I can't think Garmin's altitude is very good. I ride with quite a few people with Garmins, mostly 500s and 800s. Group ride, everybody together, everybody will get different elevation readings. If there's much climbing, the readings may differ by 100s of feet. Descents are effected by speed ... do a steep climb with a rapid descent, then turn around and retrace your route in the other direction. The profiles will not be mirror images. Finally, there was the time I did a ride with a guy who forgot his Garmin, so he asked me to send him my .fit file for the ride. Same file, his showed 160+ feet less climbing.
I use Delorme Topo if I want elevation readings. I don't know if it's the most accurate, just what I use.
Last edited by roadbuzz; 06-02-13 at 08:20 PM.