Interesting question.
First point: as far as I know, the only way to see the equator on a map is to look at the resource balloons (the arrow of the balloon points to the equator).
I'm going to cover the simple case for maps - but understand that if the map wraps in the X direction instead of the Y direction, then the equator is supposed to run north south. If the map wraps in both directions, the equator is east west. If you like, think of top wrapping maps as simply being turned sideways.
Now, each map generator defines a top latitude, and a bottom latitude. The latitude of the bottom plot on the playing map is the bottom latitude specified by the generator, and the top plot has the to latitude specified by the generator (both of these subject to rounding, which I ignore for simplicity). The plots in the middle have latitudes that use a linear interpolation for these two values. Translation: degrees of latitude per plot depends on the height of the map, but every step covers the same latitude.
As usual, latitudes are handled by the code as integers, so you may have multiple plots with the same latitude.
If the top and bottom latitude are the same magnitude, but differ in sign, the equator will be in the middle of the map. The default map implementation uses +90/-90, so it has this property.
But otherwise, you need to eyeball it. Oasis is +40/0; so the equator is on the bottom.
Here's the list of standard maps that override the defaults:
- Balanced +70/-70
- Great Plains +45/+25
- Highlands +85/+10 (or -10/-85 if you put the ice in the south)
- Inland Sea +60/-60
- Oasis +40/0
- Team Battleground +80/-80
The Space elevator can be built anywhere in the tropics (+30/-30). So you can see that you can build it in the bottom 3/4 of Oasis, the bottom 1/4 of Great Plains, the middle 1/2 of Inland Sea, etc.