GPS Alt
Hi, I used to work for a company that specialised in GPS software and hardware solutions and think I may be able to offer you an answer. I'll try to explain in a simple terms as possible!
GPS altitude is not that accurate. It takes a minimum 3 satellites to calculate a 2D fix, and 4 for a 3D fix. Obviously the more satellies, the more accurate. However, only the satellites closer to the horizon can play any part in calculating your altitude to a higher degree of accuracy. However, the Earth model used by the GPS system is based on an 'averaged' spheroid:
Earth is not a perfect sphere like a globe would lead you to believe, it is in fact slightly more elongated across the equator than between the two poles. Plus the fact that some of the surface is covered in high terrain, while the rest is at sea level. So, if you see where I'm coming from, the developers of the GPS system 'averaged' out Earth into a spheroid taking into account these factors. Thus, you may be in a position where you are calculated to be at altitude, when in fact you are standing on a beach. Likewise, you could be calculated at being at a negative-value altitude when standing at sea level, due to the averaged Earth surface used by the system.
The only truly accurate altitude reporting GPS's on the market are those with built in barometers.
Hope that this is of use to you and I haven't confused you further!
Atb, Tri