Altimeter vs GPS altitude

[L.Adamson]

Originally Posted by paul330

Whilst the GPS dictates your track and (accurately) calculates the vertical approach path, any procedural altitudes and minima are STILL based on the baro ......... Hence for stand-alone GPS approaches, you will find a minimum temperature specified.

Don't get me wrong - I think that the GPS is a brilliant piece of kit and adds to safety and awareness. You just have to recognize that the vertical information (whilst "accurate") is not really applicable in the current regime. Just as GPS navigation has replaced radio aids, I guess that the vertical information may one day replace altimeters but it's not going to happen any time soon. In the meantime, we really don't want people trying to give the impression that we should in any way doubt the accuracy of the altimeter because the "more accurate" GPS shows a different altitude nor use the GPS in preference because it is "more accurate". Remember, we are measuring different things despite them both being called "altitude"

Unless Galelio & a WAAS equivilent are on line, then there is little point in arguing the differences between GPS in the UK, and here in the USA. That's because the difference is quite dramatic. Vertical GPS guidance is used here for GPS based approaches. That makes it very applicable. And since I'm one who has a very high interest in the "decline" of CFIT........it makes the difference between GPS & barometric altimeters ....even more important.

In the meantime, as I mentioned yesterday............I took a look at a series of in-flight photos taken in my RV. Before using the XM weather option that shows current "altimeter"settings on my GPS screen, it was normal to see variations between my GPS altitude, and the altimeter.... of several hundred feet. Perhaps more. In every photo, in which the GPS is showing the current altimeter setting.........the difference is around 50' to right on. What that means, is that I've been actively adjusting the altimeter in flight, thanks to constant updates from XM. In some cases, the setting had changed a small amount, and I hadn't re-adjusted. That just goes to show the value of XM (or another inflight weather source) & WAAS.

L.Adamson

[BobTurner]

If your gps and barometric altitude are always within 50' then either:
1. You are aways flying low, or
2. You never fly in significantly non-standard conditions, or
3. Something is wrong.

[Low Pass]

Originally Posted by BobTurner

If your gps and barometric altitude are always within 50' then either:
1. You are aways flying low, or
2. You never fly in significantly non-standard conditions, or
3. Something is wrong.

Or, 4. You're still sitting in the work shop making airplane noises!

[L.Adamson]

Originally Posted by BobTurner

If your gps and barometric altitude are always within 50' then either:
1. You are aways flying low, or
2. You never fly in significantly non-standard conditions, or
3. Something is wrong.

Let me have the pleasure of....

Telling you (and the next reply), that you only think, that you know what you're talking about....

I'm usually flying at 7500-11500' msl. On many occasions, I have shown other pilots as well as non-pilot passengers...........just how well the GPS altimeter and the aircraft's follow each other --- exactly, for a few minutes, when I've updated my altimeter setting.....usually from XM weather, or a current area report. As expected, after a few minutes, they will begin to drift apart. I have several comparison pics at 9,500' msl.

Keep the updates fresh............and you won't see that 200'+ deviation, that I've often seen myself. NOTHING is wrong. It's actually RIGHT!

Now..............go out, and do it yourself. If it doesn't work..............then something is definately wrong..

I've done this for several years...........and all times of the year.

L.Adamson

[L.Adamson]

Originally Posted by BobTurner

If your gps and barometric altitude are always within 50' then either:
1. You are aways flying low, or
2. You never fly in significantly non-standard conditions, or
3. Something is wrong.

Some more...........

First............read all of my postings concerning 50'

Second...........are you trying to tell me, that all GPS approach airports are low elevation.......because they don't have a chance in **** of any accuracy, if they're higher elevations?

Wouldn't something be wrong, if your GPS & aircraft's altimeter would be way off..........as you taxi to the runway after setting the altimeter to field elevation?

So please tell me..........at what altitude for normal GA flying, does the aircraft's altimeter not have a chance of being close to GPS altitude, if the area barometric pressure setting is current.

And, do you use XM, or any other current weather aids, with altimeter setting updates?

