Speed through the Water vs. Speed over Ground as it relates to True Wind is an interesting discussion topic. The following is an extract from Raymarine:
Various Raymarine displays are capable of displaying a variety of Wind data types, which sometimes cause confusion, so we’ll explain a little about what each data type is and what other data is required in order to calculate and display.
Angle or direction?
When we refer to a data item as a wind angle (e.g. Apparent Wind Angle, AWA), we mean relative to the bow (0 to 180 degrees port or starboard.) When we refer to something as a direction (e.g. Ground Wind Direction, GWD) this is relative to North.
Apparent Wind
Apparent Wind Angle (AWA) and Apparent Wind Speed (AWS) are the default displayed data on dedicated wind instruments such as i60 and predecessors. This is directly measured by the masthead sensor and doesn’t depend on any external data. Apparent Wind will vary depending on your boat’s motion. As examples, if you’re motoring at 10kn dead upwind into a 15kn breeze, your AWS will be 25kn and your AWA will be 0 degrees (wind on the bow.) On the other hand, if you’re motoring dead downwind at the same speed, in the same wind, you’ll see 5kn AWS from astern (180deg AWA.)
True Wind
What Raymarine refers to as True Wind (TWA, TWS) is always wind-over-water, not wind-over-ground. We calculate this from AWA and AWS, plus speed-through-water (STW, from a paddle-wheel or equivalent.) It’s not possible to calculate proper True Wind from GPS speed (SOG.) If you’re looking at your wind instrument and you have AWA and AWS but TWS is showing as dashes (-.-kn) then you don’t have Speed data coming in. In addition to TWA and TWS, our recent multifunction displays and i70/i70S instruments also offer TWD (true-wind referenced to north rather than the bow.) This requires compass Heading as well as STW. We can’t use GPS COG for this because COG is the direction you’re moving, not the direction you’re facing (which is what Heading measures.) Lighthouse 2 MFDs also require Heading in order to display TWA/TWS.
Ground Wind
Ground Wind is what you’d measure standing on the dock, referenced to north (GWS and GWD.) This requires AWA/AWS (direct wind measurement), GPS COG/SOG (how the boat is moving over the ground) and compass Heading (where the boat/wind-vane is pointing, relative to north.)
Why is Ground Wind not very useful for sailing?
Put simply, your keel’s in the water, not on the ground, so the wind angles you sail to are wind-over-water, not wind-over-ground. On a lake or at slack water the main difference will be your leeway, but in tidal waters or in offshore currents, ground wind can be very different from true wind, which is why our sailing-oriented wind instruments only show true wind (although on faster boats, apparent wind is probably more useful still of course.)