Andy Blackburn wrote:

...since both GPS
and barometric altimeters measure distance from the
earth's surface (in different ways obviously). Therefore
vertical motion measured from these instruments will
always be relative to the ground rather than relative
to the airmass.

The way I see it, a barometric measurement will not be relative to the
ground at all: might even be related to the airmass, if only to a
pressure level.

Relating a barometric measurement to the ground is pretty arbitrary,
though I hear some folks are willing to do it down to 200' and 1/2
mile, on dark and stormy nights.

Some things you just have to take on faith.


--
Jack
----
"It is possible to fly without motors,
but not without knowledge and skill."
-- Wilbur Wright