Grossly inaccurate elevation gains...

Has anyone else noticed relatively recent grossly inflated elevation gains on activities following an update, possibly some time this year?  I now see this all the time.

Examples:- 2 hours walking around a completely flat running track and rugby field on a stable pressure sunny day.  Elevation gain 265m

2) 400m sprint intervals on a completely flat surface for 40 minutes total time.  Gain 50m

3) Different rugby field for 2 hours, 65m gain.

All runs now seem to finish 30-40m different elevation to where they started at the same spot.

Even turning the altimeter to manual correctly setting the pressure from a calibrated weather station and elevation from a proper topo map immediately before starting an activity it fails to be accurate.

The sensor itself is clean and works normally outside activities other than drifting due to pressure changes (dumb algorithm).

This is clearly barometric sensor related as if i go onto Garmin connect and change the elevation source to DEM then the 265m magically becomes a more plausible 6m, the 50m becomes 9m and 65 becomes 12.

The problem this causes is it thinks even flat activities are on a large gradient so affects pace, effort level, vo2max and so on, artificially inflating them.

Last year and earlier although there were as expected SOME inaccuracies, they were fine and well within limits.  But a 200m elevation difference on a flat surface etc are so far out of range they're a problem.

I assume theres no way to switch OFF the barometer in activities and rely on the theoretically much less accurate GPS and DEM? 

  • I noticed it too (during cycling).

    On an almost flat route (9km, 25m up) it records ~150m gain. It could be related to temperature or relative wind, because after the start the elevation slowly rises and if I stop for a while (traffic lights) it quickly drops ~20m, then continues to rise as I ride again.

    I contacted the support - they asked if the altimeter was calibrated, the port was cleaned, suggested resetting the watch. Nothing helped. Then said that the sensor is damaged and all they can do is offer a new device (FR265) with 25% disocunt.

    You can switch off the altimeter (Sensors & Accessories > Altimeter > Sensor Mode > Barometer Only), but it won't use GPS elevation. You still need to edit the activity in Garmin Connect and change the elevation source to DEM.

  • Not just me then.  FWIW it was fine until a relatively recent update.
    Im certain (despite their claims) its not the sensor.  Outside of activity level, when checked, both the barometric pressure and altitude are correct and alter as they should (albeit with the dumb algorithm as it struggles to work out whats weather and whats an elevation change due to movement...even when asleep).

    This seems to be specifically only in activities it goes wrong and badly (maybe a year ago it did vary but within what id expect, not tens or hundreds of metres in an hour).

    I also suspect the sensor isnt temperature compensated.

    I did experiment by switching to "altitude only" in that menu a few years ago but that resulted in a constant, non changing elevation so that didnt work either for me.

    The issue editing in Connect after the event is, as far as im aware, the damage has already been done in terms of training load, GAP and so on.  I dont think it re-reads previous edits and factors in adjustments.

  • Update to this... More suggesting its firmware related.

    If i set the altimeter to "altitude only" (ie disabling weather corrections) the elevation profile is correct.

    The obvious issue here is for short activities thats fine but for longer duration and/or distance ones where barometric pressure can change over the course of the activity that wont work.

    If it was a faulty sensor, it wouldnt work here either.  This is specifically algorithm related when its trying to juggle the two.

  • I see no strange altimeter readings on my FR255M in activities, neither cycling nor running. I do however "never" sync the watch with the mobile phone, only wifi. I suspect my watch has no idea about ambient pressure or weather outside.

  • Your watch has a barometer.  Thats ambient pressue and how the altimeter works.  Doesnt need to sync or talk to anything.

  • It has a pressure sensor, but it is used both as altimeter and barometer. With an external reference, it can calculate the other. GPS can give it this reference in an outdoor activity, but the altitude part of the GPS location is not very accurate, at least if it is not averaged over time. That is why it helps the unit to know the actual ambient pressure.