Definition(s)
True Heading
True heading: horizontal angle between the vertical plane passing through the true meridian and the vertical plane passing a through the craft’s fore and aft datum line. It is measured from true north (000º) clockwise through 360º.
Source: IMO Resolution MSC.116(73), Recommendation on Performance Standards for Marine Transmitting Heading Devices (THDs), 1 December 2000, International Maritime Organization. Regulatory Guidance