GNSS NMEA $GPGGA/ $GLGGA/ $GNGGA messages show two altitude fields :
$aaGGA,hhmmss.s-s,llll.l-l,a,yyyyy.y-y,a,x,xx,x.x,x.x,M,,x.x,M,x.x,xxxx*hh
1 ^ 2 ^
the first one (in meters): Altitude mean-sea-level and the 2nd one (in meters): Geoidal separation, meters; Geoidal Separation: the difference between the WGS 84 earth ellipsoid surface and mean-sea-level (geoid) surface
How can I get the true altitude from this data ?
First of all, what do you mean by true altitude. It is altitude referred to ellipsoid or Geoid or local vertical datum. This must clear first. If your true altitude is referred to ellipsoid, you can pick your altitude directly from the string. But if your true altitude is referred to geoid, you need to know what geoid model are you referred. You can used simple formula to compute your altitude (H=h -N), where H is your true altitude, h is ellipsodal height and N is geoid saperation. in your case, in the string already provide the geoid saperation value but which geoid model used? that you need to clarify. If your true altitude referred to local vertical datum, same as geoid you need to identify your vertical datum and just used that formula to compute your true altitude. Hope this can help you.
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