You would need a flying GPS similar to the units used by hang gliders which give a more accurate measure of height.
I did wear mine when I went out for and (attempted) wave sail to see the tracks afterwards, unfortunately to smaller sail and a plummeting breeze just showed how far I had to swim

Oh well, that's the first swim in over the reef out the way for the season


Will wear the little black box when I go out next to see if I can get any usable data