Strange Garmin Readings

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  • Strange Garmin Readings
  • Premier Icon fathomer
    Subscriber

    I have an older Edge 200 bought second hand, riding buddy has a new Edge 500. Since he’s had it, we’ve been on 3 rides together and whilst distance has been almost the same, the elevation gains been way out.

    First ride on the Chase I recorded 493m, him 786m. Second ride at FoD I recorded 410m, him 517m. And last weekend in the Peak I had 643m, he had 826m.

    Any ideas what might be going on? You’d have thought the 500, being the more expensive would be more accurate (though I’d expect them to be the same), but the figures I’ve recorded seem more realistic. The ride on the chase was the Dog/Monkey and from his house in Upper Longdon so nearly 800m seems well out.

    Premier Icon wwaswas
    Subscriber

    500 has barometric altitude, your 200 is using (inaccurate) satellite tracking so I’d expect his to be more accurate overall.

    bung the ride in memory map or something but even then bear in mind that’ll only pick you crossing contours and you can do a lot of up and down between contour lines without crossing one.

    Premier Icon ChrisI
    Subscriber

    Edge 200 only has GPS altimeter whereas Edge 500 has barometric altimeter which *should* be more accurate. Take both readings with a pinch of salt mind.

    Premier Icon Hooter
    Subscriber

    I have an Edge 500 which spams out sometimes and gives all sorts of strange readings. From googling around it seems like a problem with some units. The altitude readings seem to fluctuate massively – I’ve come home to find the GPS altitude profile completely flattened out a major climb.

    Premier Icon fathomer
    Subscriber

    Thanks for the responses.

    I should have said that the numbers above are what Strava is kicking out, which I guess are the bits mines missing out between the contour lines. I take the reading on the unit whilst out with a massive pinch of salt.

    FunkyDunc
    Member

    My 500 always appears to be out. I know the altitude of my house, and the local trig point on top of the moor. Always appears to be about 100ft +/- either way.

    My mobile phone was more accurate than the Garmin 🙁

    tomstickland
    Member

    The uncorrected elevation gain is unreliable on the 200. Load the fit file into Garmin Connect and you can view the corrected data (interpolated from the map database).

    Garmin Connect, Bikehike and Strava will give different answers for the same file though! It can vary by 200 or 300m in 1500mm.

    “I should have said that the numbers above are what Strava is kicking out,”
    Oh, I see. Oh well. Always quote the biggest number!

    arj256
    Member

    You know manually setting an elevation point reference on the 500 improves altitude readings and keeps them reliable.
    Or just leave the Garmin outside for 20mins before starting makes it more accurate.

    timba
    Member

    My Etrex30 with barometric altimeter varies from 1194′ climbed to 1285′ climbed on the same commute route. It generally hovers around the 1200′ mark though. Distance-wise it’s very consistent. HTH

    FunkyDunc
    Member

    Arj doesn’t work on mine

    Premier Icon andytherocketeer
    Subscriber

    Done this one before.

    I put the same file (from an eTrex) in to Strava, Everytrail and Endomondo. One simply added up all the differences between each successive elevation data point. One smoothed it out to about half that total. The other added in some extra total elevation (Strava iirc). Maybe one day I’ll get a map out and add up the difference between all the contours, which will probably come out somewhere between Everytrail and Endomondo’s interpretation.

    Oh and that’s with barometric data, with an eTrex that had been warmed up for half hour befor the ride, and manually calibrated at the start point, and all data from that warm-up period discarded.

    Friend’s GPS device measured betweeen -50ft and +50ft riding along a beach. Ride that enough and you can get a significant total elevation gain/drop for the day.

    edit: so really it’s not so much “strange garmin readings”, but “strange online logging website interpretation” (although the actual garmin device will says something different, and probably not really the number I’d expect or want to see either)

    mduncombe
    Member

    soooo many things going here.

    Different devices may report differnt elevation gains just down to the algorithm used when capturing the data.

    Barometric elevation is better than GPS elevation data, especially when the weather is settled.

