Part One.

The Mapping of Great Britain
by Chris Knights.
Many navigators, (as well as any other folk that may be interested in maps,) may like to know the story of how our Country came to be surveyed and mapped, ending with the Ordnance Survey Maps without which, for instance, the sport of rallying would be a rather difficult task for the organisers! Let’s start with a little bit of history…


Early Days...


he first set of maps of Great Britain was produced by the great printer Saxton between 1574 and 1579. Other than this, all mapping was a local activity funded by local subscriptions, but these maps were usually of poor quality and inconsistent.

By 1750 Britain was falling behind other European countries in the extent and quality of its mapping, notably France, and The Society of Arts tried to persuade the Government to organise a full national survey. This failed so in 1759 the Society itself offered a prize of £100 to anyone prepared to undertake an accurate mapping of their own county at a scale of one inch per mile. 

This scale continued to be the standard scale of British mapping until the Ordnance Survey announced that it was changing to metric scales in 1965. The first OS 1:50,000 metric map was produced in 1974. 

The Society’s prizes were taken up with enthusiasm and a large number of individual county maps were produced over the next thirty years; but it soon became clear that these maps were unsuitable for military purposes which was the Government’s only concern. The idea of a full scientific national survey must be credited to William Roy (1726-1790) who was a civilian draughtsman with the army’s Board of Ordnance at its Edinburgh Office. 

Following the Jacobite uprising which culminated in the battle of Culloden in 1746, George II realised that accurate maps of Scotland were necessary to pursue his aim of pacifying the Highlands of Scotland and he authorised a new military survey to be undertaken under the guidance of William Roy who was given a military rank in the Corps of Engineers. The survey of Scotland proceeded well but in 1755 the Seven Years war against France and Spain intervened and Roy and his mapmakers were sent to France to prepare maps of war. 

In 1763, the war now over, Roy realised how vulnerable Britain was to invasion and proposed a full national survey for defence purposes, but the Government declined on the grounds of cost. Roy was disappointed but continued to experiment with the technical aspects of surveying. For instance he developed a new barometer, which he used as an altimeter for assessing heights. In 1775 he measured the height of Snowdon as 3568 feet (only 8 feet too high!) and his mathematician friend, Dr Charles Hutton, who was doing some of the calculations for him, invented the use of contour lines. 

In 1784 Roy became aware that any future national survey had to be based on ‘triangulation’ as this was the only method, which would provide an accurate indication of the relative positions of the country’s features. Triangulation allows distances between features to be calculated without having to measure them since measurement of distances on the ground is very time-consuming and almost impossible on other than totally flat terrain. (See part Two for a detailed explanation of Triangulation.)
But Roy also realised that even triangulation required a precisely measured base line a few miles long as a starting point and he chose the flat lands of Hounslow Heath, the site of Heathrow Airport today, as his starting point.

Roy tried and rejected several measuring methods such as rods of wood and iron and eventually settled on the use of glass tubes 20’ long and 1” in diameter which were unaffected by temperature and humidity. This was the first precisely measured base line in Britain. 

By this time the new King, George III, was anxious to cooperate with the French, who were disputing the exact position (ie latitude) of the Greenwich Observatory whilst promoting their own surveys based on the Paris Observatory. The King agreed to fund a survey of the south east corner of England which could match up to the surveys already completed in northern France by triangulation across the Channel. 

Roy therefore ordered at the king’s expense a new super-accurate theodolite that would be needed for the triangulation, from Jesse Ramsden the famed instrument maker. Whilst this instrument was being manufactured Roy roughly surveyed the countryside between Hounslow Heath and the south coast and pinpointed and cleared all the hilltops and other high points that he proposed to use for his triangulation. Ramsden’s theodolite was delivered in July 1787 and the first stage of the survey began. 

Roy died in 1790 aged sixty four just as the King’s survey of the south east was completed. 

Charles Lennox, 3rd Duke of Richmond had been appointed Master General of the Board of Ordnance in 1782. The role of the Board was to deal with the army’s fortifications, small arms and munitions. Richmond had always had a personal interest in mapmaking and had long been impressed with Roy’s professional approach to the task and his use of scientific methods. In 1791, soon after Roy’s death, Richmond set up ‘The Trigonometrical Survey of the Board of Ordnance’ and appointed Major Edward Williams as its first Director.

The Ordnance Survey was born.

The First OS Maps.

