Garmin Edge - elevation & mounting

We were comparing notes on our ride yesterday - myself, someone else with another Edge 705, and someone with a different brand of computer (don't know what kind). I had 3600 feet of elevation gain, the other 705 had almost 4100, and the third computer had 4300.

The person with the other Edge told me that it makes a difference whether the computer is mounted level, and that after he leveled his, his elevation numbers more closely matched the other person's.

Is this true? Anybody know? I used the wedge to mount my computer fairly flat, but I never checked it with a bubble. My stem has a 7 degree rise so there might very well still be some slope to the computer. (bike is at LBS now so I'll have to check it when it comes back!)

Oakleaf, I don't have a great understanding of the electronics, but I don't think the mounting has anything to do with how it records elevation. I have an Edge 305 and from what I've read on it, it uses both the satellites and barometric pressure to record elevation change, neither of which would be effected by how it's mounted.

As for the discrepancies, I've noticed that what the unit displays after a ride is different from what is shown in the Training Center software once it's uploaded, and I always go by what the software tells me the elevation was, not the unit itself. I believe that the unit uploads the track information to the software, which then plots it, so that should be the more accurate reading.

Don't think it makes a difference. Garmin gets the absolute elevation from the satellites, then uses barometric pressure to get the difference in gain. I've sometimes nixed the mount altogether and just tossed the Edge into my jersey pocket and still get readings OK.

That being said, I do find that Garmin elevation gets off if you don't let it "warm up" enough before hitting start (that is, you're standing still and the elevation is still changing even after locking into the satellites), or if a weather front moves in.

-- gnat! (movin' on up)