Settling a city can never decrease the yield of a tile. So if the base yield of a tile is higher than 2/1/1 in any category the city center will keep it. For example a wine tile on a plains hill with a river is 0/2/2 so after settling you get 2(city minimum)/2(tile)/2(tile) .
and +0.4
to a Desert tile (technically any tile, but on regular maps it only spawns on riverside deserts). Not sure why it was programmed that way, but it's what it is.
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