The data collected during the triangulation of the south east coast was expanded by detailed ‘topological’ survey work on the ground and the results were passed on to the mapmakers at the Survey’s headquarters in the Tower of London. In 1801 the first OS map (Number 1) covering the county of Kent was published. 

The surveying continued, first along the south coast because the maps were still essentially for military defence purposes, and then gradually northwards. The process was the same throughout: first the primary triangulation with triangle ‘legs’ about 30 miles long, then the secondary triangulation with approximately 5 mile legs inside the primary triangles, then the detailed topographical survey on the ground prepared a survey at a scale of 6 inches to one mile. All this data was passed to the engravers and printers who produced the one-inch maps. 

Copyright was always a contentious issue and there were many infringement cases against commercial mapmakers who merely copied the OS maps. For most of the time the OS maps were sold to the public though few could afford them at the equivalent of £50 per map at current prices. 

This first national survey was completed in 1841. 

Over the next hundred years the initial survey was revised continually and was extended to include Ireland (which was of course part of the Kingdom at the time). Map production was advancing all the time and by the start of the 20th century the look of the one-inch maps would be familiar to those who use the latest versions today. Early experiments with aerial mapping were tried but without much success. 

By the 1930s, however, massive changes to the landscape, especially in urban areas, made it clear that a complete new survey was needed, and a new national primary triangulation was started under the auspices of Captain Martin Hotine in 1935.

The New Survey. 

Hotine realised that war was looming again so he pushed the survey ahead quickly. First the triangulation sites were recce’d, often using the same sites as the original triangulation 100 years before. The hilltop sites were cleared of undergrowth and new sites established where the original could not be used. Next the pillars were constructed on the sites – these are the still familiar concrete ‘triangulation pillars’ better known as ‘trig points’ so beloved by Navscat organisers. 

The Second World War intervened but the national re-triangulation was completed by 1957. A new base line for the re-triangulation was measured along the Ridge Way in Wiltshire using invar tape strung on catenaries with precise weights taking up the slack. At the conclusion of the survey a base line was measured in Caithness on the north coast of Scotland to check that the triangulation calculations were accurate. This 15-mile line was found to be just 20 metres shorter than the calculated distance based on the base line 800 miles to the south. 

An embarrassing mistake. 

Throughout this time, secondary triangulation and on-the-ground surveys followed the main observations. This revealed an interesting mistake made a century earlier. The Greenwich Meridian (0° longitude) had been defined by Bradley’s transit instrument in the Greenwich Observatory up to 1850 and this position had been used by the OS as the definitive 0°. The new Astronomer Royal, Sir George Airy, built a new transit instrument in 1850 that he positioned at Greenwich in the room next to Bradley’s earlier instrument. Airy’s instrument was accepted by the Washington Conference in 1884 as being the precise location of 0° longitude for international geographical and navigation purposes (much to the disgust of the French who wanted the meridian to run through the Paris Observatory). Unfortunately the Survey never reacted to the Washington agreement and ever since 1850 the position of 0° longitude marked on all the OS maps has actually been 5.79 metres west of the true meridian! 

Up to date. 

Aerial photography suffered in its early days from lack of sufficiently good equipment, but advances in techniques have ensured that the days of the ground survey were soon to be over. It is interesting to note that the aerial surveys still relied on the trig points to locate precisely the overall positioning controls for the aerial maps. In fact, trig points were repainted in bright white just so that they could be easily seen on the aerial photographs. It didn’t matter that many of the trig points could no longer be seen from adjacent triangulation points so long as they could be spotted from the air! 

Measuring methods also advanced from the 1950s. The Swedish Geodimeter issued a pulsed light beam that could be reflected back to the observer and this was tested by the US Army Mapping Service on the Ordnance Survey’s Ridge Way base line in Wiltshire, as was the South African tellurometer which worked similarly but used microwave pulses. The Ridge Way was used because it is considered to be the most precisely measured base line in the world. In fact, as a result of these tests, the speed of light was brought into question and after independent proof by the National Physical Laboratory the new figure for the speed of light was internationally accepted. What a tribute to the OS men with their measuring rods! 

Since the 1980s Global Positioning Systems (GPS) using satellites have been widely used and since 1990 the two hundred year old triangulation methods have no longer been used for the primary survey. For the future the mapping scientists are looking to intergalactic bodies such as pulsars and quasars to enable distances such as the complete length of Great Britain to be measured to within a single millimetre.

Home...       Part Two...      Part Three...