GPS altitude again is close to actual

Mxsmanic wrote:
[...]
Note that WAAS and LAAS will _not_ compensate for either of the above
types of error. Differential GPS systems like this work best when you
are at exactly the spot used as a reference for the corrections. If
you are anywhere else, the corrections may not be right for your
position. The further away you are from the surveyed reference
position used to generate the corrections, the more likely it is that
your position will be incorrect.

While correct for the case of LAAS and DGPS, this is not correct in the
case of WAAS.

Hint: W != L

Regards,
Jon