Thread: QNH or QFE ?
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Old 9th Aug 2018, 12:20
  #47 (permalink)  
Discorde
 
Join Date: Jan 1999
Location: UK
Age: 71
Posts: 476
Originally Posted by BackPacker View Post
However, there is no way to convert an accurate barometric-based altitude reading into the equivalent GPS-based altitude reading and vice versa unless you know not only the local QNH, but also the exact environmental lapse rate at your given position. Heck, you even need to know the actual humidity across the whole column of air below you. Using the standard ISA lapse rate for this instead may lead to hundreds of feet of error if you're flying at FL300.
Adjusting GPS altitude to match indicated barometric altitude would be accomplished electronically within the apparatus without pilot input required.
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