In Tropico 4, when I select to construct a mine I get an overlay showing the location and density of ressources.
Do I build the mine right on top of that or rather beneath the ressource field?
I am pretty sure I once built a gold mine directly on top of the ressources and it was unable to mine anything.
Same with farms: When I want to build a farm, I see which area is best for growth. Do I build the farm into this area, or right beneath it so the field will be there?
According to experiments and documents referred to in this Tropico 3 thread on the official forums:
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.
Although the Tropico Wiki says it otherwise (and does not give any proof for their claim), I've always placed my mines directly on top of the resources (both salt and mineral mines) in Tropico 4 and all the mines eventually depleted all of their reserves. I can't say for sure whether miners work faster if you place the mines next to the resource or right atop of it, but I am certain that it is not true that miners cannot get resources that are right below the mine building.
Check this screenshot below. There was an iron patch in the place of the army base before and there were two mines on top of the patch. So practically most of the iron ore was located directly under the mine buildings but still, that resource got depleted after a few years:
Image is from the sixth extra mission named "Super Hero" (Or "The Masked Avenger" according to the Wiki). You can check the initial state of the island where you can spot the iron patch in this place.
A similar correction for farms: If there are natural trees on top of fertile land and a farm is near, farmers clear the trees and plant their crops. Natural trees don't prevent farmers from working that specific square with the trees. I didn't try this with the trees that are planted by the order of El Presidente.
I personally always build a mine on both sides right next to the indicated resource marker so I have a lot more miners hired and working on the spots like gold, since there's not as much as iron.
This is relevant to Tropico Paradise Island but may also apply to other Tropico versions.
In order for mines to be productive the mine has to be built close to, but not on top of, the yellow area for the minerals. But the mine also has to be built BELOW it. Why? Because the miners move half as fast when they're carrying minerals, speed is a lot slower when moving uphill (and makes them tired quickly) so it follows they will be a lot less productive when carrying minerals uphill.
You know that a mine's not going to be profitable anymore when they start going below the mine for minerals, although gold sells for so much that it can run for maybe ten or fifteen years after that.
Mines typically are profitable for about 25 years. And if you have a huuge mining plot you should build mines on the outside then tear them down when they're not profitable, rebuilding them working your way in. On flat islands only gold mines are worthwhile, and they don't last long.
