[sailoog.com]

Ups! es al revés:

http://en.wikipedia.org/wiki/Magnetic_declination
http://en.wikipedia.org/wiki/Magnetic_deviation

entonces:

HDG - Heading - Deviation & Variation

$--HDG,x.x,x.x,a,x.x,a*hh

Field Number:

1. Magnetic Sensor heading in degrees
2. Magnetic Deviation, degrees
3. Magnetic Deviation direction, E = Easterly, W = Westerly
4. Magnetic Variation degrees
5. Magnetic Variation direction, E = Easterly, W = Westerly
6. Checksum

Donde...

1. valor que marca el IMU (Ra)
2. Desvio (∆)
4. Declinacion magnetica (Dm)