GPS Altimeter Accuracy in airplanes

[Seldom]

My OR300 altimeter was set to "fixed".  The pilot said our cruising altitude would be 39000 feet.  My tracklog shows 41000+.

Is the pilot stating the altitude above the ground, or above sea level? If he's stating above sea level, is 2000 ft. vertical a reasonable error for GPS based altimeter?

[Indrid Cold]

Is the pilot stating the altitude above the ground, or above sea level?

Neither, he's referring to flight level, or FL390 in this case, an apparent altitude of 39,000 ft which is a pressure altitude.

[jbensman]

What's the proper way to set the baramoter when in a commerical jet?  You say you have it set to fixed.  What's the difference between the fixed and variable?  I wish there was a setting (or even a field) to show GPS elevation when you are flying.  (Being from the ST Louis area, I am glad I was not flying last night-but it would be interesting to see what the barameter would read during a tornado)

[Seldom]

I'm pretty sure you set it to "fixed" rather than "variable".  If it's set to "variable" the altimeter will register barometric pressure, which in the jet should be limited to around 5,000 ft.  Unless you're flying an old 737.  Then it could read the actual altitude on some planes.

[Indrid Cold]

What's the proper way to set the baramoter when in a commerical jet?  You say you have it set to fixed.  What's the difference between the fixed and variable?  I wish there was a setting (or even a field) to show GPS elevation when you are flying.

You set it to fixed, this logs the GPS reading not the barometric altimeter.

[Boyd]

Heh, I was driving in my car yesterday and the elevation reading was -350 feet. Actual elevation there is about 70 feet. The GPS altitude readings always seem wildly inaccurate to me.

[maps4gps]

barometric pressure, which in the jet should be limited to around 5,000 ft. 

Under ideal conditions.  I took a hiking altimeter along on trips and it was closer to 6,000 feet when in flight but would rapidly change after the plane decended below that level.  Some trips in winter could be brutal - we were stacked in a circular path in snowy considitons with limited visibility, every few minutes the pressure would rapidly (in 15 seconds or so) varry between about 3,000 and 9,000 ft.  The person next to me was a commercial pilot and explained that in such conditions the entire flight crew was 'watching' for unexpect planes, etc.  and that the cabin pressure was NOT being reset to adjust for the changing speeds necessary to keep in the flight pattern as directed by ground conttrol.

[Dickfoster]

In aviation they are talking about altitude above mean sea level (average sea level whatever that is) it's in no way and a truly exact thing this mean sea level stuff.
   
Flight levels are referenced to a barometric altimeter MESL again reading set to a barometer that has been set to a standard pressure of 29.92" of mercury or 1013.25 hPa. Pascals is just another measure of pressure but on the metric system.
When you arrive at the destination airport the cockpit altimeter is reset to the current local barometric pressure at the airport to get an altitude reading that more closely matches the actual altitude of the field and all the local air traffic does the same. It helps if everyone is working from the same page. BTW when you leave an airport you are again setting your altimeter to the local current pressure which of course changes with the weather. When in flight and flying at FL thus and so, you set your altimeter to 20.92 and hopefully so does everyone else. 

Some day it may change over to a more exact measurement that GPS systems make possible as they actually reference distance from the mass center of the earth, a common global datum.
Then you'd look up what the actual ground level is for the position over which you'd happen to be at the moment. By measuring your altitude (distance from the center of the earth) with the GPS then subtracting the ground level, you'd know your actual altitude above the terrain at any moment at any place. It's always handy to know that kind of stuff since airplanes don't fly though rock piles very well. However, it may take them a while to give up thier old pitot static instruments. Old ways and habits always die hard no matter how nonsensical they may be given a new technology.     

My OR300 altimeter was set to "fixed".  The pilot said our cruising altitude would be 39000 feet.  My tracklog shows 41000+.

Is the pilot stating the altitude above the ground, or above sea level? If he's stating above sea level, is 2000 ft. vertical a reasonable error for GPS based altimeter?