Defining Areas in Level

Hi
Im in a planing phase of a game i want to create using MMF2.
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The game should be an open world game consisting of one ( 1 ) frame with a size of 10000x10000 pixles. The frame is a top-down map with lots of different terrains areas like grass, forrest, snow, sand. ( image below )
http://i55.tinypic.com/x7nmg.jpg
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The player should be able to wander round the map. I want the player to be able to know what area he is currently standing in at any time. ( image below )
http://i53.tinypic.com/1sz2fr.jpg
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I can alocate areas using x and y coordinated, but that will only creatate square areas, and seeing that my map has irregular shaped areas ( image above ) that wont work for me.

How can i allocate areas in such a map?

Use square tiles, but create some transition tiles as well, to soften the edges.

eg.
http://www.gamedev.net/page/resources/_/reference/programming/isometric-and-tile-based-games/298/tilemap-based-game-techniques-handling-terrai-r934

eg.
http://www.create-games.com/article.asp?id=1869

use your image ^^ as an Active Object, and then compare the getRGB(PlayersX,PlayersY) of it. Then you can tell what colour of the map the player is on.

Exellent! The get RBG method seems perfect :D
Thanks