I suppose most of us use OS maps when developing routes, especially if they're historic ones, like The Friargate Line, that were closed and lifted many years ago. I've trusted them to be realistic with dates until recently but a couple of anachronisms made me start to wonder whether that was altogether wise. A colliery that was closed and demolished by the given date is shown as open and operating on one map but the same spot from a map with similar date at a different scale showed this area after closure. Ooo-err, Missus!
I went to Derby Local Studies Library last Saturday and found a few interesting snippets but didn't have time to check their map collection so I made a return visit yesterday. They didn't have many large-scale maps but a librarian brought me one that was labelled as 1947 and showed the course of the line through Derby from Chester Green, on the north-eastern outskirts, to just before Friargate station. As I traced the course of the line I spotted that it was tagged as "GNR Derbyshire and Staffordshire Extension", which made me look at the Midland lines and I found they were tagged "Midland Railway North Midland Line". Tilt!! A 1947 map with pre-Grouping IDs? An older librarian showed me that the legend at the bottom of the map stated it was the result of a 1912 survey and had been issued in 1914, which made sense. Then she pointed to another legend, at the top this time, recording that this was a 1947 revision and said that the OS would have compared a later survey with earlier ones and only altered what had to be altered, leaving anything else alone.
I suppose the lesson to draw from this is that the map will be more or less correct for the nominal date but that there will be occasional anachronisms we need to be aware of. On the other hand there's always the maxim that Hertsbob told me "You're doing the research so you know what's right or wrong. How many players will know what you know?"
Keith