Businessman Charles Booth was sceptical about a claim in 1885 that a quarter of Londoners lived in extreme poverty, so he employed people to investigate.
They found the true figure was 30 per cent. The findings were entered onto a ‘Master Map’ using seven colour categories, from black for ‘Lowest class, semi-criminal’ to gold for wealthy.
The authorities were terrified into action, and the first council houses were built soon afterwards.
Peter Barber, Head of Map Collections at the British Library, gives us 10 maps that changed the world. That is number 6.
Number 5? Google Earth.