Here are the wind measurement acronyms. I assume OpenCPN
is using them in this way:
AWS = Apparent Wind Speed (as measured from your boat under sail)
AWA = Apparent Wind Angle (relative to your bow, as measured from your boat under sail)
TWS = True Wind Speed (the speed that would be measured if your boat was motionless on the water).
TWD = True Wind Direction (the compass
heading from which the wind is coming, as measured from your motionless boat). This is the wind direction the way a land-based weather
station reports it.
TWA = True Wind Angle (the wind angle, relative to the bow, after removing the "boat motion" component.
When your boat is motionless in the water
, TWS = AWS, and TWA = AWA.
Wind Angles can be displayed in three ways:
Bow = 0
Starboard beam = 90, or 90, or 90 Starboard
Stern = 180
Port beam = 270, or -90, or 90 Port
There are subtleties in the True Wind measurements. Traditionally, the frame of reference is the water's surface, so currents can affect the TW numbers. Some instrumentation uses GPS-derived boatspeed for the Apparent-to-True calculations, and this gives a ground-based, rather than a water-based reference. This gives different numbers in the presence of current
To be useful, TWD, in my opinion, should be ground-referenced, since this is how the GRIBs and weather charts
provide it. It can be referenced to true or magnetic north.