Originally Posted by
Carbonfiberboy
AFAIK a Garmin doesn't get altitude from GPS but rather from a barometric sensor. Changes in barometric pressure during a ride will affect altitude readings. If you don't understand explanations here, time to do some studying. A Garmin is not the most user-friendly device ever invented.
Some Garmins don't have the barometric sensor, and just use GPS for elevation. It's hard for GPS to be accurate on elevation without averaging a lot of readings, so it's not quite as accurate as the barometer ones.
But I did have my 705 jump up 200 feet in 10 minutes when I was stopped and a thunderstorm front rolled through.