question about altitude correction

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skully fox

question about altitude correction

Post by skully fox » Tue Sep 17, 2019 3:52 pm

Hi,
I think purpose of EGM96 correction is to get accurate MSL(HAE) altitude.
and I'm curious whether smartphone gps chip receives "ellipsoidal height" from satellite. (not MSL height??)

When I see raw NMEA $GPGGA sentence from smartphone NMEA logging app, 9th value is MSL height and 11th is geoid separation.
So, sum of these two value represents ellipsoidal height at the smartphone.

Does Alpinequest use this(9th value+11th value) ellipsoidal height? and subtract EGM96 value from this ellipsoidal height ?

then where came from $GPGGA geoid value? satellite signal or device?
($GPGGA geoid value was different from EGM96 bilinear interpolated value a little with various smartphones at the same place.)

Thank you so much.

Psyberia-Support
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Re: question about altitude correction

Post by Psyberia-Support » Sat Sep 21, 2019 5:32 pm

Hi and sorry for the delay.
Actually the application doesn't use the NMEA data, but the locations from the Android location API, allowing use of other sources of location like network or mock.
Those Android API locations have an WGS84 elevations (the GPS raw elevation), and the application convert them to Geoid (MSL) elevations using it's own EGM96 model (if this setting is checked).
However in some very few devices, the Android API locations are already converted, in this case you can disable this correction in the app to avoid a double correction.

Since the application uses it's own EGM96 interpolated values and not the NMEA geoid separation, you may see a little difference in the final elevation.

Hope it helps,
Best regards
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