[pct-l] Follow-up to the earlier GPS discussion

[Carl Siechert]

I don't get this. Putting aside my question about how gravity is
mapped...Where is this elevation mapping data stored? In the receiver? And
with all the other mapping minutiae stored in most receivers, why can't the
elevation be more accurate? Btw, in limited observations with automotive
receivers, I haven't noticed that elevations on a (presumably economically
justifiable) interstate highway are any more accurate than those seen far
afield.

I always assumed that vertical position was calculated on the fly by
triangulation, just like lat/long. And I further assumed that the reason for
the usual error is simply that the high angle of the satellite signals
doesn't allow for as much precision when triangulating on the Z-axis. Am I
lost?

On Wed, May 19, 2010 at 2:44 PM, <gwschenk at socal.rr.com> wrote:

>
> Vertical position is based on mapping of the earth's gravity. This mapping
> is densified in more urban areas where it is economically justified. In
> remote mountain regions it is not considered as important. And so, every so
> often you will read where someone will complain that the elevation of
> Whitney is wrong, because his Magellan says so!
>
>