The National Library of Scotland has an absolutely phenomenal online resource for those of us with ancestors from nineteenth-century London, Ordnance Survey Maps, London, Five feet to the Mile, 1893-1896. Yes, you read that right, five feet to the mile – the maps are so detailed they even show the location of public urinals! And because one of the viewing options is as an overlay on a modern online map (you even have a choice of background maps) you can match up the nineteenth-century locations to the modern street plan – I’ve just used it to locate three vanished churches in Bethnal Green.