Re: Altitude reading on aircraft

Date:         30 Mar 98 04:31:16 
From: (CL6013R)
Organization: AOL
References:   1
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>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 ?

YES , the airplanes altimiter is set 29.92 inches of mercury during cruise (any
time above 18,000').  True altitude may vary by a couple of thousand feet due
to air pressure changes , but if we are all at 29.92 then we all read the same
altitude and maintain proper verticle seperation.