Jim Logajan writes:

His new portable would almost certainly use WAAS when available and would
not be off by 500 ft.

WAAS is independent of GPS. It's one of many GPS augmentation schemes.

WAAS is accurate to 25 ft vertically 95% of the time. Standard aneroid
altimeter have no adjustment for changes from non-standard temperature. A
temp difference of ~4 degrees Celsius from standard atmosphere would show a
500 ft difference.

So if you depend on your WAAS GPS for altitude, you might collide with traffic
that is depending on barometric altitude.

Which is why WAAS was developed.

WAAS was developed to improve GPS accuracy overall, not to allow GPS to
replace traditional barometric altimeters.