I have a garmin edge 705. Every week I ride the
The reason is linked to startup of the device probably. I have same issue.
2 ways to improve :
1/ Startup your device
You need to startup device, and wait automatic change of the screen to the ride mode, from the sattellite search screen.
You also can check you have more than 4 sattellites before starting a ride. Altitude can only be computed when at least 4 sattellites are synchronized. So start your device in the open air, not indoor, nor close to a building.
Garmin devices start automatically and have probably a smoothing function that make altitude converge to the reality slowly from a started erroneous capture.
2/ Upload your ride on Garmin connect site, then apply altitude fix algorithm proposed. This algorithm will fix the altitude of your waypoints, both fixing pure altitude start issue, and also fixing denivellation computation, which is erroneous because of cumulation of errors.
Hope this helps
Garmin EDGE 705
on Oct 11, 2010