Yesterday my roommate at work was watching the 1982 Thames documentary. Towards the end there is this scene where Dick and Roy posed themselves in front of Roy's gigantic drawing of the city. You can see that it would take up an entire wall. Roy did this and a lot of his other work for the Thief after hours and on weekends, on top of the work he did during the day on commercials or title sequences.
I found this photo of the drawing on Michael Sporn's blog:This drawing was used as a map to help them figure out the geography of the film's story. It was also used as a style guide for the backgrounds and parts of it where copied into BGs that made it into the film, the scene "Mouth to Mountain" for example, that we've mentioned in earlier posts.