Placing mines and farms

According to experiments and documents referred to in this Tropico 3 thread on the official forums:

• Farms should be placed such that the area surrounding the building has the highest yield potential. Ideally, you place your farm on a spot with horrible yield but that has high yield around it to maximize use of farming land.
• Mines need to be placed on top of the deposit for peak efficiency.

Since there isn't much about the mechanics of the game that has changed between the Tropico 3 and 4, it's unlikely that Tropico 4 is any different. There are posts about Tropico 4 that say pretty much the same thing about farms, although they are on other non-official sites.

FWIW: My observations through my own playthrough in Tropico 4 supports the above statements. I don't have screenshots of overlays vs. crop placement or mine placement vs extraction rate, but the most productive mines for me have been right on top of the overlay area and the most productive farms were surrounded completely by green overlay areas with the road 3-5 squares away. They will build across the street to get to good land if they have to, but I usually place my road down a stretch of spread out farms for maximum effectiveness.

According to the Tropico Wiki:

Mines must not be located directly on top of resources, since the building will hinder the mining process, but they must be located very close to maximise production.

This matches with what the graphics indicate - when the mine was on top of the deposit, only the squares not covered were being mined, but when the mine was just off to the side, almost the whole area had miners.

Workers cannot mine or harvest crops where building and trees are placed. So I would avoid placing mines and farms directly on the green area, and instead place them a little further away from the green area, as this lowers their efficiency. Placing the buildings on the yellow area seems to be the most efficient way of doing it, as the workers gain easy access to the green areas, while still having some yellow areas around them.