mike regish writes:

Do those amounts vary with location?

Yes, but unfortunately that is only one of the variables. They also
vary dramatically with the changing positions of the satellites above
(and the satellites move significantly from one minute to the next).
Atmospheric conditions and other factors also come into play.

Overall, though, the problem is that the satellites are positioned to
optimize lateral navigation--but that also positions them in a way
that is unfavorable to vertical navigation. The vertical accuracy can
never be as high as the lateral accuracy; it can't even come close.
That's the way the system is designed.

Perhaps one day GPS will provide altitudes accurate to within a few
feet; but if it does, then by that time the lateral positions will be
accurate to within millimeters.

How large a difference is there between the 2?

It varies by location and conditions. I'm not sure of the exact
differences. One problem is that the geoid altitude is fixed, whereas
pressure altitudes depend on atmospheric conditions.

If everyone used GPS for altitude, it would probably work out okay in
some cases, particularly at high altitudes. But as long as anyone is
using a conventional altimeter, the differences are large enough to be
dangerous.

But you're right. If that's true, and I don't doubt it is, GPS would be
better suited to terrain avoidance and less so to aircraft separation.

I prefer to reserve GPS for lateral navigation only. For that it
works quite well, on a par with other navaids (depending on various
factors). For altitude it's a waste of time. Better to have a
standard altimeter and a radar altimeter for terrain.

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