The Ordnance Survey

Ordnance Survey (Welsh: Arolwg Ordnans), an executive agency and non-ministerial government department of the Government of the United Kingdom, is the national mapping agency for Great Britain, producing maps of Great Britain (and to an extent, the Isle of Man), and one of the world's largest producers of maps.

The name reflects its creation together with the original military purpose of the organisation (see ordnance and surveying) in the first instance in mapping Scotland at the time of the creation of the British United Kingdom following many centuries of conflict and confirmed later during the Napoleonic Wars when there was a threat of invasion from France, and its logo includes the War Department's broad arrow heraldic mark. Ordnance Survey mapping is usually classified as 'large scale' (i.e. showing more detail) or 'small scale'. Large-scale mapping comprises maps at six inches to the mile or more (1:10,560, superseded by 1:10,000 in the 1950s); it was available in sheet-map form till the 1980s, since when it has become digital. Small-scale mapping comprises maps at fewer than six inches to the mile and includes the "leisure maps", such as the popular one inch to the mile and its metric successors, still available in traditional sheet-map form. Ordnance Survey of Great Britain maps are in copyright for 50 years after publication date. Some of the Copyright Libraries hold complete or near-complete collections of pre-digital O.S. mapping.

Northern Ireland, although part of the United Kingdom, is mapped by a separate government agency, the Ordnance Survey of Northern Ireland.

The roots of Ordnance Survey go back to 1747, when Lieutenant-Colonel David Watson proposed the compilation of a map of the Scottish Highlands to facilitate the subjugation of the clans following the Jacobite rising of 1745. In response, King George II commissioned a military survey of the Highlands, and Watson was placed in charge under the command of the Duke of Cumberland. Among his assistants were William Roy, Paul Sandby, and John Manson. The survey was produced at a scale of 1 inch to 1000 yards. The labours of Watson and Roy, in particular, resulted in The Duke of Cumberland's Map, now in the British Library. Roy would go on to have an illustrious career in the Royal Engineers, and he was largely responsible for the British share of the work in determining the relative positions of the French and British royal observatories. This work was the starting point of the Principal Triangulation of Great Britain (1783 – 1853), and led to the creation of the Ordnance Survey itself. Roy's technical skills and leadership set the high standard for which Ordnance Survey became known. Work was begun in earnest in 1790 under Roy's supervision, when the Board of Ordnance (a predecessor of part of the modern Ministry of Defence) began a national military survey starting with the south coast of England.

By 1791, the Board received the newer Ramsden theodolite (an improved successor to the one that Roy had used in 1784), and work began on mapping southern Great Britain using 5 mile baseline on Hounslow Heath that Roy himself had previously measured and that crosses the present Heathrow Airport. A set of postage stamps, featuring maps of the Kentish village of Hamstreet, was issued in 1991 to mark the bicentenary.

In 1801, the first one-inch-to-the-mile (1:63,360 scale) map was published, detailing the county of Kent, with Essex following shortly after. The Kent map was published privately and stopped at the county border while the Essex maps were published by Ordnance Survey and ignore the county border, setting the trend for future Ordnance Survey maps.

During the next twenty years, roughly a third of England and Wales was mapped at the same scale (see Principal Triangulation of Great Britain) under the direction of William Mudge, as other military matters took precedence. It took till 1823 to re-establish a relationship with the French survey made by Roy in 1787. By 1810, one inch to the mile maps of most of the south of England were completed, but were withdrawn from sale between 1811 and 1816 because of security fears. It was gruelling work: Major Thomas Colby, later the longest serving Director General of Ordnance Survey, walked 586 miles (943 km) in 22 days on a reconnaissance in 1819. In 1824, Colby and most of his staff moved to Ireland to work on a six-inches-to-the-mile (1:10,560) valuation survey. The survey of Ireland, county by county, was completed in 1846. The suspicions and tensions it caused in rural Ireland are the subject of Brian Friel's play Translations.

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.

The British Geological Survey was founded in 1835 as the Ordnance Geological Survey, under Henry De la Beche and remained a branch of the Ordnance Survey until 1965. At the same time the uneven quality of the English and Scottish maps was being improved by engravers under Benjamin Baker. By the time Colby retired in 1846, the production of six inch maps of Ireland was complete. This had led to a demand for similar treatment in England and work was proceeding on extending the six inch map to northern England, but only a three inch scale for most of Scotland.

When Colby retired he recommended William Yolland as his successor, but he was considered too young and a less experienced Lewis Hall was appointed instead. When after a fire in the Tower of London, the headquarters of the survey was moved to Southampton, Yolland was put in charge, but Hall sent him off to Ireland so that he was again passed over when Hall left in 1854 in favour of Major Henry James. Hall was enthusiastic about extending the survey of the north of England to a scale of 1:2,500. In 1855, the Board of Ordnance was abolished and the Ordnance Survey was placed under the War Office together with the Topographical Survey and the Depot of Military Knowledge. Eventually in 1870 it was transferred to the Office of Works.

