of Southampton, Hampshire.
Ordnance Survey (OS) is an executive agency of the United Kingdom government. It is the national mapping agency for Great Britain, and one of the world's largest producers of maps. The name reflects the original military purpose of the organisation in mapping Britain during the Napoleonic Wars when there was a threat of invasion from France. OS is widely regarded as the most systematic and thorough mapping institution in the world, detailing every corner of Britain long before satellite technology made quality maps of the same standard available elsewhere in the world.
- 1747 King George II commissioned a military survey of the Scottish highlands following the Jacobite revolt of 1745. William Roy was the engineer responsible for this pioneering work; one of the staff involved was noted artist Paul Sandby. The survey was produced at a scale of 1 inch to 1000 yards.
- 1790 It was not until 1790 that 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 in anticipation of a French invasion.
- By 1791, the Board had purchased the new Ramsden theodolite, and work began on mapping southern Great Britain using a baseline that Roy himself had previously measured and that crosses the present Heathrow Airport.
- 1801 The first one-inch-to-the-mile (1:63,360) 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 ignored 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.
- 1819 It was gruelling work: Major Thomas Colby, later the longest serving Director General of Ordnance Survey, walked 586 miles in 22 days on a reconnaissance 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 was completed in 1846.
- 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.
- 1830s After the first Irish maps came out in the mid-1830s, the Tithe Commutation Act 1836 led to calls for similar six-inch surveys in England and Wales.
- 1841 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 at the Tower of London in 1841, OS 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 National Manuscripts.
- Between 1861 and 1864 a 'facsimile' of the Domesday Book was issued, county by county.
- After the fire, OS relocated to a site in Southampton, and the twenty-five inch to the mile survey was completed by 1895. Just under 400 towns with a population of over 4000 were surveyed at a scale of 1:500. Funding was agreed in 1855 and publication completed by 1895.
- WWI. During the First World War, OS was involved in preparing maps of France and Belgium for its own use.
- Post-WWI. After the war Colonel Charles Close, then Director General, developed a marketing strategy using covers designed by Ellis Martin to increase sales in the leisure market.
- 1920 O. G. S. Crawford was appointed Archaeology Officer and played a prominent role in developing the use of aerial photography to deepen understanding of archaeology.
- 1929 Listed Exhibitor - late entry - British Industries Fair. Survey Maps etc. (Stand No. R.6) 
- 1935 The Davidson Committee was established to review Ordnance Survey's future. The new Director General, Major-General Malcolm MacLeod, started the re-triangulation of Great Britain, an immense task involving erecting concrete triangulation pillars (trig points) on prominent (often inaccessible) hilltops throughout Great Britain. These were intended to be infallibly constant positions for the theodolites during the many angle measurements, which were each repeated no less than 32 times.
- The Davidson Committee's final report set OS on course for the twentieth century. The national grid reference system was launched, with the metre as its unit of measurement. An experimental 1:25000 scale map was introduced. The one-inch maps remained for almost forty years before being superseded by the 1:50000 scale series, as proposed by William Roy more than two centuries earlier.
- WWII. Many more maps were created during World War II, including:
- 1:40000 map of Antwerp, Belgium
- 1:100000 map of Brussels, Belgium
- 1:5000000 map of South Africa
- 1:250000 map of Italy
- 1:50000 map of Northeast France
- 1:30000 map of the Netherlands with manuscript outline of German Army occupation districts.
- 1969 OS had outgrown its site in the centre of Southampton (made worse by the bomb damage of the Second World War), and moved to the suburb of Maybush, towards the edge of the city, where it remains today. Some of the remaining buildings of the original city-centre site are now used as part of the court complex.
- 1995 Ordnance Survey digitised the last of about 230,000 maps, making the United Kingdom the first country in the world to complete a programme of large-scale electronic mapping. OS is now a civilian organisation with executive agency status.
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