GPS altitude vs altimeter altitude

It is my understanding that even if a GPS is reading perfectly and your
standard altimeter is reading perfectly even on a standard day that the
2 values will not be the same. Does anyone know about how far off they
are and if there is a way to calculate what the difference should be?
My guess is that at different points around the earth, the correction
will be significantly different, making a generic formula difficult if
not impossible to write.

--
Chris W
KE5GIX

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If you have an perfectly calibrated barometric altimeter and
have a perfect altimeter setting taken just a few seconds
before, there are mechanical errors. Altimeter settings on
the ground will vary in just a few miles. The lapse rate is
not constant. But the GPS errors are very small and are
calculated and corrected in real time. But aircraft
separation is based on barometrics, so GPS is not used to
establish cruise altitudes.

GPS could be used for instrument approaches since the ground
and obstructions are fixed and generally known quantities.
GPS is used for photography and surveying.

GPS and terrain mapping will keep you safe from the ground
and the barometric altimeter is still needed to keep the
cruise altitudes set.


--
James H. Macklin
ATP,CFI,A&P

--
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But government sees it as an obstacle to be overcome.
some support
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See http://www.fija.org/ more about your rights and duties.


"Chris W" wrote in message
news:q2x0g.917$9c6.179@dukeread11...
| It is my understanding that even if a GPS is reading
perfectly and your
| standard altimeter is reading perfectly even on a standard
day that the
| 2 values will not be the same. Does anyone know about how
far off they
| are and if there is a way to calculate what the difference
should be?
| My guess is that at different points around the earth, the
correction
| will be significantly different, making a generic formula
difficult if
| not impossible to write.
|
| --
| Chris W
| KE5GIX
|
| Gift Giving Made Easy
| Get the gifts you want &
| give the gifts they want
| One stop wish list for any gift,
| from anywhere, for any occasion!
| http://thewishzone.com

The altimeter is a barometer and the GPS is a computerized stopwatch.
Considering that they measure such different things, amazing that
there's any realationship at all, huh?

The mathematical model of the earth the the GPS software uses is
idealized somewhat and provides only an approximation of the bulging of
the surface of the earth relative to its geometric center point
(points, actually!). So, at any particular location you're over, your
actual distance from the ground can be different from what the GPS's
software thinks it is. So point A is that the GPS altitude can be
inaccurate.

Point B is that your altimeter is only as accurate as the setting you
have in the Kollsman window. If it's based on old data or data for a
distant location or if rapid weather change is occurring, it won't
exactly reflect the pressure corrected for MSL for the location you're
above.

If your GPS has WAAS enabled, you're in an area where the model of the
earth closely matches reality and you have a current altimeter setting
for the exact area, they should come very close to matching. If any of
the above isn't true, you'll see some degree of difference between
them.

Chris W wrote:
It is my understanding that even if a GPS is reading perfectly and your
standard altimeter is reading perfectly even on a standard day that the
2 values will not be the same. Does anyone know about how far off they
are and if there is a way to calculate what the difference should be?
My guess is that at different points around the earth, the correction
will be significantly different, making a generic formula difficult if
not impossible to write.

--

Grumman 236 wrote:
The altimeter is a barometer and the GPS is a computerized stopwatch.
Considering that they measure such different things, amazing that
there's any realationship at all, huh?

The mathematical model of the earth the the GPS software uses is
idealized somewhat and provides only an approximation of the bulging of
the surface of the earth relative to its geometric center point
(points, actually!). So, at any particular location you're over, your
actual distance from the ground can be different from what the GPS's
software thinks it is. So point A is that the GPS altitude can be
inaccurate.

But note that most consumer GPS receivers do not report altitudes based
only on the idealized mathematical model of the earth's shape
(generally the WGS-84 ellipsoid). They also include correction tables
that reflect the difference between the geoid shape and the ellipsoid
as a function of your position over the earth's surface. So the
reported altitude is with respect to sealevel, not relative to the
idealized ellipsoid. Naturally there will still be measurement
uncertainties and the correction table is also limited in precision,
but in terms of aircraft altitudes the errors are pretty small.

Point B is that your altimeter is only as accurate as the setting you
have in the Kollsman window. If it's based on old data or data for a
distant location or if rapid weather change is occurring, it won't
exactly reflect the pressure corrected for MSL for the location you're
above.

True, but then there's also Point C. Even if you've entered the
correct current value into the Kollsman window for your location, it
still just reflects the sealevel-adjusted pressure at ground level
below you and your altimeter then uses an idealized standard atmosphere
model to determine your altitude. If the temperature and lapse rate of
the actual atmosphere doesn't match the model then the reported
altitude can be significantly different from reality; hundreds or even
a thousand feet or more of variation.

But, as mentioned, aircraft vertical separations are based on
barometric altitudes and as long as everyone in a given area is off by
the same amount the separations will still be maintained.

On 16 Apr 2006 13:52:58 -0700, "peter" wrote:

Grumman 236 wrote:
The altimeter is a barometer and the GPS is a computerized stopwatch.
Considering that they measure such different things, amazing that
there's any realationship at all, huh?

The mathematical model of the earth the the GPS software uses is
idealized somewhat and provides only an approximation of the bulging of
the surface of the earth relative to its geometric center point
(points, actually!). So, at any particular location you're over, your
actual distance from the ground can be different from what the GPS's
software thinks it is. So point A is that the GPS altitude can be
inaccurate.

