The Ordnance Survey began with war in mind, in the shadow of the Jacobite Rising of 1745. The country continued to be mapped with an eye to military strategy and resources, although the Battle of Culloden in 1746 was the last major pitched battle on British soil.
Starting with strategically important coastlines in the southeast of England, considered vulnerable to invasion during the Napoleonic Wars, the maps were drawn at a scale of 1 inch to the mile (1:63,360, roughly equivalent to modern OS Landranger maps). Over the next few decades surveyors gradually worked their way across England and Wales. By 1810, most counties of southern England had been mapped but they were not available for sale for another half decade, after a fractious period of war, financial difficulties, and Luddite unrest.
By the 1840s all of Wales and most of England had been mapped at 1 inch to the mile. In the second half of the century, the threat of invasion having abated and the Industrial Revolution in full swing, Ordnance Survey mapping began to be guided more by economic than military concerns. The War Office conceded control of the Ordnance Survey in 1870 to the Office of Works (responsible for forestry and royal palaces), and in 1890 to the Board of Agriculture. With taxation and industry in mind, the OS County Series was born: mapping Great Britain in its entirety at the much larger and more detailed scale of 6 inches to the mile (1:10,560), with urban areas mapped at 25 inches to the mile (1:2,500). The new survey began in the 1840s, and revised editions were published until the 1950s. Created county by county, these new maps included an unprecedented level of detail.
With detail came risk. Although the maps were published and available to the public, some information was deemed too sensitive for general consumption. This was particularly so during the World Wars, when the threat of invasion loomed once more, and aerial bombardment was a new and frightening reality.
Military and industrial locations were surveyed in the Ordnance Survey’s usual detail, and were available to the military, but were omitted from the published maps.
Sometimes, the change was subtle, as in this map of Weedon Barracks in Northamptonshire. Built during the Napoleonic Wars as a store of military equipment close to the strategic Grand Junction Canal, the store was later expanded to include barracks, and extra storehouses and workshops, which were added during the First World War. The site remained in use from 1804 to 1965. The barracks are shown in detail on both maps, but on the published sheet, labels that show the site’s military use are not included. The street name ‘Ordnance Road’ remains, however, which might have given the game away!
Lavernock Fort, in Glamorgan, was a gun battery built in 1870. It was used in the Second World War to defend the Severn, an important route for Atlantic shipping, and was used as a lookout post for volunteers of the Royal Observer Corps, responsible for spotting German aircraft. All evidence of the battery was removed from the published map.
Lavernock Fort is a fairly small military installation, but some much larger sites were given the same treatment. In northern Kent, on the Thames Estuary, a 128-hectare site manufactured cordite, nitro-glycerine, and gelatine dynamite for Curtis’s & Harvey, a gunpowder company which controlled half of the British gunpowder industry in 1898. The factory, and the battery to its south, disappeared from the published map, leaving sheepwashes as almost the only landmarks.
You might be forgiven for wondering about the point of our final map if you had access only to the published version. No physical geographical features are shown, only the administrative boundaries of Yorkshire and Lincolnshire in the Humber Estuary.
At first glance the secret map does not appear any more detailed. For military eyes only, an inconspicuous cross has been added, marking Bull Sand Fort. The fort is the larger of two Humber sand forts, built on sandbanks during the First World War and extensively used in the Second World War to protect the entrance to the Humber Estuary. The fort is marked only with a cross as it was not surveyed in detail by the Ordnance Survey, but it was a significant fort, able to support 200 people, with fresh water pumped in from a natural source of fresh water under the sand. Armour on the seaward side was a foot thick. An anti-submarine steel net was stretched between the two forts, making a formidable barrier.
The removal of military installations from OS maps was at its height in the 19th century and the World Wars, but throughout the Cold War and beyond, many sensitive sites were left off the maps entirely. It took the public availability of high-resolution satellite imagery at the turn of the 21st century to render this type of censorship largely ineffective, although labels are still omitted in some cases.
Certain areas are still removed entirely from digital maps and satellite imagery, including some US military bases in the Middle East. Despite efforts to restrict access to sensitive information, new developments in mapping technology and data visualisation sometimes reveal what governments prefer to keep hidden.
In 2017, the fitness tracking app Strava released a global heatmap, aggregating data from its millions of users, each using GPS technology to record their exercise routes. In parts of Syria and Afghanistan, the only users of Strava were foreign military personnel, with the result that repeated runs around military bases created bright spots of activity, clearly identifying their location.
Trainee Assistant Map Curator
References & Further Reading
The photograph of Bull Sand Fort was taken by Wikimedia Commons user Neoskywalker and is available at https://commons.wikimedia.org/wiki/File:Bull_Sand_Fort.jpg. Accessed 29 April 2020.
The Strava Heatmap can be explored here: https://www.strava.com/heatmap
Cliffe History n.d., The Explosives Works at Cliffe. Accessed 29 April 2020 from http://www.cliffehistory.co.uk/the_explosives_works.html
Hern, A. 2018. ‘Fitness tracking app Strava gives away location of secret US army bases’, The Guardian, 28 January. Accessed 30 April 2020 from https://www.theguardian.com/world/2018/jan/28/fitness-tracking-app-gives-away-location-of-secret-us-army-bases
Higley, C. 2011. Old Series to Explorer: a field guide to the Ordnance Map. London: The Charles Close Society. Available in NLW reading rooms. Catalogue record: https://discover.library.wales/permalink/f/1norb00/44NLW_ALMA21753597350002419
Historic England n.d., Bull Sand Fort. Accessed 30 April 2020 from https://historicengland.org.uk/listing/the-list/list-entry/1083477
Historic England n.d., Curtis’s and Harvey Ltd, Explosives Works, Cliffe. Accessed 29 April 2020 from https://www.pastscape.org.uk/hob.aspx?hob_id=1517194
McCall, K., n.d., ‘Where We Play’. Strava Stories. Accessed 30 April 2020 from https://blog.strava.com/galleries/heatmap/