Re: Altitude reading on aircraft

Date:         30 Mar 98 04:31:17 
From: (James Matthew Weber)
Organization: Customer of Access One Pty Ltd, Melbourne, Australia
References:   1
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On 24 Mar 98 11:38:49 , wrote:
>On a flight from Frankfurt to Johannesburg I used my Garmin GPS 12 XL and I
>compared the readings with those on the movie screen when "flight tracker" is
>The speed and heading matched, the position as well (I checked the GPS
>readings on a map which map position matched on the flight tracker graph),
>but the altitude sometimes deviated by several hundred meters (up to 2000
>ft). Is this because aircraft use "logical" altitude, determined by local air
>pressure and GPS'es by satellite geometry ?

The earth is not a perfect sphere. It is a very complex ellipsoid. As
a result, there are  several hundred  different sets of survey data.
In order to accurately translate a GPS position into Long, Lat, and
Altitude and make it match a local map, you have to use the same
survey data the map makes used.

 Most GPS's (at least the American ones) default to  WGS84, which is
the standard for North America. The higher end GPS's  support many
other references, and some of the Magellans allow you to input the 5
constants that make the corrections. The point is the use of WGS84
outside North America can easily produce errors of several hundred
meters, especially in Altitude.  The GPS measures the distance from
the Sats, not from the ground!!! It accurately locates your position
in space, however due to irregularities inthe surface of the earth,
this is not necessarily accurately translated into altitude above the
earth if you don't have the local corrections for the shape of the

The Aircraft probably does wander a little at cruise, particularily
over water, where there is no reason to adjust the altimeter very ofte
as long as everyone uses the same settings. There may be no local
barometric data from sea level, so it may not be practical either.