Well now that depends on how you aim to use it. To cut a long story short it's not wise to transmit entire maps whilst players are trying to play the game. Therefore it is definately best to have a predefined world with all the map data in it, and then if it is to be expanded, release these as patches to the client. As for the actual data structure the two best methods (which are kinda similar in structure anyways) is to either use an array(s) or a data bank.