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Geoid height approximation

Umberto Uderzo shared this question 7 years ago
Answered

Hello, after enabling auto height correction and downloading the geoid info I notice that the height reported is way better, but differs by 1 meter from AlpineQuest app and my Garmin GPS. Is it something related to the geoid being used? Is there some way to change it to a different model?

Thanks! Umberto

Best Answer
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Good day Umberto,

to be true, value "1 meter" made a nice smile on my face. Mainly because

- height accuracy of modern GPS devices (phones/tables) is close to 10 meters and worst

- accuracy of geoid model is around 2 metres

- method how to interpolate values from EGm96 model is more then one and depend on developers


So all this together ... I'm surprised that You notice only! 1 meter difference :). 1 meter is fine, really and even if conditions are same, even if all three applications use same model of geoid, there is still chance that other apps/devices compute geoid difference by different method.


Hope I've answered You, otherwise feel free to ask.

Replies (6)

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Good day Umberto,

to be true, value "1 meter" made a nice smile on my face. Mainly because

- height accuracy of modern GPS devices (phones/tables) is close to 10 meters and worst

- accuracy of geoid model is around 2 metres

- method how to interpolate values from EGm96 model is more then one and depend on developers


So all this together ... I'm surprised that You notice only! 1 meter difference :). 1 meter is fine, really and even if conditions are same, even if all three applications use same model of geoid, there is still chance that other apps/devices compute geoid difference by different method.


Hope I've answered You, otherwise feel free to ask.

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Yes i know that all considered the question is quite silly.

But i initially tought that every app should report the very same height, as computd by the GPS unit (being correct or not). And now i discovered that on the same mobile phone different apps may report different values. What made me think is the fact that the Garmin and the AlpineQuest app were reporting (at the time i tested them) the same value (maybe a coincidence) and Locus not.

To tell the entire story, i was searching caves on mountain and the description on the book were something like "at height 1557m leave the mountain path and turn to the left into the woods", which is pretty silly way to describe how to find something as the error percentage is huge.

I mainly asked for personal knowledge. :)

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Hello,

- elevation value from GPS ... this value is based on WGS84 ellipsoid, not a "real" geoid. Anyway even here will be results different in various applications. In "Altitude manager" screen is possible to enable various filters for measured elevations, so these filters has influence on value you see in Locus.

- and elevation recomputed to geoid is completely different story. Not sure if Garmin or AlpineQuest do this. Even here are options to recompute this value based on information received in GPS information (NMEA messages) or directly from EGM96 geoid model as Locus do now.


This topic for me looks like on at least huge blog post, better small book :). Anyway I think it is really complicated to compare elevation values for different applications on same device as there is many factors that influence results. And even more complicated it's to compare different applications on different devices.

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Hi, excuse me if i should have another thead anyway, which are best setting for height calculation?

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Good day Luca,

"best" is really relative. Anyway in most cases, most useful should be

  • offset set to "Automatic correction", this is really important to get values close to what you may see on paper maps or outside
  • if you have downloaded SRTM Data for your area, set this option to "Optimize GPS values" or even replace. This may increase consistency of values and remove unwanted peaks
  • altitude filter is not so important mainly in case of using SRTM files

More about this topic may be found in our manual.

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Yes, I know "best" meaning ;-)

For me is "to get values close to what you may see on paper maps or outside" and don't have unwanted peaks.

...but I don't understand if "Optimize GPS values" need SRTM file?! (...and SRTM approximation???)


Thanks, as always!

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Please, check mentioned manual page. All necessary information should be there. If not, we will gladly improve manual web page. Thanks

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