comes in many guises and our freedom to know precisely where we are and where we might be going was born on a long straight track between TQ 077768 and TQ 138709 – on Hounslow Heath.

“All you need is the plan, the road map, and the courage to press on to your destination.”

Earl Nightingale

“A road map always tells you everything except how to refold it.”

Ralph Waldo Emerson

— from Martyn Day