How come GPS reads different

GeorgeB wrote:

On Sun, 12 Jun 2005 06:27:43 -0400, Cub Driver wrote:


On Sat, 11 Jun 2005 20:54:01 -0700, "Aluckyguess" wrote:


I am getting over 500ft. difference and more.

Oh,. good grief! I'm sure I've never had as much as 100 ft of
variance, and more often it's 20 feet or so. (Garmin 296)


I frequently drive by 2 signs marking the "eastern continental divide"
with the elevation. With my WAAS equipped, non-aviation GPSR
(Meridian Gold), I habitually flip to a screen with elevation
displayed and have yet to differ by more than 40 ft. As Dan says, I
am usually within 15 ft.

But sometime and someplace you may not be:
http://gpsinformation.net/main/altitude.htm


I took two trips across parallel bridges in eastern Virginia 3 days
apart; the reading was stable and within 5 ft of what I visually
estimated it should be. It differed by 2 ft betweent he 2 days; my
eyes told me the bridge heights did also ...

I would trust a WAAS equipped GPS, tracking 5 or more birds in a clear
environment, to be more ACCURATE than a barometric altimeter. I would
USE my barometric altimeter if it were working as other aircraft in
the area are, adjusted to local barometer per information gained by
radio from official sources.

Yes, augmented systems are a different matter.


Matt