How are trade route yields calculated?

[A Space Ostrich]

I think they're calculated based on the opposite yield ie high food equals more production. But that's just a wild hunch.

[Kuciwalker]

I believe it is based on the delta for each good between the two cities. So a high (surplus) food city trading with a low surplus food city = a lot of food.

[regexcellent]

Trade routes also seem to expire after a number of turns so you have to start shifting them around. I'll go into the lua file to see if there's some magic.