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Ordnance Survey

The roots of the United Kingdom Ordnance Survey go back to 1790, when the Board of Ordnance[?] (the historical counterpart of the Ministry of Defence) began a survey of the English south coast in anticipation of French invasion.

Today, it is a civilian government agency which covers its own costs through the sale of maps. Its mapping information has been fully digitised since 1985, and it supplies information in various paper and computerised formats.

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. 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. 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 are only available for parts of the country not covered by Explorer or Explorer OL maps (currently only some Scottish islands). These maps are smaller than the new ones and generally have 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 are being phased out.

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

The original maps were made by building short (approx four foot high), square, concrete pillars ontop 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 photographs, but the pillars, or trig points remain.

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