| Code Value | Definition |
|---|---|
| geodetic | Referenced to the direction of the geographic North Pole. |
| Magnetic | Referenced to the direction of the magnetic North Pole. |
| militaryGrid | Referenced to the direction of "north" in a military grid established by a suitable map-projection, for example Universal Transverse Mercator (UTM). |
| Relative | Referenced to a direction established by the context of use (for example: the heading of a vessel). |