In this article we shall consider the origins of Ordnance Survey, the national mapping agency for Great Britain (so excluding N Ireland who have OSNI). As the name suggests, this is a military history, largely based on the initial work of just two military engineers and 12 other soldiers of the 12th Regiment of Foot.
After the Jacobite Rebellion was put down in 1745 at the Battle of Culloden, the last battle fought British soil, the Duke of Cumberland wanted to track down the supporters of the ‘Young Pretender’, Bonnie Prince Charlie, and the others who wished to replace George II with the ‘Old Pretender’, James Stuart, but the maps they had of the Scottish Highlands owed more to the imaginations of the map makers than any accurate study so Cumberland’s troops were usually evaded. In 1747 Lieutenant-General Watson, a military engineer, decided to rectify the situation with a 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 their work was interrupted by the Seven Years War.
With relations with the French back on better terms, in 1783 a proposal by the French astronomer, Jacques Cassini, whose father identified the rings of Saturn and after whom the NASA probe to the planet was named, was sent to the Secretary of State of England, Charles Fox. In this 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 from these two sites. The proposal 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 extremely 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 very accurate measurement of 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 we sometimes see on high ground across the UK mark the corners of these survey triangles. This triangulation method is the same as the one subsequently employed by George Everest (pronounced ‘Eve-rest’) when he started to survey the Great Indian Arc of the Meridian south to north through India in 1800, thereby completing the longest measurement of the Earth’s surface ever attempted – and identifying the mountain that carries his name as the tallest in the world (although he would not have thanked us for pronouncing it incorrectly).
The place where General Roy decided to lay his initial base line was Hounslow Heath, chosen because of its flatness and proximity of Greenwich. On 16 April 1784 Roy and Banks conducted a preliminary examination of the site. They decided that the baseline should run south-eastwards on the site now largely occupied by Heathrow Airport. 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 26 May 1784 and was completed in early July.
To measure the length of his baseline 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 weren’t accurate enough either and so he used glass tubes laid on wooden cradles. This was a painstaking process: three rods of about 20 feet were supported on trestles and the ends aligned to an accuracy of one thousandth part of an inch. The first rod was then carried to the end of the third, an operation to be repeated 1 370 times. The final measurement gave the length of the baseline as 27 404.01 ft (8 352m). Eight years later when the baseline was measured again it was found to be only 2¾ inches out (7cms).
By 1787 Roy had extended his triangles to the Kent 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.
Today the Hounslow baseline, upon which all subsequent Ordnance Survey maps are based, is covered with houses and the world’s second 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 (pictured) – I passed it every time I went for a meeting with the Heathrow Expansion team when we were preparing for their DCO and was much relieved to see that the expansion plans for the airport were to leave it unaltered.
Next time we’ll look again at trig points as well as other markers to be found on the ground that might provide some useful land referencing clues.
This article is written by Ashley Parry Jones, Director – Planning, WSP. The opinions expressed are those of the author and do not purport to reflect the opinions or views of WSP or SoLR or its members. The information provided does not and is not intended to constitute legal advice and instead is offered for general purposes only. It does not constitute the most up to date legal information. Any links and references provided are for the readers’ convenience only and do not constitute a recommendation of those sources.