But there is a GPF file, (as opposed to a GPX) which appears to be in something like an XML format, and about which I can't find any info, hence my query here. As mentioned, this kind of editing does work on the Garmin 595, so there has to be something similar with Montanas.
I believe that the maps themselves incorporate a TYP file, which can be extracted, modified, and put back, but I've yet to experiment with that angle.
Bookmarks