L.Adamson -- Garmin 696

[Alan Carroll]

Originally Posted by paul330

To a certain extent. The principle that needs to be appreciated is that an altimeter measures pressure, not altitude. We therefore fly at pressure levels, not heights. The fact that it is displayed in feet gives us a warm, fuzzy feeling and is an approximation to our true altitude given the multitude of errors that the instrument is prone to (when I was a QFI in the RAF I used to know them all - there's something like a dozen ). The instrument could just as well be calibrated in "whaloons" and we could report our level as "1500 whaloons".

Hence, to say that the GPS altitude is "more accurate" is really meaningless. You simply are not measuring the same thing. Hence, the bottom of controlled airspace is not really at 3000'. It is at an atmospheric pressure equivalent to a reading of 3000' on our calibrated barometer.....

I agree with most of this. However, I'd argue that "accuracy" does in fact have a specific meaning. It is a measure of how closely the airplane indication matches its actual elevation above a datum (MSL). Its true the altimeter only measures pressure, but it reads out in feet (or meters) so it seems fair to judge it by that standard.

If you're flying an instrument approach the accuracy of the elevation indicated by the altimeter is certainly not meaningless!

[Sid Lambert]

It's just math guys.

If you are at sea level, standard pressure but at 40c... What does your alt read? Do the same at 10,000. I bet your gps is off (doesnt match the alt) by about 500' at each end of that spectrum.

[loopfuzz]

Hot topic! I enjoy reading it all!

When it comes down to it, I trust my gps altitude more during x-country. (certified or not).

Christopher.

[N402RH]

The Anywhere Map people have a good explanation:

http://docs.controlvision.com/pages/gps_altimetry.php

Barometric Altimetry Errors

At higher altitudes, the GPS altitude and Indicated (barometric) altitude often diverge. This is caused by the limitations of barometric altimetry, not by any error in the GPS calculations. All pilots know that temperature affects air density, and that on a hot day, the air is less dense – reducing aircraft performance.

This also affects the accuracy of the altimeter. Setting the Altimeter setting makes the indicated altitude agree with field elevation when the aircraft is on the ground at that airport. At ISA Standard temperature, the altimeter will be accurate as the aircraft climbs away from the airport elevation as well. However, when the air is very cold (more dense) or very hot (less dense) the altimeter will accumulate errors as we climb away from the field elevation where the altimeter setting was taken.

On a day that is 30 degrees Celsius colder the altimeter will show the aircraft about 10% higher than it’s AGL altitude above the station altitude where the altimeter setting was recorded. 30 higher results in the indicated altitude being 10% lower than the actual TRUE (GPS) altitude would indicate. Thus, on hot summer days, the GPS will show a higher cruise altitude than the altimeter, typically by about 500 feet at 10,000 feet. This has an impact on using the VNAV target altitudes and should be compensated for by the pilot. Air traffic control is based on Indicated altitude, not on GPS altimetry.

Rob Hickman
N402RH RV-10

[BobTurner]

Mr. A.,

I really don't like flashing credentials, but as a CFII and a Ph.D in physics yes, I do think I know what I'm talking about.
A standard barometric altimeter uses a model of the atmosphere to change pressure, which it measures, to altitude. There is a single adjustment to the model, the Kollsman window. But the atmospheric model has at least two variables, the sea level pressure and the rate at which pressure decreases with altitude. The AIM says that you set the equivalent sea level pressure into the Kollsman window, but that is not correct. The altimeter setting is whatever it takes to make the altimeter read the true MSL altitude at the reporting station.
A good GPS will read true MSL altitude with good accuracy. So as you approach an airport GPS altitude and barometric altitude will agree, if you have a current altimeter setting from the airport. As you climb, the barometric and GPS altitudes will agree only if the rate of pressure decrease with altitude follows the model. On a non-standard day, that won't be the case. I can be flying right over the airport, with a current altimeter setting, it doesn't matter. Remember your private training, when you had to use an E6B to convert your barometric altitude to true altitude? This is exactly what is going on here. Check GPS altitude for clearing the hills; use barometric altitude to stay at the proper altitudes for flying.