How do I restrict building placement?

Heya! I have a good concept for an Empire/City builder type map. So, I've decided to try my hand at WC3 map making. I decided a good place to start is the economy system. Players will have to build various industrial / agriculture buildings in order to increase their income, but I want to limit the number of these buildings.

I want to make it so that certain buildings can only be built in certain areas. For example: Farms will only be built on fertile land, lumber mills can only be built near trees, mines can only be built near ore.

If anyone can give me any suggestions, on how this might be accomplished within the world editor, I'd be very grateful.

Never mind, I found the answer I was looking for. Thanks anyway. :P

For the rest of us who are curious, can you tell us what you settled on doing or what other thread you found to help you?