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In most electronics these days, this is a sailing term that measures your speed made good in the direction of the wind. There are different ways to define and measure it.

With just wind instruments and a knot meter, you can compute the true wind direction (relative to the water) and then project your boat speed into that direction using the true wind angle, computed from the measured apparent wind angle.

Adding a GPS and heading sensor, you can compute the true wind direction (relative to fixed earth–its normal definition) and then project the SOG onto the true wind direction. This is the most accurate measure of your progress in the direction of the wind from a navigation point of view.

See related discussion at true wind from apparent

In the past, and maybe still in some GPS displays, VMG is used to represent WCV, the waypoint closing velocity. This is the SOG projected in the direction of the next waypoint.

Hence we need to check our instruments for VMG-Wind or VMG-waypoint

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