Whilst researching the Custodian of Marvels, I spent many hours reading about the rookeries of London and exploring the city via Google Street View. I chose the Cable Street / Cannon Street area for Elizabeth, Fabulo and the others to disappear into. But I had two problems in fixing the precise geography:

1) Significant areas of the street plan were wiped clean with the clearance of the slums.

2) There is no railway in the Gas-Lit Empire version of London, so the line to the north of Cable Street would never have been built.

I turned to maps dating from before the slums were cleared. It would have been possible to make up a fictional road layout. (It's unlikely that anyone would check up on the arrangements of now-demolished streets to make sure I had it all correct.) But having a definitive map was helpful. It made the place feel real to me. It is my conviction that if a location feels precise and vivid to the writer...

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