This is a section of the third edition Ordnance Survey map showing Heddon-on-the-Wall in greater detail. The scale of the map is 25 inches to one mile. The map was published in the early 1920s. The third edition Ordnance Survey map was a further revision of the first edition. The revision started in 1912 and was completed by 1922. The first edition Ordnance Survey map was the first survey of the county of Northumberland on such a large scale [one inch to one mile, six inches to one mile and twenty five inches to one mile]. The survey began in 1855 and was completed by 1864. The Ordnance Survey was set up in 1791 to produce large-scale maps of the southern coast of England. At this time the British government was worried about the threat of invasion by the French. It was therefore decided that a national mapping survey should be undertaken. In 1840 Ordnance Survey was asked to produce a six-inch to one mile map for all of the country.