OK, got it. It was really just a matter of definition.

Now back to Gilo's original question: How does one best measure VMG? Unless the wind is unusually steady in both direction and speed and there's little current, GPS based measurements might not be very good for trimming. They'll mostly tell you how well you're playing the shifts -- valuable information in its own right, but not the poster's original intent.

This kind of leaves those who have to train with a lone boat in a difficult spot. To know VMG accurately, you have to know both vessel velocity and wind velocity with respect to the Earth...at the position of the vessel. Hmmm...