GPS altitude discrepancies

I've been using a Garmin GPS396 unit for a couple of months now. I am
amazed at the accuracy of the elevation information. It always seems
to be within a few feet of the listed airport's elevation, for example.
I took it up in my old Cessna 170. The elevation at the airport was
right on. At altitude, however, it was off by as much as 600' from my
altimeter at 14,000'. Initially, I figured that my altimeter was off.
Then, I took it up in a friend's Malibu, and discovered similar
discrepancies from his altimeter. Again, the GPS is very accurate at a
number of ground points. I'm at a loss to explain this. I'm using
altimeter settings very near my location - so why would the altitudes
vary by so much?

In following up on this, I checked the GPS altitudes in the jet I fly,
but it was inconclusive. Using a nearby altimeter setting up in the
flight levels (around 35,000') the GPS altitude was different by a
couple hundred feet, and seemed to vary +/- 200'. But I don't know
how accurate the elevation is in this unit (Airbus 320), or even
whether it is corrected for differential. It is used primarily for
lateral navigation, and since the unit is about 5 years old, it may not
even have the accuracy of the latest from Garmin.

Does anybody out there have some thoughts about this, or maybe similar
experience?

a google search on "GPS altitude discrepancies" comes up with a lot of
information...
the best explination I found is below, so it is a quote from another
source. BT

1) GPS altitude is TRUE altitude, not barometric altitude, above sea level
in reference to the center of the satellite's orbit, which should be the
center of the earth, but it isn't (see #2 below),

2) The idea of TRUE is in reference to a few different models of the
"geoid," The models have to fudge an average to get an idea of where
"ground" is. It is NOT the height above the surface of the earth - including
sea level. The geoid is is not a sphere. The geoid is not exactly where sea
level is. Where the real sea level is determined by gravitational
abnomalities.

http://www.esri.com/news/arcuser/0703/geoid1of3.html

BUT to some extent what we call MSL now, the barometric altitude is based on
the same inaccurate models with more fudging. We already know that the
altimeter setting where my plane is is going to be slightly off from what
the ATC is telling us to use - which was taken at a point in time earlier at
a place not where we are.

3) GPS receivers need WAAS to get good accuracy in the Z-axis. They are much
more accurate in the other two axi.

As was mentioned, some GPS receivers have also barometric altitude
capability so they they can reference the number that you and ATC expect.

You wanted an easy explanation?


"Pablo" wrote in message
oups.com...
I've been using a Garmin GPS396 unit for a couple of months now. I am
amazed at the accuracy of the elevation information. It always seems
to be within a few feet of the listed airport's elevation, for example.
I took it up in my old Cessna 170. The elevation at the airport was
right on. At altitude, however, it was off by as much as 600' from my
altimeter at 14,000'. Initially, I figured that my altimeter was off.
Then, I took it up in a friend's Malibu, and discovered similar
discrepancies from his altimeter. Again, the GPS is very accurate at a
number of ground points. I'm at a loss to explain this. I'm using
altimeter settings very near my location - so why would the altitudes
vary by so much?

In following up on this, I checked the GPS altitudes in the jet I fly,
but it was inconclusive. Using a nearby altimeter setting up in the
flight levels (around 35,000') the GPS altitude was different by a
couple hundred feet, and seemed to vary +/- 200'. But I don't know
how accurate the elevation is in this unit (Airbus 320), or even
whether it is corrected for differential. It is used primarily for
lateral navigation, and since the unit is about 5 years old, it may not
even have the accuracy of the latest from Garmin.

Does anybody out there have some thoughts about this, or maybe similar
experience?

Good points made by Btiz. Also, we fly barometric altitudes. You may
have a non-standard lapse rate and the pressure up high in the flight
levels is something different than what the altimeter expects. After
all, the local altimeter setting you are using is on the ground, you
are 6 miles ABOVE that. You may not have all the air (or may have more)
above you than the altimeter expects.

On Thu, 15 Dec 2005 22:18:14 -0800, "BTIZ"
wrote:


3) GPS receivers need WAAS to get good accuracy in the Z-axis. They are much
more accurate in the other two axi.

Garmin 396 does have WAAS capability. On my Garmin 295 this was a
feature that had to be turned on via menu. WAAS mode = on, is
something the OP should check.

Thanks for the input. I understand that true altitude is what the GPS
is reading. But why then would the elevation at airport reference
points be so highly accurate, while the reading at a few thousand feet
above the surface varies so much against the altimeter??

"Pablo" writes:

Thanks for the input. I understand that true altitude is what the
GPS is reading. But why then would the elevation at airport
reference points be so highly accurate, while the reading at a few
thousand feet above the surface varies so much against the
altimeter??

It's inherent in the way barometric altimeters use an idealized model
of the atmosphere. Altimeter settings are specifically computed to
cause correct (approximately "true") readings at the surface. As the
actual departs from the idealized model atmosphere, so do the
barometric readings higher up.

- FChE

Because the altimeter setting you dial into the Kolsman window on the
altimeter is based on the pressure on the ground. The altimeter assumes a
standard pressure lapse rate as you go higher. The further you get from the
ground and the more non-standard the lapse rate really is, the less the
altimiter reading corresponds to the actual MSL altitude.

At airports that don't report altimeter settings I set my altimeter
according to my GPS altitude and this has given me a reasonable ground MSL
according to the published altitude of the airport.

--
Best Regards,
Mike
http://photoshow.comcast.net/mikenoel
"Pablo" wrote in message
oups.com...
Thanks for the input. I understand that true altitude is what the GPS
is reading. But why then would the elevation at airport reference
points be so highly accurate, while the reading at a few thousand feet
above the surface varies so much against the altimeter??

Mike Noel wrote:
At airports that don't report altimeter settings I set my altimeter
according to my GPS altitude and this has given me a reasonable
ground MSL according to the published altitude of the airport.

How is this better than the traditional way of just setting your
altimeter to indicate the published field elevation?

THere's only two reasons you want to know your altitude. One is to
know how high you are relative to the ground, the other is to know how
high you are relative to other airplanes. Setting your altimeter by
the GPS gives you neither.

Sometimes the GPS is closer at hand than the map. Also, I don't live in
Kansas so my airports aren't flat.

I would never set my altimeter while aloft with the GPS since the barometric
altitude provides the necessary separation from other aircraft. I would
check my GPS altitude with charted high terrain altitudes to guage VFR
clearance margins.
--
Best Regards,
Mike
http://photoshow.comcast.net/mikenoel
"Roy Smith" wrote in message
...
Mike Noel wrote:
At airports that don't report altimeter settings I set my altimeter
according to my GPS altitude and this has given me a reasonable
ground MSL according to the published altitude of the airport.

How is this better than the traditional way of just setting your
altimeter to indicate the published field elevation?

THere's only two reasons you want to know your altitude. One is to
know how high you are relative to the ground, the other is to know how
high you are relative to other airplanes. Setting your altimeter by
the GPS gives you neither.