The primary triangulation of the United Kingdom of Roy, Mudge and Yolland was completed by 1841, but was greatly improved by Alexander Ross Clarke who completed a new survey based on Airy's spheroid in 1858, completing the Principal Triangulation.  The following year he completed an initial levelling of the country.

After the first Ireland maps came out in the mid-1830s, the Tithe Commutation Act 1836 led to calls for a similar six-inch to the mile survey in England and Wales. Official procrastination followed, but the development of the railways added to pressure that resulted in the Ordnance Survey Act 1841. This granted a right to enter property for the purpose of the survey. Following a fire at its headquarters at the Tower of London in 1841  the Ordnance Survey relocated to a site in Southampton and was in disarray for several years, with arguments about which scales to use. Major-General Sir Henry James was by then Director General, and he saw how photography could be used to make maps of various scales cheaply and easily. He developed and exploited photozincography, not only to reduce the costs of map production but also to publish facsimiles of nationally important manuscripts. Between 1861 and 1864, a facsimile of the Domesday Book was issued, county by county, and in 1870 a facsimile of the Gough Map.

From the 1840s the Ordnance Survey concentrated on the Great Britain 'County Series', modelled on the earlier Ireland survey. A start was made on mapping the whole country, county by county, at six inches to the mile (1:10,560). From 1854, to meet requirements for greater detail, including land-parcel numbers in rural areas and accompanying information, cultivated and inhabited areas were mapped at 1:2500 (25.344 inches to the mile), at first parish by parish, with blank space beyond the parish boundary, and later continuously. Early copies of the 1:2500s were available hand-coloured. Up to 1879, the 1:2500s were accompanied by Books of Reference or "area books" that gave acreages and land-use information for land-parcel numbers. After 1879, land-use information was dropped from these area books; after the mid-1880s, the books themselves were dropped and acreages were printed instead on the maps. After 1854, the six-inch maps and their revisions were based on the "twenty-five inch" maps and theirs. The six-inch sheets covered an area of six by four miles on the ground; the "twenty-five inch" sheets an area of one by one and a half. One square inch on the "twenty-five inch" maps was roughly equal to an acre on the ground. In later editions the six-inch sheets were published in "quarters" (NW,NE,SW,SE), each covering an area of three by two miles on the ground. The first edition of the two scales was completed by the 1890s. A second edition (or "first revision") was begun in 1891 and completed just before the First World War. From 1907 till the early 1940s, a third edition (or "second revision") was begun but never completed: only areas with significant changes on the ground were revised, many two or three times.

Meanwhile funding had been agreed in the 1850s for a more detailed survey of towns and cities. From 1850-53, twenty-nine towns were mapped at 1:528 (10 feet to the mile). From 1855 1:500 (10.56 feet to the mile) became the preferred scale. London and some seventy other towns (mainly in the north) were already being mapped at 1:1056 (5 feet to the mile). Just under 400 towns with a population of over 4000 were surveyed at one of these three scales, most at 1:500. Publication of the town plans was completed by 1895. The London first edition was completed and published in 326 sheets in the 1860s-70s; a second edition of 759 sheets was completed and brought out in the early 1890s; further revisions (incomplete coverage of London) followed between 1906 and 1937. Very few other towns and cities saw a second edition of the town plans.

From 1911 onwards (mainly 1911-1913), the Ordnance Survey photo-enlarged to 1:1250 (50.688 inches to the mile) many 1:2500 sheets covering built-up areas, for Land Valuation / Inland Revenue purposes. About a quarter of these 1:1250s were marked "Partially revised 1912/13". In areas where there were no further 1:2500s, these partially revised "fifty inch" sheets represent the last large-scale revision (larger than six-inch) of the County Series. The County Series mapping was superseded by the Ordnance Survey National Grid 1:1250s, 1:2500s and 1:10,560s after the Second World War.

From the late 19th century to the early 1940s, for War Office purposes, the O.S. produced many "restricted" versions of the County Series maps and other War Department sheets, in a variety of large scales, that included details of military significance, such as dockyards, naval installations, fortifications, and military camps. These areas were left blank or incomplete on standard maps - though for a brief period in the early 1930s, during the Disarmament talks, some of the blanks were filled in. The War Department 1:2500s, unlike the standard issue, were contoured. The de-classified sheets have now been deposited in some of the Copyright Libraries, helping to complete the map-picture of pre-Second World War Britain.