Blue Station Door/Structure ID Number Signs on the London Underground

Outside Green Park station on Piccadilly, there is a curious blue sign near the busstop where I wait for the bus to school. I realized it must be a tube specific sign since I had also seen it inside the tube station. But what do the numbers mean? I decided to collect a few to see if there was a pattern.





















A couple of signs from Baker Street Station, Green Park Station, South Kensington Station, and Seven Sisters Station.
I'm afraid some of the shots were blurry as tube stations were probably not made for people to stand around and take photos in the corridors and it seemed to me as if people wanted to intentionally bump into me to show how annoyed they were at me for standing in the middle of the corridor taking pictures of weird numbers - preventing them from running a few milliseconds faster. Oh london commuters, you so funny.

I noticed that some were on doors, but some were on walls. All doors had these signs, but sometimes there would be a different sign right next to the door. Sometimes the numbers were sequential, and then at other stations, they were not in sequence! It was not hard to see that the first number referred to the level number, but what was the other number? How was it determined?

Eventually, I've traced it to the TFL's official London Underground signs manual. It says this is the "Standard door and level number sign"; aka "Station area ID codes". Searching for more information on this online seems to suggest, from discussions on various forums, that the first number stands for level below ground (Ground level is 1). The second number apparently refers to a room/structure number which is for the reference of the London Fire Brigade, which can refer to these numbers on a special station plan.





But... how is the second number derived? Why are the numbers so different?


I tried to look for a detailed station plan but could not find one (I tried to find it for one example, Baker Street); I guess the unavailability of publicly available maps might be for security reasons. However, in the process I did find another great map - Detailed map of London Tube, Underground, Overground, DLR, Tramlink & National Rail:



Further searching led me to this 3D visualisation of the station maps for certain stations by Andrew Godwin. Its a cool project but was apparently created from memory so it does not have the data that I am seeking:



Nevertheless it is fantastic to have the complete overview of the position of the stations and all of their platforms. I like the idea of having the grand overview of how these transportation lines work together, bringing us one step closer to finding out the meaning behind each and every sign on the underground...

See Also:
Collection of 3D maps of London Underground/DLR stations
Detailed map of London Tube, Underground, Overground, DLR, Tramlink & National Rail
London Underground signs manual