It would be interesting if someone who had access to Garmin map data could try this out and see exactly what data this returns

I've got one of their CDs somewhere. I'll see if I can find it tonight. There isn't much documentation in this package, and it isn't compiling for me at the moment (seems to be dependent on an outdated version of libgd). Also, it seems to be primarily a library and I don't see anything that looks like a test program so I am not sure what to do with it if I can get it to compile...

-Mike