Source? Any more information on how it is known the layout was like this?
London
"who’d a thunk it"
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I don't know where that specific image is from.but there are many like it based on what we know about Roman London (like this from the Museum of London), which is quite a bit given the bits that still exist (there are various guides to visiting them but the Temple of Mithras and Roman Wall are good ones) and the extensive history of excavations across London.
If you want a book in this then Lacey Wallace's The Origin of Roman London is from 2014 and seems one of the most recent more academic books on the subject (it's a bit expensive but a library could order it for you). If you just want a quick read then the Wikipedia article is well-sourced and looks to be a decent overview