New Popular Edition Maps is an attempt to produce a copyright-free database of British postcodes. It does this by asking people to hunt around on a clickable, zoomable map of the UK for places for which they know the postcode (e.g. their home), and then enter that postcode at that spot. It's a bit like a stripped-down Google Maps; and you can search the map by placename or postcode. But what's interesting about this is that the maps used are out-of-copyright Ordnance Survey maps (1 mile to the inch) from the 1940s and early 1950s, which could be useful for historians or teachers, though these are obviously not the intended audience. Unfortunately Northern Ireland and most of Scotland is missing. (The National Library of Scotland has the OS maps of Scotland from the 1920s.)

Finding this inspired me to do a bit of a search for other online historical maps of Britain which similarly attempt to cover the whole country. (There's a useful list of out-of-copyright maps here.) Old-maps.co.uk has been around a while and uses OS maps from the late 19th century. Vision of Britain (which site has lots of historical statistics which you can slice various ways, and which I must explore more thoroughly one day) is more sophisticated, and has a neat trick of switching between different maps depending upon the zoom level: for example going from a 1921 large-scale map to a 1904 OS one to a NPE map. It also has 19th-century maps and a 1930s land utilisation map. But possibly the most interesting is Old Ordnance Survey Maps, which is based upon OS maps from the 1910s, 1920s, 1930s and 1940s. The coverage is very much incomplete; but it uses the Google Maps API, which means that it has a familiar interface for users, and could be used for mashups. It already overlays the regular Google Maps satellite and street maps. There are also handy links to take you to the same location at old-maps.co.uk and Vision of Britain. I can think of some improvements (for example, printing the publication date on each map) but this approach has tremendous potential.

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