Thanks for that Paul, that was very helpful to me. I did some digging and found that the Mac version of Java is bugged in certain ways, one of which is the way it responds to ShowSaveDialog, returning an incorrect path which in turn is giving me the substring error. I'll have a play around and try and get a fixed version up soon. It may be that the program will lose the ability to create a new folder for saved files as this is the easiest fix.

Ravs: Heh. The ability to separate the layers was the reason why I took up the programming of this in the first place and has been there since the very first version that I implemented. I'm impressed that you got such good results in your last map with just the single layer and also realise now that you probably had no idea what I was talking about when I said you should have used the mask layer to underpaint your trees to stop coasts and rivers showing through.