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GPS elevation query

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  • March 13, 2012 at 11:48 am #15329
    MCLK

    I tend to spend quite a lot of time using sites like Bikely and MapMyRide to plan out longer century rides and then often compare them to the results on my Garmin afterwards. I’ve noticed that the Elevation figures seem to vary quite drastically from site to site, to the point where I don’t know which one to trust – Anybody else had similar problems?

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  • March 13, 2012 at 11:42 pm #670049
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    MCLK

    nick_rearden wrote:You’re

    nick_rearden wrote:
    You’re quite clearly overthinking this; it’s the biggest number, obviously. Unless someone can produce NASA data to the contrary.

    I’m not sure I am. I just wanted a reliable way to study a route before jumping on the bike.

    March 13, 2012 at 10:26 pm #670047
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    nick_rearden

    You’re quite clearly
    You’re quite clearly overthinking this; it’s the biggest number, obviously. Unless someone can produce NASA data to the contrary.

    March 13, 2012 at 3:39 pm #670045
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    miffed

    Id go with the biggest
    Id go with the biggest number.

    March 13, 2012 at 3:39 pm #670043
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    MCLK

    malmesburyclarioncc wrote:I

    malmesburyclarioncc wrote:
    I use a Garmin and elevation figures vary even on the same routes but ridden on different days. How ever you measure it you ‘only’ climb what you climb and not really knowing just adds to the adventure.

    I guess you’re right. I’ll go with the highest figure and be pleasantly suprised if it works out to be less 🙂

    March 13, 2012 at 3:33 pm #670041
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    malmesburyclarioncc

    I use a Garmin and elevation
    I use a Garmin and elevation figures vary even on the same routes but ridden on different days. How ever you measure it you ‘only’ climb what you climb and not really knowing just adds to the adventure.

    March 13, 2012 at 3:07 pm #670039
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    MCLK

    I’ve just had a look at the
    I’ve just had a look at the ride I did on Sunday, 109 km in total. The elevation figures are as follows

    Garmin Barometric 2263m
    Garmin non Bar 2366m
    Bikemap.net 2050m
    MapMyRide 1677m

    So quite a range for the same route. Normally I would probably just go with what the Garmin says but when entering an organised ride, a AAA audax for example, it’s a little hard to get feel for what I’m up against beforehand, when I don’t know which figure to go by – the extra 600m variation (above) would change the nature of a ride quite considerably.

    March 13, 2012 at 2:00 pm #670037
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    Mr Will

    Three different methods of
    Three different methods of calculating elevation:

    1. GPS derived.
    2. Barometric (I believe this is what Garmins use).
    3. Map data.

    All have their plus and minus points, and which is more accurate depends on the exact situation and the quality of the data.

    March 13, 2012 at 12:52 pm #670035
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    MCLK

    I was thinking it might be a
    I was thinking it might be a sampling precision issue, too, as it is more noticable on longer routes.

    Most of the sites I’ve tried use google for mapping so I wonder if the GPS information calculated independently as exporting a common route between such sites still produces different elevation results?

    March 13, 2012 at 12:01 pm #670033
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    thereandbackagain

    Yes.
    I get a lot of
    Yes.

    I get a lot of variation between my Garmin, bikemap.net and bikeroutetoaster. Like you I don’t know what to trust.

    I suspect it’s to do with the sampling frequency used to calculate heights. For example, if the maps are only sampling elevation every 100m then they may miss small undulations that the Garmin picks up, if it’s calculating more frequently.

    However, that’s speculation on my behalf. I’d like to know if anyone has a better insight into what’s happening.

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