Hmmm, so wouldn't the NMEA ($GPVTG - Track made good and ground speed) serve as the current heading? It seems as though the GPS
is reporting TMG via Magnetic heading. Jeeze I could be all wet here. But I remember even my ancient old GPS
's would provide the current Magnetic heading.
Track Made Good and Ground Speed.
054.7,T True track made good
034.4,M Magnetic track made good
005.5,N Ground speed, knots
010.2,K Ground speed, Kilometers per hour
1 = Track made good
2 = Fixed text 'T' indicates that track made good is relative to true north
3 = not used
4 = not used
5 = Speed over ground in knots
6 = Fixed text 'N' indicates that speed over ground in in knots
7 = Speed over ground in kilometers/hour
8 = Fixed text 'K' indicates that speed over ground is in kilometers/hour
9 = Checksum
The actual track made good and speed relative to the ground.
x.x,T = Track, degrees True
x.x,M = Track, degrees Magnetic
x.x,N = Speed, knots
x.x,K = Speed, Km/hr