For some reason, pinpointing the location of fictional cities in our real world seems to be a shared obsession among comic book fans. It's not as tongue-and-cheek as the intentionally ambiguous location of Springfield in The Simpsons ("Oh hi-ya, Maude"), but the debate about the geographical coordinates of cities like Metropolis and Gotham City is one about establishing canon and wrapping our heads around the logic of that fiction. While Marvel's heroes roam around a fictional New York, DC's icons each have fictional domains of their own--even though they were originally conceived as different shades of the Big Apple.
In comic book canon, DC Comics has identified Gotham and Metropolis as being cities on the east coast, in New Jersey and Delaware, respectively. But their location isn't enough; these cities need canonical geography of their own, and that's why artist Eliot Brown was hired in 1998 to map out Gotham City for the Cataclysm story arc. That mapping has endured as a foundation for storytelling in Batman comics and films, most recently used as guides for Christopher Nolan's Dark Knight films. The upcoming Gotham TV series may tap into that geography as well. What's interesting to me is the connection that cartography establishes between reader and story--the visual mapping of imagined spaces makes the fiction that much more believable, whether it's a map of Middle Earth in Lord of the Rings or Westeros in Game of Thrones. In the inherently visual medium of comics, the "rules" established by cartography are even more resonant. Just think about how many times you've seen a huge two-page spread detailing the layout of the Batcave. (h/t Smithsonian Mag)