I use an Android GPS cycling app to track my bike rides and I would like to get a reasonably accurate estimate of the total ascent I did on a ride. I understand that GPS is very inaccurate at determining elevation, so I have been importing the resulting GPX files into several sites that use mapping data to determine elevation, and I've found they're ridiculously inconsistent. For example, from a 30-mile ride I get the following climb data from four sites using the same GPX file:
CardioTrainer 980 Strava 1085 MapMyRide 554 RideWithGPS 2131
The results are consistent over multiple rides, and I can add more sites with an equally mixed set of results.
I think I can easily toss RideWithGPS as a badly flawed outlier based on subjective experience alone, but MapMyRide is consistently about half the ascent of the others, so it makes me wonder who is off by 2-to-1.
My question is, how can I determine whose algorithm is the most accurate without spending hundreds on a device with a barometric altimeter?