But note that most consumer GPS receivers do not report altitudes based
only on the idealized mathematical model of the earth's shape
(generally the WGS-84 ellipsoid). They also include correction tables
that reflect the difference between the geoid shape and the ellipsoid
as a function of your position over the earth's surface. So the
reported altitude is with respect to sealevel, not relative to the
idealized ellipsoid. Naturally there will still be measurement
uncertainties and the correction table is also limited in precision,
but in terms of aircraft altitudes the errors are pretty small.

Point B is that your altimeter is only as accurate as the setting you
have in the Kollsman window. If it's based on old data or data for a
distant location or if rapid weather change is occurring, it won't
exactly reflect the pressure corrected for MSL for the location you're
above.

True, but then there's also Point C. Even if you've entered the
correct current value into the Kollsman window for your location, it
still just reflects the sealevel-adjusted pressure at ground level
below you and your altimeter then uses an idealized standard atmosphere
model to determine your altitude. If the temperature and lapse rate of
the actual atmosphere doesn't match the model then the reported
altitude can be significantly different from reality; hundreds or even
a thousand feet or more of variation.

But, as mentioned, aircraft vertical separations are based on
barometric altitudes and as long as everyone in a given area is off by
the same amount the separations will still be maintained.

Have a look at the following url:
http://sps.unavco.org/geoid/

David

wrote:
On 16 Apr 2006 13:52:58 -0700, "peter" wrote:

Grumman 236 wrote:
The mathematical model of the earth the the GPS software uses is
idealized somewhat and provides only an approximation of the bulging of
the surface of the earth relative to its geometric center point

But note that most consumer GPS receivers do not report altitudes based
only on the idealized mathematical model of the earth's shape
(generally the WGS-84 ellipsoid). They also include correction tables
that reflect the difference between the geoid shape and the ellipsoid
as a function of your position over the earth's surface.

Have a look at the following url:
http://sps.unavco.org/geoid/

Yes, that URL gives you the correction between geoid and ellipsoidal
heights. But as I already said, similar correction tables are already
built into most consumer GPS receivers (incl. all Garmins and
Magellans) so the altitudes they report are not what the above website
refers to as "GPS Elevation" but rather the elevation with respect to
MSL.

If you look at the NMEA data output of a GPS receiver, the altitude is
given in the $GPGGA sentence and one of the data fields specifies the
value being used by the receiver for the 'Height of geoid above WGS84
ellipsoid' when reporting the elevation.
See http://aprs.gids.nl/nmea/#gga for details.

AIM 1-1-19a(8) tells you to not to rely upon GPS altitude. Until the AIM is
changed, I'm not concerned with the details.

Bob Gardner

"Chris W" wrote in message
news:q2x0g.917$9c6.179@dukeread11...
It is my understanding that even if a GPS is reading perfectly and your
standard altimeter is reading perfectly even on a standard day that the 2
values will not be the same. Does anyone know about how far off they are
and if there is a way to calculate what the difference should be? My
guess is that at different points around the earth, the correction will be
significantly different, making a generic formula difficult if not
impossible to write.

--
Chris W
KE5GIX

Gift Giving Made Easy
Get the gifts you want & give the gifts they want
One stop wish list for any gift, from anywhere, for any occasion!
http://thewishzone.com

OOps. AIM 1-1-19 (a) (4). Doggone feds changed the numbers on me!

Bob Gardner

"Chris W" wrote in message
news:q2x0g.917$9c6.179@dukeread11...
It is my understanding that even if a GPS is reading perfectly and your
standard altimeter is reading perfectly even on a standard day that the 2
values will not be the same. Does anyone know about how far off they are
and if there is a way to calculate what the difference should be? My
guess is that at different points around the earth, the correction will be
significantly different, making a generic formula difficult if not
impossible to write.

--
Chris W
KE5GIX

Gift Giving Made Easy
Get the gifts you want & give the gifts they want
One stop wish list for any gift, from anywhere, for any occasion!
http://thewishzone.com

To respond to the original posters question. I have often asked the
same thing. My observations are this. My plane is sitting in the hangar
at 6412.37 msl. The airport was just recently surveyed so I am sure
this is a real close number. I set my altimeter to the current setting
and it shows within 3 feet. I fire up my King KMD150 MFD and it will
almost always show 8- 12 feet higher then that, never seen it lower. It
must be that mathmatical modeling thing. Then I fire up my Garmin 196
and it varies between 15 feet higher and 10 feet lower. I attribute
that to a less sensitive circuitry in the handheld unit altho I might
be wrong surmising that. I would be curious to see what others see in
their respective areas.?

Ben
www.haaspowerair.com

The barometric altimeter is calibrated to zero at the wheel
contact point with the ground. The GPS measures at the
antenna. Like all things made by man, there are tolerances.

Reason the baro-altimeter is wheel height, think of a 747,
do you want the cockpit or the wheels to clear the trees at
the end of the runway?



--
James H. Macklin
ATP,CFI,A&P

wrote in message
oups.com...
| To respond to the original posters question. I have often
asked the
| same thing. My observations are this. My plane is sitting
in the hangar
| at 6412.37 msl. The airport was just recently surveyed so
I am sure
| this is a real close number. I set my altimeter to the
current setting
| and it shows within 3 feet. I fire up my King KMD150 MFD
and it will
| almost always show 8- 12 feet higher then that, never seen
it lower. It
| must be that mathmatical modeling thing. Then I fire up my
Garmin 196
| and it varies between 15 feet higher and 10 feet lower. I
attribute
| that to a less sensitive circuitry in the handheld unit
altho I might
| be wrong surmising that. I would be curious to see what
others see in
| their respective areas.?
|
| Ben
| www.haaspowerair.com
|