GPS altitude again is close to actual

I made another flight to KFTG this afternoon and periodically checked
the GPS receiver provided altitude versus my aircraft altimeter. GPS
was within 40' every time I checked (closer at times). I need to
check my last altimeter check to see if it has any errors but it is
certainly within standards so GPS is proving to be close enough for
VFR in these cold weather conditions.

Ron Lee

In article ,
(Ron Lee) wrote:

I made another flight to KFTG this afternoon and periodically checked
the GPS receiver provided altitude versus my aircraft altimeter. GPS
was within 40' every time I checked (closer at times). I need to
check my last altimeter check to see if it has any errors but it is
certainly within standards so GPS is proving to be close enough for
VFR in these cold weather conditions.

Ron, is your GPS WAAS enabled?
My non-WAAS Garmin 195 will only get within 150 feet vertically.
Vertical is the least accurate of the three planes.

john smith wrote:

In article ,
(Ron Lee) wrote:

I made another flight to KFTG this afternoon and periodically checked
the GPS receiver provided altitude versus my aircraft altimeter. GPS
was within 40' every time I checked (closer at times). I need to
check my last altimeter check to see if it has any errors but it is
certainly within standards so GPS is proving to be close enough for
VFR in these cold weather conditions.

Ron, is your GPS WAAS enabled?
My non-WAAS Garmin 195 will only get within 150 feet vertically.
Vertical is the least accurate of the three planes.

Good question. It is a Garmin GNS 430 without WAAS. I should try my
Etrex as well.

Of course I am not advocating using GPS to establish flight altitudes.

Ron Lee

I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location? From my
understanding these two planes of reference are not the same but are
closer in some spots than in others.

Marco

Ron Lee wrote:
john smith wrote:

In article ,
(Ron Lee) wrote:

I made another flight to KFTG this afternoon and periodically checked
the GPS receiver provided altitude versus my aircraft altimeter. GPS
was within 40' every time I checked (closer at times). I need to
check my last altimeter check to see if it has any errors but it is
certainly within standards so GPS is proving to be close enough for
VFR in these cold weather conditions.

Ron, is your GPS WAAS enabled?
My non-WAAS Garmin 195 will only get within 150 feet vertically.
Vertical is the least accurate of the three planes.

Good question. It is a Garmin GNS 430 without WAAS. I should try my
Etrex as well.

Of course I am not advocating using GPS to establish flight altitudes.

Ron Lee

"Marco Leon" wrote in message
oups.com...
I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location? From my
understanding these two planes of reference are not the same but are
closer in some spots than in others.

Marco

Pressure based altitude measurements also assume a standard pressure change
as a function of altitude that will not always be correct. This will cause
the difference between the altimeter and the GPS to change with altitude.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

Marco Leon writes:

I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location?

In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS. The system is not designed to provide
highly accurate altitude, as the original users (soldiers in the
field, ships) didn't require it. For most users, it's a lot more
important to know where you are on the map than to know how high you
are above sea level.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Mxsmanic wrote:
Marco Leon writes:
I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location?

In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS. The system is not designed to provide
highly accurate altitude, [..]

And a simple Google search with "gps altitude accuracy" will bring up
quite a few interesting write-ups on the topic. All urge extreme
caution about trying to use GPS as an altimeter.

Kev

"Kev" wrote:


Mxsmanic wrote:
Marco Leon writes:
I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location?

In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS. The system is not designed to provide
highly accurate altitude, [..]

And a simple Google search with "gps altitude accuracy" will bring up
quite a few interesting write-ups on the topic. All urge extreme
caution about trying to use GPS as an altimeter.

Kev


Altitude accuracy is not as good as horizontal for reasons already
mentioned...not because of a design issue. It is not the accepted
form of altitude information because baro inputs do that. However, it
can be an indicator if somehow a pilot lost all altitude information.

Ron Lee

"Ron Lee" wrote in message
...
"Kev" wrote:


Mxsmanic wrote:
Marco Leon writes:
I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location?

In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS. The system is not designed to provide
highly accurate altitude, [..]

And a simple Google search with "gps altitude accuracy" will bring up
quite a few interesting write-ups on the topic. All urge extreme
caution about trying to use GPS as an altimeter.

Kev


Altitude accuracy is not as good as horizontal for reasons already
mentioned...not because of a design issue. It is not the accepted
form of altitude information because baro inputs do that. However, it
can be an indicator if somehow a pilot lost all altitude information.

Ron Lee

Never say never, as the old saw goes....
Having said that, I would never use my GPS altitude indication for
anything other than driving my car over the Rockies or Sierras.
On the other hand, the KLN-89B in the 172SP I used to fly had the
ability to dial in the local baro setting. I don't recall the delta between
the 89B and the legal altimeter being more than 20 feet with the baro date
set in. Biggest problem was the 89B altitude info was all but useless for
anything other than audible altitude deviation alert. That and having to dig
out the book and flip through the pages to remember how to input the baro
data. And, as I recall the altitude alerting was not legal for Part 91
either

Mxsmanic wrote:


In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS.

You are the absolute last person, flying a sim, to make this statement.
This is simply not borne out by those of us who actually do compare
the GPS to the altimeter.