GPS altitude is quite a bit less accurate than the latitude and longitude if you look at typical variation sitting in backward (use UTM coordinates to express horizontal as meters of easting and northing, and compare to meters of altitude).
Even more important for vertical separation of aircraft is that EVERYBODY must use the same technology to avoid collision. A pilot controlled by ATC must be on the pressure contour corresponding to the altitude he was told to fly. If he is at "true altitude" when everyone else is flying to an agreed on, but slightly incorrect, principle, someone will die. Air traffic control DEPENDS on every aircraft from the smallest Cessna to the Airbus 380 agreeing on the altitude equation. Standardization is more important than correctness. Pilots are trained that altimeter reads pressure altitude not true altitude, and not to place over-reliance on it for clearing mountains, etc. The Standard Atmosphere and the altimeter equation are fairly accurate for mid-latitudes (45°) in spring and fall. At the equator and in summer, pressure altitudes expand upward from true altitude, at the poles and in winter, downward. At high altitudes, the errors can exceed the spacing between assigned flight levels. ________________________________ From: "mech...@illinois.edu" <mech...@illinois.edu> To: U.S. Metric Association <usma@colostate.edu> Sent: Sun, April 18, 2010 11:32:32 PM Subject: [USMA:47172] Re: Air flight altitudes in meters What is the contribution of GPS data to the navigation of large aircraft? Do GPS data dominate barametric data? ---- Original message ---- >Date: Sun, 18 Apr 2010 18:10:10 -0700 (PDT) >From: "John M. Steele" <jmsteele9...@sbcglobal.net> >Subject: [USMA:47170] Re: Air flight altitudes in meters >To: "U.S. Metric Association" <usma@colostate.edu> >Cc: "U.S. Metric Association" <usma@colostate.edu> > > Actually, it is nominally based on height above > sealevel. Sitting on the runway, with altimeter > correction dialed in, it will read the published > height of the runway above sealevel. > > At cruise levels, no altimeter correction is used > and reading is called flight level. It is the > height above sea level IF sea level were at 15 °C, > 101.325 kPa, and a lapse rate of -6.5 K/km to the > stratosphere (11 km), and zero lapse rate above that > to 20 km. Further it it uses a height variable > called geopotential height, the height that would be > true if gravity were constant with height. There is > a transformation between that and geometric height > in the standard. > > ------------------------------------------------ > > From: James R. Frysinger <j...@metricmethods.com> > To: U.S. Metric Association <usma@colostate.edu> > Cc: U.S. Metric Association <usma@colostate.edu> > Sent: Sun, April 18, 2010 8:47:07 PM > Subject: [USMA:47168] Re: Air flight altitudes in > meters > > Altimeters work off of atmospheric pressure > readings, Pat. But the readout is in terms of height > above terrain. So assignments and reports are always > in length units. No human pressure to altitude > correlation procedures are used. > > Jim
