Digital watches are rubbish for height readings....they go on air pressure and its all highly unacurate.....
If you get any decent gps it should have a height function:) ...but the question is are you going to get the thing out when your 20 foot up, read it, put it away, and then at the same time fly your kite....and prepare yourself for landing....
Best just to say you were bloody high:eek: