Wow, I downloaded it and viewed it full size, and the graininess disappeared completely! It's a really cool look you have achieved actually, it's unfortunate that it doesn't come across when zoomed out. I'd recommend taking advantage of that, and adding more locations. At that size you can put in quite a lot without making it too crowded, especially if you make the font a little smaller. I also suggest that you change the texture for the ocean – it works really well to convey vegetation, soft hills, and sharp mountains, but it doesn't read as waves to me. Not sure what to replace it with though, so not very useful advice maybe

Anyway, this is a great map. Have some rep.