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  1. #1
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    Issues with elevation data from Etrex30

    Just got my first gps and promptly took it out for the maiden voyage. The track I recorded started and ended in the same place and my elevation in Basecamp shows 697 at the beginning and 661 at the end. From the look of the graph the elevation slowly drops that 36 feet over the course of my 30 min track (but obviously shouldn't of).

    What type of accuracy with regards to elevation should I expect out of this unit? I thought I read somewhere that Basecamp does something to correct discrepancies in elevation data. Is that an automatic process or are there manual steps involved?

    The barometric altimeter was one of the reasons I went with the 30 so needless to say my first outing was a little disappointing. And to make things worse it looks like I have a stuck pixel!

    Whatever floats your bike, dude

  2. #2
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    elevation accuracy/error is well-covered in here.

    suffice to say, error in elevation measurements is not measured as a % of the absolute elevation measured, but rather in a linear amount +/- the measured amount.

    that can be affected heavily by a variety of factors. one being your calibration (or lack thereof). I do not know how the new etrex models handle this. many of the Edge models do an auto-calibration only which does an okay job. other more detailed models allow you to manually set calibrate the elevation at intervals of your own choosing.

    there is also the issue of atmospheric drift. if a weather front is coming in or leaving, your elevation would drift over time. in your case, with the elevation values decreasing by 30ft, could be explained by the barometric pressure rising with a high pressure system moving in.

    but in short, what you see there is well within expected performance. on a good day, you'll still see a few feet of variation. on a bad day, you could see more than you did today.

    Garmin Connect will do corrections, but not Basecamp. keep in mind, however, that while the start and end points will be the same, overall accuracy will not be likely to improve unless your elevation profile is total garbage due to failure to calibrate correctly, or a fast-moving weather system.

  3. #3
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    Thanks for the reply. I will have to look into if my model has a calibration feature. I checked the weather history and the barometric pressure shows flat or slight decreasing over the course of my track. So if I am not mistaken that should of had the opposite effect, if any, of what I saw. I uploaded it into connect and it did correct it for me. Glad you mentioned connect, I completely forgot about it.

    Now just to decide if that stuck pixel is worth the hassle of a warranty claim...
    Whatever floats your bike, dude

  4. #4
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    Quote Originally Posted by Dustin Mustangs View Post
    Thanks for the reply. I will have to look into if my model has a calibration feature. I checked the weather history and the barometric pressure shows flat or slight decreasing over the course of my track. So if I am not mistaken that should of had the opposite effect, if any, of what I saw. I uploaded it into connect and it did correct it for me. Glad you mentioned connect, I completely forgot about it.

    Now just to decide if that stuck pixel is worth the hassle of a warranty claim...
    a drop of 30ft would be a pretty small drop in barometric pressure that may be more local in nature. it is worth noting that there is spatial variation in atmospheric conditions depending on land cover. the one you would be most likely to be aware of is the urban heat island effect where temperatures are higher in cities because paved surfaces and buildings absorb/emit more heat than natural surfaces like leaves and grass. urban heat island effect is one aspect of albedo, or the reflectance of surfaces on the earth.

    how this effects air pressure....temperature and pressure are directly (but inversely) related to each other.

    net result - maybe there wasn't a large regional weather system moving in you'd see from looking at the weather on tv or the web. but it is possible your barometric altimeter was measuring a real variation in air pressure.

    also, depending on how the etrex 30 handles calibration, it may be an artifact of that. in the past, Garmin's auto calibration has sorta kicked in slowly over time, which would show up in the track as drift, rather than an on/off (calibrated/not calibrated) sort of jump. this is a big part of the reason the Edge models have a waypoint calibration, where you can set a waypoint with a known elevation and when the GPS passes it, it will auto calibrate to that point. that would be an on/off situation and would be really obvious in the elevation profile if it was far off to begin with.

    as for the stuck pixel, it might be worth a call to Garmin's tech support to see what they offer as a solution. I have done warranty claims with Garmin before and they're not that big of a deal. biggest hassle is not having the GPS for a week or two, and with that, it's easily manageable if I arrange the timing of the replacement right.

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