Jonathan Stott wrote:

John_H wrote:

Differential GPS is very accurate.

Ordinary hand held GPS isn't too bad if the satellites are in the
right position -- mine usually gets within a metre a so whenever it
gives a steady altitude reading (if the reading drifts it won't be
accurate).

Ordinary GPS can be significantly out! The first thing I did when I got
my device was to take my height - it reckoned (after letting it settle
for a while) that I was at -14m. While that's only 22m out, it's still
quite inaccurate!

It certainly can be a long way out, and will be if the reading has any
tendency to drift over a time period (say five minutes). Allowing
time to 'settle down' won't achieve accuracy -- you must get a
consistent identical reading over time. It may take several attempts
at different times of the day to find the favourable satellite
distribution needed.

I do have the advantage of having set up (and rechecked) my weather
station with DPGS so I know its altitude exactly. I've also taken
numerous readings against it with various handhelds, which have
cosistently confirmed the above.

--
John H