It's tough to build a great fantasy world without a map—there's a reason why the Game of Thrones credits are basically a Google Earth flyover—and comic book worlds are no exception. While Batman's "Gotham City" has been around since 1940, it wasn't properly mapped until 1998's "No Man's Land" arc. Over at the Smithsonian, there's a great profile of how this design, created by illustrator Eliot Brown, came to be.
Brown's design draws heavily from the landmarks of New York, and his city layout can be seen in more recent editions of the comics, as well as Christopher Nolan's Dark Knight films.
[Smithsonian, via FlowingData]