If I had CorelDraw, I could do this. Unfortunately, I only have GIMP to work with for now. There has to be a trace function using GIMP, separating the traced layer from the original. I haven't been able to figure it out, and the GIMP user manual isn't much use. Grrrr ..... I was hoping to avoid investing in software .....
Adobe Illustrator has a Live Trace function which might be tweakable for what you're trying to do - inkscape is the open source version, perhaps it can do something similar...
What you need is to create a new layer, trace by hand over the original on the new layer, maybe in red or something like that to distinguish the two, and then delete the old layer. The exact details about which menus and buttons to push for creating or selecting which layer your on I will leave to the Gimpers. I think Devin probably has the easiest solution given the type of map your trying to extract - unless this is a pretty simple part of a very extensive or complex set of them.