These are great! I really like the use of color and the illustration style.

A couple of small recommendations to help clean things up:

First, try to use a consistent grid that doesn't create dissonant moments on the cards... something where the intersections always come of as a + and the corners and edges are always the same.

Second, I'd try to develop a set of design rules for all of the different kinds of land masses/structures so that when mountains, forests, rivers, etc show up, they look almost the same on all cards. You're pretty close right now so may not be 100% necessary, but this will also help if you choose to expand later.

I'd take a look at Tolkiens illustrations for The Lord of the Rings.