I didn't know you could get erasable water based marker pens - do they stay on quite well? (ie. does the mark stay on without being accidentally rubbed?)
WH Smith or any other similar store sells them right alongside the permanent markers (look and write exactly the same). The line can be accidentally "smudged" but from experience that is rare and does not cause me any problems (not easy to erase the complete track line).
Plus point is that you can easilywipe the chart after the trip without going in search of the meths.
