I have just started using the direwolf20 1.6.4 pack which includes Opis Map writer and I have noticed something which I don't know if it is a bug or something else. I am the sort of person that when I create a world I tend to run around, like most people I assume, and look for a suitable place to setup shop and if I don't find somewhere I like I usually just delete the world and start again. So the problem is that if you delete the world then create a new one with the same name when you get into the new world the map has everything you discovered from the previous world still there. If you do this multiple times then you have a pretty screwed up map. To stop this I deleted the "map writer sp worlds" folder from inside the saves folder.