How come GPS reads different

How come the GPS reads a different altitude than the Altimeter?

Aluckyguess wrote:
How come the GPS reads a different altitude than the Altimeter?



For many reasons. The main reason is that they derive altitude by two
completely different methods and this inevitably introduces errors.
Then each method has its own inherent errors.

Even two altimeters side by side won't read the same altitude other than
by accident.


Matt

I am getting over 500ft. difference and more.
"Matt Whiting" wrote in message
...
Aluckyguess wrote:
How come the GPS reads a different altitude than the Altimeter?

For many reasons. The main reason is that they derive altitude by two
completely different methods and this inevitably introduces errors. Then
each method has its own inherent errors.

Even two altimeters side by side won't read the same altitude other than
by accident.


Matt

"Aluckyguess" wrote in message
...
I am getting over 500ft. difference and more.
"Matt Whiting" wrote in message
...
Aluckyguess wrote:
How come the GPS reads a different altitude than the Altimeter?

For many reasons. The main reason is that they derive altitude by two
completely different methods and this inevitably introduces errors. Then
each method has its own inherent errors.

Even two altimeters side by side won't read the same altitude other than
by accident.


Matt

The number of sattelites being received is one of the variables -- how many
are you registering on the GPS? What brand/model is the GPS. Is the GPS
certified for navigation? Does the GPS have a Kohlsman equivalent input?
What are the altitude specifications of the GPS? Are you comparing it to an
altimeter in an airplane? Is the calibration data on the altimeter valid?
Have you compared the altimeter to the GPS while parked at a known reference
(i.e., the benchmark of your local airport)? My Magellan handheld takes
about five mintues of averaging to get a reasonably close altitude
correlation -- how much time are you giving yours?

Bottom line, forget the GPS altitude reading, dial in the local pressure
data into the airplane's altimeter and trust it.

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)



-- all the best, Dan Ford

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Aluckyguess wrote:

I am getting over 500ft. difference and more.
"Matt Whiting" wrote in message
...

Aluckyguess wrote:

How come the GPS reads a different altitude than the Altimeter?

For many reasons. The main reason is that they derive altitude by two
completely different methods and this inevitably introduces errors. Then
each method has its own inherent errors.

Even two altimeters side by side won't read the same altitude other than
by accident.

Not that unusual. GPS isn't very accurate vertically. Most GPS manuals
tell you not to rely on the altitude in any serious way.


Matt

"Matt Whiting" wrote in message ...

Not that unusual. GPS isn't very accurate vertically. Most GPS manuals tell you not to rely on the altitude in any
serious way.


Matt

Unless the GPS is WAAS capable?

"Matt Whiting" wrote:

Not that unusual. GPS isn't very accurate vertically. Most GPS
manuals tell you not to rely on the altitude in any serious way.

It's usually more accurate than the altimeter, which is not corrected
for non-standard temperature.
--
Dan
C172RG at BFM

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.

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.

Aluckyguess,

How come the GPS reads a different altitude than the Altimeter?


Main reason: Altimeter indication is corrected for barometric pressure.
First order of business before comparing is to set the altimeter to
29.92. Even then, 500 feet off is not unusual IF the GPS is not WAAS
capable.

--
Thomas Borchert (EDDH)