Bistro Didi
It's a cool idea but why not do a large cake and then cut the pirate map shape out of it? If you want to do the cupcakes I suppose you can butt them up to one another and ice the overall design onto them.

viv
YES!! it will be a wonderful idea. Make a 9 x 13 cake to be the main island and then decorate it with iced cupcakes to make the cliffs, the mountains, the ocean and the beach. the beach and the waves can be around the edges of the 9 x 13 on a very large platter or large cookie sheet ( 10 x 15 inch) You will need to make several batches of icing: Brown for the 9 x 13, blue for the water cupcakes, white for the beach cupcakes and green for the foliage and tree cupcakes. Add sprinkles of white and brown sugar on top of the white cupcakes to make it seem sandy - crunch crunch. Use a palette knife or a butter knife to lift the blue icing up in wave patterns and if they all go in the same direction then it will look like the wind is blowing over the waves. Make a couple of green icings, one yellowish green, one darker green and one light green. Buy a small bag of each of jelly candy fishes, trees, rocks - sometimes these are chocolate filled. and birds. The fish can top some of the wave cupcakes. The rocks can go on the beach or at the base of a "cliff". You can also buy "gold" chocolate coins to use in your scene. Make one cupcake with an X on top(made with red and black licorice) to denote the Probable location of the treasure. Make mini cupcakes for the border of your map, or use licorice all sorts - they make a great edge and use gelled icing in tubes to write a hidden message and the usual North/South compass points and the Compass Rose (the decoration on a map that has the North South sign. It will be wonderful. Take lots of photos and when you are making and icing and building your masterpiece make sure that you are washing your hands frequently especially after you lick your fingers!!! The hardest part is to not buy too many candies and spend all of your money in a candy shop. You just need a small school of fish - you could use "Goldfish" crackers - a box isn't too expensive. I once made a gingerbread coastal community and found blue jelly dolphins to put in my water. HAVE FUN!!!

ranebo1000
Sanding sugar comes in many colors, so you can use it for the water, the sand, the grassy areas. www.wilton.com has tons of colors. They also have gels, icings, and even cake pans that make pirate ships. Some Walmarts have their stuff. Try a local cake decorating or baking supply store for toppings and pirate themed decorations. Happy birthday!

Steve
You could make part of the map on each cake, so you can only see it when all the cupcakes are together. So one will have the X to mark the treasure, and then there will be lines, and trees and caves and stuff on all the others. You'll need a variety of colors to pull it off.