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Elevation drift upwards

corvid1728
During the last month or so, the 'home' elevation shown on my watch has risen approximately 1000 feet (from ~5300' to ~6300'). (I record a walking track to and from my gym.) I understand that the barometric altimeter is better at relative changes than absolute ones, but the change shown in my 'base' elevation seems much larger that I should expect. The elevations shown before about a month ago were pretty consistent and reasonable accurate. (Currently on 7.0, earlier versions were automatically updated by the watch.)

Do I need to reset the barometric altimeter, and if so, how? A keyword search in the user manual doesn't show anything useful.


Also, on a related note, I sometimes run the stars at my gym as part of a workout; unfortunately, the stairs are only one story so I need to run up and then run down again, etc. However, the stair count on the watch doesn't match the number of flights I run up and down. Frequently, the watch will show something like 5 flights up and 20 flights down (where the actual number should be approximately 20 both directions). I seem to get somewhat more accurate stats if I pause for 30 seconds at the top and at the bottom, but this doesn't seem to be terribly consistent. Is there a minimum response time in the altimeter that I should know about, or some other factor I need to take into account?
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    The problem is that every time the ambient pressure changes, the watch has to guess whether it was caused by you moving to another altitude or caused by the pressure at your altitude changing.

    I will not claim to understand the algorithm behind those guesses, but it seems to me that the watch will act like this (as long as the altitude is not being forced to a certain value by manual input or by using the GPS altitude reading):

    Quick changes in ambient pressure are assumed to be caused by elevation changes. In this case the watch will show a changed altitude, and it will keep showing an unchanged barometric pressure (which is Garmin terminology is the equivalent air pressure at mean sea level).

    Slow changes in ambient pressure are assumed to be caused by a changed pressure at constant altitude. In this case the watch will show an unchanged altitude and a changed barometric pressure.

    It would be fair to assume that sometimes, these guesses are wrong. If the wrong guesses keep accumulating to the same side, your watch may end up showing an increasing altitude and an increasing barometric pressure, even though the ambient pressure sensor is working correctly, and the average ambient pressure over time is unchanged.

    The cure is to reset the altitude reading, either from GPS altitude or from a manually selected altitude. This will give you the correct altitude and the correct barometric pressure in one go (assuming that the ambient pressure sensor is working correctly) I don't know the 935, but I can tell you how it is done on a Fenix 3. With a little luck, it is the same procedure:
    Open the altimeter widget.
    Press the Start button.
    Select Calibrate.
    The watch will ask "Enter current elevation?" with the reply options "Yes" and "Use GPS".
    If you choose Yes, you can manually enter the elevation.
    If you choose Use GPS, the watch will turn on the GPS, wait for a GPS fix and then start showing the GPS altitude, which will change a few meters up and down while you look at the screen. When it seems stable, press the Start button to use the GPS altitude for the calibration.

    There is also an Auto Calibration option. I think the intent is that it will auto calibrate the altimeter whenever the GPS is turned on for other reasons. But this behaviour seems to be changing with new firmware releases. With the current firmware for my watch, it doesn't seem to be working. I don't know about the 935.
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    Thanks; that was quite helpful. (However, the calibration is in Sensors & Accessories, not Widgets, at least on my 935 with 7.00.) I've gone ahead and calibrated the watch with "Use GPS" and it's now much closer to reality. I also found this online in Garmin Support which was helpful: https://support.garmin.com/faqSearch/en-US/faq/content/fFqaolJqep3QqZY5UqOCa6

    Regarding Auto Calibration, the watch was set to do that, and as noted I regularly engage the GPS as part of my walk tracking so the watch should have reset the altimeter. (It certainly used to most of the time since I bought the watch.) However, as you observe, the Auto Calibrate doesn't seem to be working with recent firmware updates (the past two updates, I would guess, because my issue has been going on for a month with my 935).
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    On my 935, in the last months I have noticed regular altitude drift also, and this seems to be related to changes in weather/barometric pressure. I think that the algorithm which assigns pressure changes to be either due to elevation or weather needs to be tweaked a bit.

    If I am at home for several hours and the local air pressure starts to drop, it thinks I have gone up in altitude and vice versa. Not a big problem of course, but definitely something which can be improved upon.