Originally Posted by
IO540
I think you mean EGNOS or WAAS, but yes that's about it.
My understanding of GPS fixes is that having the altitude assumed to be right gives a quick fix, but once you have a multi-satellite fix it is equally accurate in all directions (so say +/- 50 meters on a bad day - +/-10 on a good day for a non-EGNOS/WAAS receiver). We just perceive 50 meters vertically to be a bigger error than 50 meters laterally (and of course it is if you are trying to avoid terrain ;-)
The much bigger error is if the GPS doesn't have a clear view of where local sea level is on this part of the earth. I believe all of the TAWS ones have an accurate map of exactly where sea level is and how high the surface is above sea level. The EGNOS ones clearly know this well enough to get you to precisely 250 ft above this surface point for the LPV approaches.
A camping one may use the basic ellipsoid (thank you LH2) and be out by a considerable distance.