GPS altitude : will not work below 3000 ft because the earth is not a perfect circle . Your C152 will find itself 1000ft below the ground in some places..
As said before WGS84 is an North American reference point made artificially to ensure positive altitude above its continent . Will not work in other parts of the world.
Wow, really. It is not a reference point, WGS84 appoximates the Earths Ellipsoid. It has been adopted by ICAO as the worldwide standard for aviation.
The grid is based on UTM, with 60 zones. Each gridline is its own unique reference line, and is ultimately accurate.
Zone 1 and 60, moving from West to East, and letters from South to North., and does not relate to the Prime Meridian, nor 85W.
Max difference between geoid and ellipsoid is 105m. IF you have a GPS on the aircraft, or wherever, the system calcs the difference between the Geiod (MSL) and the Ellipsoid (GPS alt) by means of the lat/long and corrects, so you dont have to.
It does not give you AGL, it gives you altitude based on MSL.