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  1. #1
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    what one is more accurate?

    My friend and I did a ride together today. We did the same route, but the data seems quite different on my brand new Forerunner 910xt. Is this much variation normal? Thoughts?

    Garmin Edge 500

    12.6mi
    Distance

    2,028ft
    Elevation

    Garmin Forerunner 910xt

    13.7mi
    Distance

    2,421ft
    Elevation

  2. #2
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    pretty typical variation.

    no way to know which is closer to the truth on the ground unless you know what the trail distance really is. and you will never be able to get a measurement without error. it's just not possible.

    elevation measurements are even more error-prone.

    you can look at a map showing both tracks overlaid and see if one of them dropped a more accurate track in the corners or whatever but that only gives you a rudimentary idea.

  3. #3
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    Are there different setting in the garmin devices for "data smoothing" or similar?

  4. #4
    trail projectile
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    Check the wheel size settings.

  5. #5
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    that's less than 10% error between the two. that's not bad.

  6. #6
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    I don't have a setting for wheel size in my 910xt, but I seem to remember that setting on my old Edge 800.

    There is a setting for data recording to be every second vs. smart recording, but I don't think that will change things much except the file size.

    It was more like a 20% variation earlier on in the ride.

    Oh well, I guess this is normal variation. Thanks for the comments.

  7. #7
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    Quote Originally Posted by fitnessgeek View Post
    I don't have a setting for wheel size in my 910xt, but I seem to remember that setting on my old Edge 800.

    There is a setting for data recording to be every second vs. smart recording, but I don't think that will change things much except the file size.

    It was more like a 20% variation earlier on in the ride.

    Oh well, I guess this is normal variation. Thanks for the comments.
    recording interval DOES make a difference in the output. I ALWAYS use 1sec recording on mtb rides. there's a lot of aliasing (shortcutting of corners due to recording interval) that occurs. The more frequent the recording interval, the less it occurs.

    Here's an exaggerated example. See how the GPS track shortcuts the curve?



    FWIW, the GPS with the less frequent recording interval will always underestimate the actual distance.

    this is only one potential source of error. there are others, and some of them will result in an overestimate of the actual distance.

  8. #8
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    I just changed the recording interval to every second and I'll see if the two units compare more closely next time we ride together.

    I read on a different forum that this setting didn't make much difference, but I'll give it a shot for myself. Cheers!

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    Quote Originally Posted by fitnessgeek View Post
    I just changed the recording interval to every second and I'll see if the two units compare more closely next time we ride together.

    I read on a different forum that this setting didn't make much difference, but I'll give it a shot for myself. Cheers!
    The shorter the distance traveled and the straighter the trail, the less difference it makes. But on long rides on very twisty trails, it can make a good bit of difference.

    Just make sure both units are set to their maximum recording intervals when mtb riding. For fitness data collection, anyway, they should be. If you're collecting HR, speed, or cadence data, it's the sort of thing that can change quickly and often, and missing those changes with a too-infrequent recording interval can throw off your fitness assessments based on a given ride/run.

    If you are only interested in location information and your trail is not terribly twisty or you are walking (a hike, for example), then it makes sense to reduce your recording interval to something more manageable. It doesn't take all that long at 1sec recording intervals to blow out the 10,000 track point limit on most units' active log. On my Oregon, doing that requires that I do some file manipulation on the computer before I put the resulting track online.

  10. #10
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    Quote Originally Posted by fitnessgeek View Post
    I just changed the recording interval to every second and I'll see if the two units compare more closely next time we ride together.

    I read on a different forum that this setting didn't make much difference, but I'll give it a shot for myself. Cheers!
    It does make quite a difference, especially on shorter windy trails and when paired with Strava. I rode with a friend for a month, him having a 910xt and me an Edge 500. Mine was on one second while his was on smart and he kept coming out with better speeds (average and segment times) even though I would always be ahead and finishing some sections with a decent gap. We switched his to 1 second and it made it almost identical to my Edge.

    He got an Edge 500 a couple weeks later anyways but the 1 second interval switch should put them both more in line with each other.

  11. #11
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    Great, thanks for the input. I'll give it a shot.

  12. #12
    sheep in FOX clothing
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    Quote Originally Posted by NateHawk View Post
    Just make sure both units are set to their maximum recording intervals when mtb riding. For fitness data collection, anyway, they should be. If you're collecting HR, speed, or cadence data, it's the sort of thing that can change quickly and often, and missing those changes with a too-infrequent recording interval can throw off your fitness assessments based on a given ride/run.
    The "smart recording" on the Edge does attempt to record all the changes in logged data (other than small changes in position obviously), so if you are mountain biking using a HRM and a wheel.cadence magnet, there is enough changing data values to log that it effectively ends up recording once every few seconds anyways.

    All of that said, I do use 1-second recording, because short of 1,000-km rides, I don't know how you'd ever max out the log.

  13. #13
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    yeah, with a HRM or cad sensor, I think the Edges default to 1sec recording. auto on my Oregon produces fine results while hiking, but crap when mt biking. also, when my active log hits 10,000 points, the Oregon dumps 2,000 points at a time into auto archive files and they disappear from the screen.

  14. #14
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    Quote Originally Posted by ghettocruiser View Post
    All of that said, I do use 1-second recording, because short of 1,000-km rides, I don't know how you'd ever max out the log.
    10,000 points with 1 second recording only allows 2.7 hours of recording.

    Quote Originally Posted by NateHawk View Post
    On my Oregon, doing that requires that I do some file manipulation on the computer before I put the resulting track online.
    I just bought a Oregon 450, can you provide some details on how you combine the archived points with the active track?

  15. #15
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    Quote Originally Posted by Slash5 View Post
    10,000 points with 1 second recording only allows 2.7 hours of recording.
    The Edge 500 has more than 10,000 trackpoints. Way more.

    Documentation is vague, but I haven't hit it yet, longest ride about 10 hours.

  16. #16
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    Quote Originally Posted by Slash5 View Post
    I just bought a Oregon 450, can you provide some details on how you combine the archived points with the active track?
    that depends on the computer program on your computer that you use for file manipulation

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