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Ascent variance against Edge 800

gofbang
I went for my first ride yesterday with my 820 along with a friend using his 800
Both devices showed 72.5 miles, but whereas my 820 showed 4,900 ft of ascent, his 800 showed nearly 6,100 ft, 25% variance!
Can anyone explain this and whether one of us need to change any settings?
Many thanks
Mark
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    I would also be interested in the issue. As I am getting more and more satisfied with the device, that aspect is still far away from what I'd expect.
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    They are barometric altitude devices If I recall. They therefore use air pressure to estimate altitude. Did you stop and leave them in the sun? Did the temperature change during the ride.
    I had an 800 and have had an 820 for a couple of years. I'd say the 820 is better than my 800 was. But if I stop and l put the device in standby or turn it off then large jumps in altitude can occur. This is particularly noticeable if I take it to a cafe or leave it in the sun.

    I think the reason GPS is not used to estimate altitude is because it is not accurate in the vertical and the changes in elevation are relatively small compared with the changes in position on the ground.

    I'm sure someone will correct me if this is wrong.

    Nick
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    We did stop for half an hour or so for a bite to eat. But you say it's the 820 which can 'gain' altitude when left in the sun; it was the 800 which showed the much higher ascent. I'm going out for another hilly ride with my friend today; I'll see if the issue recurs.
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    We did stop for half an hour or so for a bite to eat. But you say it's the 820 which can 'gain' altitude when left in the sun; it was the 800 which showed the much higher ascent. I'm going out for another hilly ride with my friend today; I'll see if the issue recurs.


    Have you checked which value is closer to the real value in that terrain?

    My 820 tends to give lower values than any of my friend's devices
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    We did stop for half an hour or so for a bite to eat. But you say it's the 820 which can 'gain' altitude when left in the sun; it was the 800 which showed the much higher ascent. I'm going out for another hilly ride with my friend today; I'll see if the issue recurs.


    Golfbang,
    I meant that either of them can lose or gain altitude due to air pressure and/or temperature changes. If you look at the altitude profile of the ride on both. (On Garmin Connect or Strava if you upload to that) you’ll probably see a step change when you stopped.
    Nick
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    Thanks for the replies.
    We did 55 hilly miles together today and my friend's was more or less the same (hard to be exact because we met at the 5 mile point and split at 50). The ride was non-stop this time.
    I've attached profiles of our ride on Monday. They're look more or less identical, but total ascent differs.
    My friend reckons his device over measures from previous experience. I'm just surprised that they can differ by so much.
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    The brew stop was at 35 miles by the way
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    Have you checked which value is closer to the real value in that terrain?


    What is the "real value"?

    Should one include every little bump?

    The gain provided by route planners (like RWGPS) use elevation data for points that are far enough apart to miss gains that cyclists would consider as significant.

    It's not that straightforward to measure gain.
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    It's difficult to see what is going on without the data files. The profiles you posted are against distance. If they were against time differences my be more obvious. You could then see any stops etc.
    If you think of the difficulty of recording elevation it is not a trivial task. I guess anything within 5% is pretty good. So two devices being 10% differnt is not a surprise.

    There is a good discussion of the problems and how it affects barometric sensors as well as other stuff in this link.
    https://support.strava.com/hc/en-us/articles/216919447-Elevation-for-Your-Activity

    Nick
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    Thanks Nick. It sounds like the best thing to do is to not compare to other devices but just compare elevation to my previous rides with the same device. Cheers