Elevation data

I can only speak for the 705, which is probably similar. The 705 has a barometric altimeter which is actually very accurate. The PROBLEM is that you need to adjust a barometric altimeter every time you use it. Since it works on air pressure, the weather can throw it off. So with a barometric altimeter, you need to calibrate it when you start by either entering in the sea level barometer reading, or entering in a known elevation (i.e. I know my house is at 120 feet, so that's a solid calibration point).

The 705 doesn't allow for a calibration, but instead uses GPS to figure out your initial altitude, which is sketchy at best. GPS is a pretty poor altitude reading. The good news (at least with the 705) is that over time, the multiple GPS readings allow the unit to calibrate itself pretty accurately. Still, with rides under an hour, it can still be kind of off. I do find that after an hour or so of running, however, my Garmin spits out pretty similar numbers to my calibrated wrist altimeter.

Finally, the real important thing (at least to me) is not actual elevation, but gain or loss overall. In other words, I really don't care if my ride took me up to a max elevation of 2000' or 3000', what I DO care about is that overall I climbed a total of 5000' combined. So even if the elevation is consistently off by a percentage, as long as it's consistent, I still get good numbers.

Anyway, not sure if your unit works the same way, but if it's got a barometric altimeter, this would apply. If, on the other hand, your altitude numbers are pure GPS, they're not going to be very accurate, especially while moving.