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Elevation accuracy in Garmin Forerunner 945

AngelaFrench
over 4 years ago

I use GaiaGPS to plan routes in the backcountry, mostly for hiking.  I planned a route that was 7 miles long and supposedly had 1892' of elevation gain.  I wore my Garmin 945 on the hike yesterday and its finally reading was just under 7 miles, but only 1007 feet of gain.  How could it be so wildly different than GaiaGPS?  

  • 0 over 4 years ago

    Because GPS functionality in 945 is not so good as we expect.

    Shame on you Garmin

  • 0 over 4 years ago in reply to MarinusvD

    As we don't know:
    a) Where
    b) GaiaGPS's elevation data source

    I can't say it's Garmin to blame. GaiaGPS could have as bad elevation data as Garmin Connect Web has, so the 1007 could be more realistic than the 1892. 

  • 0 over 4 years ago

    Elevation gain is a tricky topic and no two devices or methods will give you the same results.

    People here will complain that the watch GPS is inaccurate but the fact is that while it matters for absolute altitude measurement, it doesn't matter for elevation gain as only the barometer is used and GPS is only used (if you enable automatic calibration during activities) to take out the variable part due to atmospheric pressure changes at constant isobar. And the watch barometer is actually pretty accurate IMHO.

    So, what is going on? The answer is that you cannot measure elevation gain any more than you can measure the length of a coastline: the more smoothing you apply, the smaller it gets, the more granular you go, the bigger it gets, towards infinity in theory.

    Now apps that use GPS only with very little filtering will show a lot of elevation gain (because GPS is very inaccurate on the z axis) and a device that uses barometer only and with a lot of smoothing will be a lot more conservative. Apps based on maps with elevation (DEM) will tend to have the smallest elevation gain, in general.

    Which one is right? Neither. You can only reasonably compare measurements against each other from the same device with the same method.

  • 0 over 4 years ago in reply to KubrickFr

    DEM is clearly best, if you have good data, but if you had bad global not so accurate data, it's another thing.

    Eg. I've noticed that the DEM Garmin uses is bad here. They could pay a little and use this and make some update process it as the data is continuously getting better.
    https://www.maanmittauslaitos.fi/en/maps-and-spatial-data/expert-users/product-descriptions/elevation-model-2-m

    Would probably make the elevation corrections real good.

    I don't know what data Garmin is using here, but https://www.gpsvisualizer.com/DEM_coverage.php uses much more better data. Even using that would make the situation better.

  • 0 over 4 years ago

    The way I see it we payed extra for innacurate barometer and for useless oximeter. Name one person that thinks oximeter is correct or useful. Garmin is running a scam, promoting stuff that doesn't even work correctly and charging extra for it. All watches that don't have barometer will measure the altitude just fine with only GPS. Yeah, go downvote me now but you know it's true.

  • 0 over 4 years ago in reply to Anže
    Anže said:
    All watches that don't have barometer will measure the altitude just fine with only GPS

    I don't downvote people, but this is off topic, we're talking about measuring elevation gain, and yeah, barometer will be more accurate than GPS for that.

  • 0 over 4 years ago in reply to KubrickFr

    I tried it personally and with the DEM calibrated barometer it is more precise than with gps corrections