Cool, not sure I really helped, but I’m glad you found a way, it’s always satisfying to get something working using your own initiative.
I didn’t take onboard that you were editing within an installed app, rather, I was tackling it like I was building in SDK.
Radar altimeter if you need to know AGL.
I modified one of these in grad school to remotely measure properties of ice pack.
Those geologists are using far more sophisticated GPS systems that we have in our phones and will be using Ground Based and probably Spaced Based augmentation systems for accuracy.
Basic GPS is notoriously unreliable for giving accurate altitude. It was developed for lateral accuracy not vertical accuracy.
Anaerobic barometers are very accurate when corrected for local QNH or air pressure.
With GeographicLib it’s a matter of
Geoid egm96("egm96-5");
double lat = 50.0, lon = 7.0;
double geoid_height = egm96(lat, lon);
geoid_height holds the difference. For my place it almost perfectly matches the offset in measurements with XA2 and 10 V for the Sony side and Garmin Zumo XT or QStarz BL1000ST for dedicated GPS systems.