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NMEA Altitude Broken?

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7 comments

  • James H. Reinholm

    It seems like your NMEA altitude calculations are based on a reference model that represents the Earth’s surface rather than a geoid that models the actual sea level on the Earth, surface, which is what the NMEA algorithm is supposed to do. This would cause the altitude readings to be about the same, regardless of actual altitude. Do all the altitude readings stay about the same when you gain or lose altitude?

    The default algorithm for altitude is much simpler, since it just calculates the height above a reference ellipsoid that represents the Earth’s surface. The NMEA calculation then adds a geoidal separation factor to get the altitude, which in your case is way off.

    I’m not sure if a recent firmware release is causing this problem, or if the data source for the geoid has changed. This problem might also be peculiar only to certain Android devices. I’ll check into this.

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  • Brian Wagner

    Thanks for the feedback!

     

    I'm on a Samsung Galaxy 8+.

     

    And, yes, the altitudes when using NMEA staying the same and/or changed only a few times.  This resulted in what looked like a square-wave pattern when looking at the altitude graph.  Clearly not right when climbing non-square mountains ;)

     

    Anyway, it's still not clear to me why I or anyone would prefer to use that over the non-NMEA method.  At any rate, if I can help provide any further input to help diagnose the problem, just let me know.

     

    Cheers!  

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  • James H. Reinholm

    The NMEA method is actually supposed to be more accurate than the standard method if it's working right. I don't know of any other BCN user that had a problem with the NMEA method. I'll run a few tests, and see what I come up with.

    I should also mention that using the NMEA method adds to battery drain because of the extra calculations. If you happen to have a "power saving" mode enabled in your device, it could restrict certain calculations, as explained in: https://crittermap.zendesk.com/hc/en-us/articles/115002511183-Power-Saving-Modes-

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  • Brian Wagner

    Ok, makes sense.

     

    I checked and verified no power saving modes enabled on device.

     

    Again, if you need access to my device (or any data from it) just let me know.  I'd be happy to help get this diagnosed/fixed.

     

    Here's how I determined NMEA was my problem: I was standing on top of a ~9200' peak and the trip stats for my current track recording was showing I was at ~4700' for my current altitude.  I decided at that point to hunt through the settings and see if something could be affecting it (again, I've been using this app for years and never seen such wrong elevations).  When I went into the GPS section I noticed the NMEA option and that it was enabled.  As soon as I disabled it, the current elevation was updated to the correct number.  I was literally that "black and white."  Interestingly, the starting elevation was ~4700'.  Its almost as if it just wasn't updating along the way or something.  Or, as you point out, the calcs were just wrong for some reason.  Also of note, the long/lat numbers were just fine...only altitude was wrong with NMEA enabled...which makes sense I guess.  I only point this out to indicate that location/gps was working fine...only altitude was wrong/affected.

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  • Ronald McAllister

    I have had the same altitude problem this week. Thanks for your solution. I am going to try it right now. Very frustrating, I too have lost some trip data.

    (Follow up): And yes, the "Use NMEA..." setting was checked -- and not by me. 

    (Follow up #2): Yes, this solved the problem.  MANY THANKS to Brian Wagner for solving this problem and telling us all so clearly how to do it.  Much Appreciated.  

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  • Shawn D Keil

    I am wondering if this was my problem too...I never had a problem on my S7 with altitude, but I don't recall if NMEA was checked.  On my S9+ it was checked and I was getting flat elevation profiles.  I will have to see if this fixes my problem.  For those of you who have lost trip elevation profiles.  You can export the file as a .gpx and load it into this site http://utrack.crempa.net/ and it will add the correct elevation based on the coordinates.  It fixed all of my messed up track logs.  

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  • Ronald McAllister

    Too bad, but Shawn's fix did not succeed at restoring my lost elevations -- it just reported out the faulty values in BCN.

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