True Wind

The wind velocity relative to the Earth’s surface rather than relative to a moving vessel.

Apparent wind is the vector combination of true wind and the vessel’s velocity through the chosen reference frame. Marine instruments calculate true wind using measured apparent wind plus speed and heading or course data, and their result depends on whether water- or ground-referenced motion is selected.

Sensor position, mast motion, calibration, current and data-source errors can distort the calculation. ‘True’ does not mean perfectly measured, and a forecast wind at a standard height may differ from the local wind experienced at the vessel.