On Sunday, August 11, 2013 10:19:40 AM UTC-7, Dan Marotta wrote:
What are you trying to say. Seems to me that the airmass is carrying the

thermal along and the glider with it. With that premise, circling in calm

air or in a wind makes no difference. I've thermalled straight up and with

a 20+ kt drift down wind. I never had to hold heading into a wind to

maintain center in a thermal.



Or do I misunderstand what you're saying? If that's the case, please

elaborate.







...

On Saturday, August 10, 2013 8:43:05 AM UTC-7, Dan Marotta wrote:

Your glider is drifting with the wind, too...









Unfortunately, the earth, and the GPS system that tracks with it, is

not.....

It makes a difference if you are trying to relate your thermaling track to a track projected to the ground. Some thermal assistants (like SYM) draw a track on the ground color coded by climb rate. If you are drifting with the wind, this track quickly drifts upwind relative to you and the airmass (which are drifting downwind). Therefore the next time around your circle, the track will not have much relevance. The only thing that matters (as you have said) is the glider relative to the airmass, which the GPS cannot accurately track and plot. That relationship has to be inertially derived, or simple inferred to be static. The GPS plot is relative to the earth, which can be thought of as moving underneath you at the speed of the wind. Such a plot is of little value when thermaling though they can be useful for ridge and wave, which are effectively anchored to the earth.

There have been some attempts to guess the drift of the thermal from an assumed or calculated wind speed, but most thermals where I fly are not so well behaved.