Instruments for Home Built

[Auburntsts]

So for navigation and proper air space between air craft the standard barometric altimeter is used, but for true how far above the ground am I for sure, the GPS comes in handy. Got it. Thanks guys.

That's not really true either. GPS height is based upon a flat reference ellipsoid modeled to approximate the geodetic mean sea level. The GPS receiver uses this theoretical sea level estimated by the World Geodetic System (WGS84) reference ellipsoid in conjunction with the GPS constellation to establish its height above the ellipsoid model, not the actual ground. In simple terms, the reason for this is the model and Earth don't match because the Earth doesn't have a geometrically perfect shape.