NMEA HDT
The NMEA HDT datagram provides the true vessel heading. The information is normally provided by a course gyro.
NMEA HDM
The NMEA HDM datagram provides vessel heading in degrees magnetic. The datagram is no longer recommended for use in new designs. It is often replaced by the NMEA HDG telegram.
NMEA HDG
The NMEA HDG datagram provides heading from a magnetic sensor. If this reading is corrected for deviation, it produces the magnetic heading. If it is offset by variation, it provides the true heading.
NMEA THS
The NMEA THS datagram provides the true vessel heading. The datagram includes a mode indicator field providing critical safety-related information about the heading data. The THS datagram replaces the deprecated HDT.