I think you haven't set the "field of view" to a level that fits your screen. Normally it is set to 42, but with an UltraHD display i. e., one needs to go to at least 50, or one will think, that one is sitting on the map, because everything is so large and only so little can be seen.
I understand what you mean with "grainyness" , but I only got that when I zoom extremely near to the terrain, during normal play, where I zoom out to the max and with changed field of view, I can't see anything that could be called grainyness, and that with 0 AA.
But if you don't like it, at least use Blue Marble, the basic civ template is really the ugliest of all options ^^ .