from wikipedia
http://en.wikipedia.org/wiki/Gotham_City
Gotham City's geography, like other fictional cities' geographies in the DC Universe, has varied over the decades, because of changing writers, editors and storylines. At various times the depiction has Gotham on the shores of "Lake Gotham". The majority of appearances, however, place Gotham on the eastern coast of the United States.
Maps shown in various comics have depicted the city in different places. Many of the maps directly use Manhattan, Vancouver, and other real coastlines as their basis, while others are completely original. One map showing Gotham City in relation to Metropolis, the home of Superman, published in New Adventures of Superboy #22 (October 1981), placed Gotham City and Metropolis on opposite sides of a large bay. In Swamp Thing vol. 2, #53 (October 1986) the geography of Rhode Island was the basis of another map of Gotham City. The current definitive maps of Gotham City are those based on the ones produced for the "No Man's Land" story arc.
The distance between Gotham City and Metropolis has varied over the years, ranging everywhere from being hundreds of miles apart to being twin cities on opposite sides of a large bay. Blüdhaven, a city that for several years was home to Nightwing, is located near Gotham City. Additionally, the Seven Soldiers of Victory series Klarion the Witch Boy, calls New York City the "Cinderella City", referring to nearby Metropolis and Gotham as its "ugly step-sisters".
One older theory was proposed by Mark Gruenwald, who later went on to be a major writer/editor at Marvel Comics, and published in the 1970s in the DC house fanzine, The Amazing World of DC Comics in an issue dedicated to the Justice League. Gruenwald suggested that Gotham City is located somewhere in the state of New Jersey while Metropolis is located in close vicinity to Washington, D.C.
A Gotham City driver's licenceMan-Bat #3 refers to Gotham City being in the Central Time zone.
According to the Planetary/Batman one-shot, a Gotham City also exists in the Wildstorm universe. It is similar to its DC Universe counterpart, but is not usually home to costumed vigilantes. In Captain Atom: Armageddon Gotham City does not exist in the Wildstorm universe.
The Atlas of the DC Universe, published in 1990 by Mayfair Games Inc. as a supplement to the DC Heroes role-playing game (under license from DC Comics), places Gotham City in southern New Jersey (and Metropolis in Delaware). This source, never officially recognized by DC Comics, has since been contradicted with regards to other locations.
In the Batgirl series, as well as in the Vertigo Comics' Sandman series, Gotham is implied to be an entire state, analgous to New York, with Gotham City as its capital. In both cases, the book refers to "Upstate Gotham".
A Gotham City driver's licence shown in Batman: Shadow of the Bat annual #1, contains the line "Gotham City, NJ", placing Gotham City in New Jersey.
Detective Comics #503 (June 1983) includes several references suggesting Gotham City is in or near New Jersey. A location on the Jersey Shore is described as "twenty miles north of Gotham." Robin and Batgirl drive from a "secret New Jersey airfield" to Gotham City and then drive on the "Hudson County Highway." Hudson County is the name of an actual New Jersey county.
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