I've recently started running in a new place that is very flat (Phoenix) compared to where I used to run before. I'm seeing weird issues with the altimeter/elevation data. Most of my running routes have no more than 30-40 feet of elevation change according to caltopo and google maps. When I do a running activity on the watch, it would track the elevation pretty well for the first 10-15 min of a run, and then the data would suddenly get very noisy. The average elevation level would still be relatively correct, but the watch records a large number of dips and spikes, which it then adds up at the end to come up with a 500ft elevation gain for the whole activity, which is completely wrong. This only seems to happen on flat road-running routes though. When I try to go on some trails with actual elevation changes, the watch tracks those very well and has never had any significant errors. I tried doing the altimeter calibration before each run, but that doesn't seem to make a difference. Here is an example - notice how the data becomes very noisy after about 12 minutes of good tracking:
tennis court :) 