Elevation seems consistantly low by about 250 feet. (Android/Nexus 6)

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    sjmj3us

    I would be curious to know if it's a constant 250. I don't  seem to be off by that much. I do however have a fair amount of jitter. The altitude calculation comes from a very complicated Kalman filter. GaiaGps as near as I can tell implemented their own version (rather than using what's available directly on Android), which probably makes sense because Gaia is optimizing for hiking.

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    Nathan

    @jaymmartin_buy

    I'm sorry there is an issue with the elevation readings in the app. We have an open bug report for this and I have included your information in the investigation.

    Can you let me know if restarting your device improves your elevations readings?

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    jaymmartin_buy

    Rebooted, same thing,  680-698 feet for 880 foot on topo map.  9-10 GPS Satellite lock.

     

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    jaymmartin_buy

    Just downloaded "Acurate Altimeter" Android.

    Gaia GPS 686-702 feet

    AA: 789 feet GPS Elipsoide

    AA: 867 feet Elevation

    Topo Elevation ~ 880 feet

    It is almost like Gaia GPS is adjusting in the opposite direction of the Geoid

    bumpy earth model.

     

     

     

     

     

     

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    sjmj3us

    That the GaiaGPS and the Accurate Altimeter (AA) GPS calculations produce significantly different results is concerning. I am somewhat familiar with GPS calculations and there are a lot of aspects you have to consider, each additional aspect you incorporate improves the estimate. Perhaps a Kalman Filter with more states would help. 

    The other issue is like AA, Google Earth incorporates DTED, if GaiaGps supplemented with DTED I think that would also help. 

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    Nathan

    @jaymmartin_buy & sjmj3us
    Thank for the feedback and including the altitude readings. I have also linked your comments to the bug report.

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    bcorwin

    I am having this problem, too.  9/3/18 using Moto E (4) Android.  I was sitting directly on the 7600 elevation line and the app said my altitude was 7453.  It was not consistently off by 150.  At one point it was off by 80 feet.  The app was always lower than the map. I was hoping that when I uploaded the track to my account that it would correct the elevations but it did not.  As I mouse over the elevation profile of the track, when the indicator button crosses the 7600 topo line on the map in Gaia Topo Layer View, the elevation profile reads 7446.  If you want I could send you the track.  It's a pretty graphic demonstration of the problem; as you mouse over the profile the elevation readings are consistently lower than the topo lines on the map. The discrepancy ranges between 50 and 200 feet.

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    sjmj3us

    The maps use GEOID/mean sea level (MSL) elevations. It looks like GAIA GPS uses elevation over the ellipsoid (that's what I suspect at least). That would put it low by around 100 feet (at leas in the southwestern part of the USA). I would like for GAIA to tell us what they use ellipsoid or geoid and switch or give us an option. That would at least give us a common reference to evaluate. 

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    sjmj3us

    By the way bcorwin, I have a request: https://help.gaiagps.com/hc/en-us/community/posts/360014519954-Add-a-Digital-Terrain-Elevation-Database-DTED-to-Gaiagps-com-to-resolve-elevation-discrepancies which sounds like it tries to address your issue once it's uploaded to the cloud. You might want to upvote or comment on it.

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    bcorwin

    Thanks, sjmj3us.  I am in the southwest and my discrepancy does hover around the 100 foot mark so that would be consistent with your hunch.

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    Nathan

    @sjmj3us
    Currently, the Android version of Gaia GPS uses geoid elevations in the app. I can see how the option to switch between ellipsoid or geoid would be helpful, but currently, this is not available.

    @bcorwin
    I'm sorry this bug is affecting your altitude readings in the app. Please click “Submit a Request” above and include a copy of your track so it can be included in the bug report.

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    sjmj3us

    Nathan, thanks that's excellent to know. Is there somewhere on the web site or app that explains more details on this, like the open cycle map topolines appear to use 1996 Earth Gravitational Model (EGM96) geoid?

    Now jmmartin_buy's comment makes even more sense....

    "It is almost like Gaia GPS is adjusting in the opposite direction of the Geoid

    bumpy earth model."

    Nathan, sounds like you are now working on this problem? Do you need more information if seems like with Google Earth Pro or Accurate Altimeter etc you should be able to figure out the problem? 

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    jaymmartin_buy

    To be more clear, from my understanding, the GPS Elipsoide value is what comes out of the phone's GPS subsystem (raw value).  The GeoID model value is used to adjust it up or down.  Accurate Altimeter shows the Raw GPS value and the adjusted value using the GeoID model.  In my location, the adjustment adds altitude.  It looks like Gaia GPS substracts altitude. My guess was that the GeoID adjustment value's sign may just need to be flipped in Gaia GPS.

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    bcorwin

    Thanks, Nathan.  I have submitted a request with the track attached.

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    sjmj3us

    jaymmartin_buy I like your theory and there is a way to test this. I agree the correct equation is probably

    elevation_above_geoid = elevation_above_ellipsoid + delta

    and we suspect GAIA is doing 

    elevation_above_geoid = elevation_above_ellipsoid - delta

    In the USA delta is always positive (see for example this image: https://www.colorado.edu/geography/gcraft/notes/datum/gif/geoid2.gif) which produces the incorrectly low elevation.

    Note however in western Europe the delta is negative so if someone from the place with a negative delta like say Great Britain compared their elevation with Accurate Altimeter then their GAIA altitude should be consistently high by twice the delta.

    By the way where I am the delta looks to be about +70 feet and the results I get are consistent with your theory.

    I hope someone with a negative delta, see the map link I posted above, reports the difference between GAIA and Accurate Altimeter and we can confirm your theory.

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    crampton

    Thanks for doing the legwork and figuring that out, makes it much easier on me! For me the altitude was ~120 feet low and the offset was about half that. A fix will be in the next release (soon) but I'm hoping I can confirm with someone in Europe that it's off in the other direction.

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    sjmj3us

    Success!

    Version 7.0.11: 

    WHAT'S NEW

    * Fix for geoid altitude offset calculation
     
     
    jaymmartin_buy, thanks for figuring it out
     
    crampton, thanks for pushing the fix
     
    Works great for me, altitudes now match the maps in my area.
     
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    bcorwin

    Looks like mine is working now, too.  Thanks everybody!

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