How come GPS reads different

"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

Dan Luke wrote:

"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.

That's not what my KLN89B manual says...


Matt

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.


Matt



GPS altitude is not accurate. That is why you need to have a baro sensor
included as input to a IFR approved GPS.

--
Regards, Ross
C-172F 180HP
KSWI

Ross Richardson wrote:

GPS altitude is not accurate.

.... except when it is, which is most of the time.

That is why you need to have a baro sensor included as input to a
IFR approved GPS.

That baro input is used as a cross-check for gross errors, to assist
RAIM. It does not "correct" inaccuracy, and thus is irrelevant to the
normal GPS altitude readout.


- FChE

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)

"Thomas Borchert" wrote in message
...
Main reason: Altimeter indication is corrected for barometric pressure.
First order of business before comparing is to set the altimeter to
29.92.

No. As Dan points out elsewhere, the main error in the barometer is not
correcting for non-standard temperature. There are other errors as well,
but none of them involve setting the altimeter to 29.92 in an attempt to
make the altimeter more accurate. Unless the correct altimeter setting is
actually 29.92, setting it to 29.92 will simply make the error worse.

Pete

Peter,

Unless the correct altimeter setting is
actually 29.92, setting it to 29.92 will simply make the error worse.


Uh, yes. Brainfart on my side. Sorry

--
Thomas Borchert (EDDH)

"Aluckyguess" wrote in message

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

Altimeters are calibrated for local barometric pressure. GPS altitude based
on a computed, simulated sphere that approximates sea level.

If the Earth suddenly disappeared (as happened in Hitchhikers Guide to the
Galaxy) and you happened to be flying at that moment, your GPS would
continue to happily display an altitude even though there was no actual
Earth underneath you.

So, for safety reasons, I think we should all use GPS altitude so that we
can all safely land if the Earth ever actually disappears.

I've always been curious about something: The Earth is not a perfect
sphere. It bulges at the equator due to forces of tidal gravitation and
centrifugal force. My question is, do GPS altitude computations take this
into effect when you're, for example, flying over the equator instead of
over the north pole?

--
Jim Fisher

Jim Fisher wrote:
"Aluckyguess" wrote in message


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


Altimeters are calibrated for local barometric pressure. GPS altitude based
on a computed, simulated sphere that approximates sea level.

If the Earth suddenly disappeared (as happened in Hitchhikers Guide to the
Galaxy) and you happened to be flying at that moment, your GPS would
continue to happily display an altitude even though there was no actual
Earth underneath you.

So, for safety reasons, I think we should all use GPS altitude so that we
can all safely land if the Earth ever actually disappears.

I've always been curious about something: The Earth is not a perfect
sphere. It bulges at the equator due to forces of tidal gravitation and
centrifugal force. My question is, do GPS altitude computations take this
into effect when you're, for example, flying over the equator instead of
over the north pole?

--
Jim Fisher



Yes - it's called the reference ellipsoid. There are actually
several of them and some GPS receivers allow selecting your
favourite one.

Actually, if Earth would disppear with all its mass, GPS would
get unusable, as the satellites would continue out of their
tracks due to the lack of gravity pull.

--

Tauno Voipio
tauno voipio (at) iki fi