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:
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

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.

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

"Stefan" wrote:
Pneumatic altimeters are reliable,

Most of the time.

In very cold weather, you can run into something by relying on a
pneumatic altimeter.
--
Dan
C172RG at BFM

Dan Luke wrote:

In very cold weather, you can run into something by relying on a
pneumatic altimeter.

Pssst, I'll tell you a secret: All that stuff you had to learn to pass
the written was somehow linked to real life.

Stefan

"Stefan" wrote:
Pssst, I'll tell you a secret: All that stuff you had to learn to pass
the written was somehow linked to real life.

Really?

Gosh!

--
Dan

"How can an idiot be a policeman? Answer me that!"
- Chief Inspector Dreyfus

Does anybody know a quick rule of thumb about how much the altitude changes
with non-standard temperature?
I guess is also has to have in consideration that the airport altimeter
setting is also kinda "corrected".
I.E., you are flying at 4000 ft, then if you set up the altimeter setting
for an airport at 3800', you'll be far more accurate than if you set it up
for an airport at 1000' (assuming that the atmosphere is exactly the same).
or, in other words, the closer you are to the airport altitude, the more
accurate your altitude will be.


"Dan Luke" wrote in message
...

"Stefan" wrote:
Pneumatic altimeters are reliable,

Most of the time.

In very cold weather, you can run into something by relying on a
pneumatic altimeter.
--
Dan
C172RG at BFM