    Barometric elevation has its own issues when you have rapidly changing air pressure, for example when a low pressure system rolls in from the atlantic. The longer the ride the more exposed to the change in atmospheric pressure. Barometric altimeters are often used in Alpine climbing and its common practice to calibrate them frequently during the course of a climb when at known spot heights (lift stations, summits, huts etc)

    Websites will give difefrent results as well depending on the amount of smoothing they apply to the data, agressive smoothing will tend to flatten the data.

    Websites will often apply “elevation corrections” based on survey data, the accuracy/resolution of that data can vary greatly depending where in the world you are. sometime these may be better than the data captured by the device, particulary if it does not have a barometric altimeter, sometimes that correction might be worse.

    My advice, stick with a single device, preferably one with a baromatric altimeter. Dont compare different websites. Do look to see if that website is applying corrections, you may be able to overide its settings.

    For example Strava recognises my Garmin Fenix as having a barometric altimeter and uses the elevation data from the Fenix but Strava does not recognise my Garmin Oregon 600 as having a barometric altimeter (even though it does),so it automatically applies corrections. So both devices might report the same elevation gain (or thereabouts) on their screens at the end of the ride but when loaded into Strava suddenly the Oregon might be as much as 50% lower than the Fenix.

    also worth noting that the weather over the last few months has involved some pretty dramtic pressure drops that would have an impact on the accuracy of a barometric altimeter over time.

    Premier Icon jambalaya
    Subscriber

    @fathomer altitude on gps is very unreliable, especially gain/loss. The gps position changes slightly almost constantly, this includes altitude. When you are moving these little errors left and right don’t make a huge difference to total distance but they do show up in altitude gained/lost in particular. I’ve sat on top of a mountain at a known elevation and watched as over 5-10 mins the altitude settled down to the correct figure, if you where looking at gain/loss it wouldn’t have shown zero. I use gps all time on boats and it’s pretty clear that the altitude doesn’t show zero, it move around. As noted pressure is better but that needs calibrating to be accurate and over a period of time changes in atmospheric pressure (eg the weather) show up as altitude changes. I use Endomondo on my iPhone and do circular walks/rides and the altitudes are way out generally massively over stated gain/loss and despite it being circular showing gain and loss being different.

    Anyway gps is quite good at position but for altitude its poor, that’s my 2 cents.

    Premier Icon fathomer
    Subscriber

    Thanks, some pretty comprehensive replies which explain it well.

    But in general terms, my 200’s crap, his 500’s a bit better and websites like Strava make it up as they go along!

    Might start saving for an 500, maybe even an 800 😀

    mduncombe
    Member

    yep, a 500 used with Strava should be resonably consistent/acurate as Strava will see it as having a barometric altimeter and use that data rather than survey data. even so expect differences from what the 500 reports to what Strava displays due to smoothing by Strava.

    Do bear in mind that while a barometric altimeter does a decent job of capturing your relative elevation it needs some help with your absolute elevation.

    Barometric elevation will vary with air pressure/weather systems even if left at a single location. It needs calibrating, which can be done a number of ways

    1. Autocalibrate, this uses the GPS elevation to set the starting elevation and then uses the barometric altimeter to record further elevation changes during the activity. Autocalibration gives you a “ball park” starting elevation. Give the GPS the best chance by leaving it with a good view of the sky for as long as you can before a ride. for example fire the gps up and leave it on a garden bench while you sort you and your bike out. or if at the trail head, start the GPS the moment you get out of the car so that while you are fannying around with your gear the GPS gets more time to get a decent fix. GPS position and elevation data come down to geometry, the more sats it can see the better job it does, even better if those sats are spread around your view of the sky. Sats low on the horizon give the best geometric data.

    2. Find out the elevation for the start of your ride, if thats your house make a note of it from an OS map and then calibrate the altimeter before setting off everytime. If starting from a car park again use an OS map to find out the elevation of the car park and manually calibrate the device. this should give pretty good results as it provides the absolute starting elevation with the barometric altimeter handling the elevation changes. This will work very well during settled high pressure weather as the ambient pressure will change very slowly.

    Option #1 is the least hassle. option #2 is more accurate but option #1 is just fine if its elevation gain/loss you are interested in rather than your actual elevation aat at given moment during the activity.

    Mark – ex GPS Field Application Engineer & occasional Alpine Mountaineer 🙂

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