A Tip:
If a civ asks if you want to trade maps, don't just settle for a straight swap, go in an see what they'll offer you. If they are approaching you, then they really want that map, and will be willing to give you more for it. When I first discovered this, not only did I end up with Joan's world map, the original offer, but also Navigation and 530 in gold.
Just keep in mind that the other civs will respond if you continually drive a hard bargain, and might get rather irritated with you over time. It always helps when you take advantage of them this way to toss a few gold, 25-100, back to them as a gift.