Here is the map of the world as he found it, as put together by Oberon Zell. You must read this article in the ERBzine. Its fascinating, noting such things as early renditions of Carter's Mars were done upside down as that is how astronomers mapped Mars. Very cool.
These two maps give you a sample of the bad assert of it all.
post script: I consider John Carter Warlord of Mars to be one of the best yarns ever penned, with I rank Tarzan of the Apes and King Kong; all these are great stories of man's struggle against the primeval world, the clash of emotionally driven civilization with the indifference of nature. Awesome stuff.