Ordnance Survey

The Ordnance Survey (OS) is now a civilian organisation and government agency in the United Kingdom, and one of the World's largest producers of maps. In addition to a wide range of UK maps, the organisation is also working in over sixty countries world-wide.

Table of contents

1 Origins
2 Mapping Britain
3 World War I
4 UK Map Range
5 Cartography
6 Eastings and Northings
7 External Links

Origins

The roots of the United Kingdom Ordnance Survey (OS) go back to 1747, when King George II of England commissioned a military survey of the Scottish highlands following the Jacobite revolt of 1745. William Roy was the engineer responsible for this pioneering work. It was not until 1790 that the Board of Ordnance (the predecessor of the Ministry of Defence) began a national military survey starting with the south coast of England in anticipation of a French invasion.

Mapping Britain

By 1791, the Board had purchased the new Ramsden theodolite, and work commenced on mapping southern Britain using a baseline that Roy himself had previously measured. In 1801 the first one-inch map was published: it was of the county of Kent, with a second of Essex following shortly after.

During the next twenty years about a third of England and Wales was mapped at the one-inch scale. It was gruelling work: Major Thomas Colby, later the longest serving Director General of the Ordnance Survey, walked 586 miles in 22 days on a reconnaissance in 1819.

In 1824, Colby and most of his staff moved to Ireland, to work on a six inch to the mile valuation survey. Colby was not only involved in the design of specialist measuring equipment. He also established a systematic collection of place names, and reorganised the map-making process to produce clear, accurate plans. He believed in leading from the front, travelling with his men, helping to build camps, and as each survey session drew to a close arranging mountain-top parties with enormous plum puddings.

After the first Irish maps began to come out in the mid-1830s, the Tithe Commutation Act led to calls for similar six-inch surveys in England and Wales. After official prevarication, the development of the railways added to pressure that resulted in the 1841 Ordnance Survey Act. This granted a right to enter property for the purpose of the survey.

Following a fire at its headquarters in the Tower of London, the OS was in disarray for several years with arguments about which scales to use. Major-General Sir Henry James was now Director General, and he saw how photography could be used to make maps of various scales cheaply and easily. The twenty five inch to the mile survey was complete by 1895.

World War I

During the First World War the OS was more involved in preparing maps overseas, but after the war Colonel Charles Close, the current Director General, developed a marketing strategy, using covers designed by Ellis Martin to increase sales in the leisure market. In 1920 O. G. S. Crawford was appointed Archeology Officer and played a prominent role in developing the use of aerial photography to deepen understanding of archaeology.

The Davidson Committee was established in 1935 to review the Ordnance Survey's future. The new Director General, Major-General Malcolm MacLeod, started the retriangulation of Great Britain, an immense task which involved erecting concrete triangulation pillars on prominent hilltops throughout Britain.

The Davidson Committee's final report set the OS on course for the twentieth century. The National Grid reference system was launched, with the metre as its measurement. An experimental new 1:25,000 scale map was introduced. The one-inch maps remained for almost forty years before being superseded by the 1:50,000 scale series, as proposed by William Roy more than two centuries earlier.

In 1995 the Ordnance Survey digitised the last of about 230,000 maps, making Britain the first country in the world to complete a programme of large-scale electronic mapping.

The OS is now a civilian organisation.

UK Map Range

Ordnance Survey maps are available in most bookshops, generally in two scales:

1:50,000
- Landranger - These are designed as road maps. They have pink covers and 204 of them cover the whole of the UK.
1:25,000
- Explorer - Designed for walkers and cyclists. There are 351 of these maps at the time of writing, but the number is increasing. They have orange covers.
- Explorer OL - Also for walkers & cyclists. These 33 maps specifically cover tourist destinations. Identified by their yellow covers and often double-sided, they predate the explorer maps. Previously known as Outdoor Leisure maps.
- Pathfinder - Pathfinders were the predecessors to the Explorer and Explorer OL maps. These maps were smaller than the new ones and generally had no overlap between adjacent sheets, meaning that even a short walk may require three or four different maps and a long one may range over even more. For this reason they have recently been gradually phased out.

Also produced are the mapping index (free), showing which parts of the country are covered by which maps, and Travel maps.

Cartography

The original maps were made by building short (approx four foot high), square, concrete pillars on top of various high points and working out the exact position of these by triangulation. The details in between were then filled in with less precise methods. Modern Ordnance Survey maps are based on aerial photographss, but large numbers of the pillars, or trig points remain.

The OS still maintains a set of master geodetic reference points to tie the OS geographic datums to modern measurement systems including GPS.

Eastings and Northings

The Ordnance Survey maps of Great Britain do not use latitude and longitude to indicate position but a special grid.

There is a difference between the grid used in the mapping of Ireland compared to mainland Britain and the Scottish islands. This sectional concentrates on the traditional mainland reference system, called OSGB36 ™ (Ordnance Survey Great Britain 1936) used after the retriangulation of 1936-1953.

The maps are based on the projection called the Airy 1830 ellipsoid, with an origin at 49 ° N, 2 ° W. The ellipsoid is a regional best fit for Britain, more modern mapping tend to use the GRS80 ellipsoid used by the GPS. Over the Airy projection of Britain a straight line grid, the National Grid, is placed with a new false origin (to eliminate negative numbers), creating a 400 km by 100 km grid. The distortion created between the OS grid and the projection is countered by a scale factor in the longitude to create two lines of longitude with zero distortion rather than one. The produced maps contain a small variation between true north and grid north.

The position of a point on an OS map is given in northings (east-west) and eastings (north-south) in metres from the origin. To prevent very large values metre values the gird is divided. At the highest level into 25 500 km by 500 km squares, each with a letter code from A to Z (omitting I) starting with A in the north-west corner to Z in the south-east corner. This is much larger than the islands mapped, only four grid squares actually contain land - S,T,N, and H. Each large square is subdivided again into 25 100 km by 100 km squares, using the same lettering system. The created grid produces the following squares containing land:

hp ht hu hw hx hy hz na nb nc nd nf ng nh nj nk nl nm nn no nr ns nt nu nw nx ny nz sc sd se ta sh sj sk tf tg sm sn so sp tl tm sr ss st su tq tr sv sw sx sy sz tv

Within each square a point can be indicated to varying resolutions numerically, usually from four digits (1 km square) down to ten digits (1 m square.)

As the above information indicates a geodetic transformation between OSGB36 and other terrestrial reference systems (like ITRF2000, ETRS89, or WGS84) can be quite tedious, if attempted manually. The process is called a Helmert datum transformation, the transformation from ETRS89 to OSGB36 is called the National Grid Transformation OSTN02.