I cycle using an Edge 820. I've found the barometric altimeter on the Edge to be very accurate over the course of rides of various lengths. In short, I trust the values that I get from the Edge. I make use of the Edge's feature to automatically calibrate the altimeter to a known value at a fixed location (my house).
Since buying a Fenix 5+ Sapphire, I wear the watch while I'm riding, but don't bother to record the activity. My default watch face includes an Altitude field, and I generally manually calibrate the Fenix altitude to my house's altitude before the start of a ride.
I'm finding that, if I compare the altitude on my Edge with that on the Fenix during a ride, the Fenix value is usually out, sometimes by over a hundred feet. This is especially true while riding a hilly route.
Is there anything that can be done to improve the Fenix altitude accuracy? Is this a known issue, or might my watch be faulty?