Why does the displayed altitude always differ around 50 meters from the real height?
A. GPS Altitude is a simplified measurement method; it’s a little different than the normal one, for “garmin etrex, they build-in “Baro-Altimeter, to get a more realistic value. You cannot compare GPS altitude, which measures by triangulation to a geodetic plane, with a baro-altimeter that measures based on an assumed lapse rate. They are measuring two different things. We have to make some assumptions about the shape of the earth. WGS84 has defined that shape to be an ellipsoid, with a major and minor axis. The particular dimensions chosen are only an approximation to the real shape. Ideally, such an ellipsoid would correspond precisely to “sea level” everywhere in the world. As it turns out, there are very few places where the WGS84 ellipsoid definition coincides with sea level. On average, the discrepancy is zero, but that doesn’t help much when you’re standing at the water’s edge of an ocean beach and your GPS is reading -100ft below sea level. The deviation can be as large as 300f
