Hmm, 100-200m away from the actual turn. That sounds like greenflag is using a different datum. Most mapdata in the US seems to be NAD27, which is about 200m skewed wrt. to WGS84 which is what the gps is reporting. Now NAD27 stands for 'north american datum 1927', so it is highly unlikely that the european map data uses the same reference grid.

Every calculation in gpsapp is based on the WGS84 datum, because that is what the gps is reporting. The datum conversion of the mapsonus route data is performed by parse_mapsonus.py. You'll notice in a couple of places I'm using NAD27toWGS84(Coord(lat, long)). Just replace those with Coord(lat, long) and thel coordinates that you feed into the parser will not get the datum conversion applied.