How come GPS reads different

Its a 296. It seems the like the higher I go the farther off it is.
I am one to think the GPS would be more accurate than the altimeter.
I was thinking that we get incorrect altimeter settings
"Casey Wilson" N2310D @ gmail.com wrote in message
news:x1Pqe.2896$9a1.185@trnddc01...

"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.

"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

Aluckyguess wrote:
Its a 296. It seems the like the higher I go the farther off it is.
I am one to think the GPS would be more accurate than the altimeter.
I was thinking that we get incorrect altimeter settings

The altimeter is only "accurate" when the temperature profile of the
airmass is equal to the ISA standard atmosphere. The rest of the time,
there will be some error which increases with altitude (wrt wherever the
source for the altimeter setting is located). This isn't a concern for
en-route operations since everybody's altimeter has the same "error".
It can be a concern for terrain clearance in the mountains or in some
cases instrument approaches in cold weather since the altimeter reads
higher than the plane actually is.

Fred G. Black wrote:

This isn't a concern for
en-route operations since everybody's altimeter has the same "error". It
can be a concern for terrain clearance in the mountains or in some cases
instrument approaches in cold weather since the altimeter reads higher
than the plane actually is.

No concern, either. When IMC, there is a safety margin built into the
approaches, and when VMC, there are windows in the copckpit.

Pneumatic altimeters are reliable, independant of any infrastructure and
work without electricity.

Stefan

AIM 1-1-19(a)(8) tells pilots not to use GPS altitude: "GPS altitude should
not be relied upon to determine aircraft altitude since the vertical error
can be quite large."

GPS altitude is measured above the GPS sphere, which is not sea level.

Bob Gardner

"Aluckyguess" wrote in message
news
How come the GPS reads a different altitude than the Altimeter?

"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

Stefan wrote:
Fred G. Black wrote:

This isn't a concern for en-route operations since everybody's
altimeter has the same "error". It can be a concern for terrain
clearance in the mountains or in some cases instrument approaches in
cold weather since the altimeter reads higher than the plane actually is.


No concern, either. When IMC, there is a safety margin built into the
approaches, and when VMC, there are windows in the copckpit.

Pneumatic altimeters are reliable, independant of any infrastructure and
work without electricity.

Stefan

There is a buffer in the procedure design however given that cold
temperature corrections are published (at least in Canada, ref CAP GEN
p19), it's not unreasonable to assume that there are situations where
they should be used.

Fred.

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)

Fred G. Black wrote:

There is a buffer in the procedure design however given that cold
temperature corrections are published (at least in Canada, ref CAP GEN
p19), it's not unreasonable to assume that there are situations where
they should be used.

Of course. That's one more reason why you must pass a test to get the
license.

Stefan