Time Travel Explorer Blog

Unmapped: why Ordnance Survey tried to hide the Post Office Tower

by Peter Watts 18. November 2010 10:22

Post Office Tower


Thanks to Rocque, Horwood and Greenwood, London mapping was already an established art by 1848, but despite some extravagant detailing, none of these commercial cartographers took accuracy quite as seriously Ordnance Survey. That’s because the OS were a military body who made their first maps (of the South Coast) in anticipation of a French invasion from Napoleon, so considered forensic accuracy to be their martial duty. Their staggeringly detailed maps of the capital took two years to produce and the results were remarkable, if completely unusable for the average punter, who really didn’t need to know the size and shape of every single office within the Bank of England – unless they were preparing the mother of all bank jobs, that is.

The OS also cartographers wanted to incorporate height differences into their maps – something that even the likes of Wren had ignored in his post-fire map – so they set up a succession of highly placed ‘control points’ or ‘observatories’ from which they could view the streets of London from above to better gauge its hills and valleys. One such vantage point was placed right on top of St Paul’s Cathedral, which must have provided an extraordinary viewing point over London in the days before tower blocks and skyscrapers. Other high points selected for this process included natural hills and factory chimneys.

Ironically given its supposed accuracy, Ordnance Survey later became best known for an absence of one particular high point. For years, the unmistakable – and unmissable - Post Office Tower was deliberately left off OS maps because it was deemed to be an ‘official secret’ and therefore of such great military importance nobody was allowed to know about it even though it had become one of the most recognisable buildings in Britain as soon as it was opened by Tony Benn, and had even appeared in early episodes of Doctor Who. It did not make it onto OS maps until well into the 1990s, by which time the tower was more than 30 years old.