I went on the identical run 6 days apart, and the first run had an elevation gain 1,125 feet, and the second run had elevation gain of 2,257 feet. I'm using a Forerunner 220. Has anyone else had this issue?
I don't think the 220 has a barometric sensor nor do I think it calculates or displays gps altitude itself. Let me know if I'm wrong on that. What you are likely seeing on Garmin Connect is a calculation made from elevation information in a map database that is compared to your gps positions in the activity log. A lot of things can affect the accuracy of the gain reported.
The accuracy of your position plots from one activity to the next. Even if the accuracy was excellent, then it is still very likely the positions were not in the same location. IE. one run plotted you half way up a small rise then half way down for a net of almost zero gain, while another run plotted you at the bottom of a small rise then all the way at the top, for a potential gain of of whatever the rise actually was.
You really have to make your best guess at what is the better value. But averaging over time will give you a good one.
