I think the GPS altitude can be very inaccurate. I went for a hike today, and stopped in a restaurant for lunch. When I went in the altitude was about 150ft. When I came out the altitude was about -630ft, and I don't think the ground subsided that much during lunch . I could upload the track, but I already overwrote the altitude information (this is a new feature I really like!).
Edit: I just realized the location of the track is way off too. It's several hundred feet on the wrong side of a river, with various loops and excursions into the middle of swamps, etc . It looks like the GPS completely lost itself. It was OK when I left the restaurant (the track is accurate), but then a little while later the track goes crazy, and is completely wrong for the next 40 minutes. So forget my comment about the accuracy: the GPS was obviously completely lost. I don't know why, because there is not much to block it and I have recorded a track there before, but it went nuts today.
Edit: I just realized the location of the track is way off too. It's several hundred feet on the wrong side of a river, with various loops and excursions into the middle of swamps, etc . It looks like the GPS completely lost itself. It was OK when I left the restaurant (the track is accurate), but then a little while later the track goes crazy, and is completely wrong for the next 40 minutes. So forget my comment about the accuracy: the GPS was obviously completely lost. I don't know why, because there is not much to block it and I have recorded a track there before, but it went nuts today.