Basically, we had an existing map of Westeros and a xeroxed hand drawn map of Essos - both done by George R. R. Martin - and I took those into Photoshop and played with their scale until they lined up perfectly. The actual dimensions, the locations and their placement, and the different terrains are all based strictly on George R. R. Martin’s maps. It was really important that we stay as absolutely true to the books as possible because of the ardent fans out there.
Wall also works as an editor, often on David Fincher films. He won two Oscars for editing The Social Network and The Girl with the Dragon Tattoo.