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Now I know the definition of transition altitude - it's the altitude where pilots change their barometric altimeter datum from regional pressure setting to 1013.2hPa. In USA the TA is 18,000ft. But I just don't understand the whole concept. Why can't they just set the datum as 1013.2hPa at ground before takeoff. Why does it have to be 18,000ft. Also isn't the atmospheric pressure at 18,000ft much lower than 1013.2hPa, so isn't it wrong to put this as the datum?

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    $\begingroup$ Have a look at this answer including the "Further reading" section there. $\endgroup$ – Bianfable Nov 19 '19 at 11:30
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    $\begingroup$ This question covers a lot of the background to your question, and we have several others on the site about transition altitudes and levels. $\endgroup$ – Pondlife Nov 19 '19 at 14:57
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The atmospheric pressure is dropping quite signigicantly with altitude - and this is the very point of the concept of the altimeter - messure the pressure and show this as an altitude.

The basic concept of a flight level is that all high flying planes share a common reference - and do not neet to consider local weather, which can change quite dramatically during a flight. The main topic here is traffic seperation - so all should use the same reference and thus can quite compare their altitudes.

But flying low there is a nice problem called "ground" - planes should mostly avoid it. On charts the minimum altitude is noted, as well as special places like airports. Here a more precise view on the altitude is needed - so pilots need to set their QNH. Before takeoff, before landing - or when flying low for a longer time quite frequently.

But where to change from one reference to another? That much depends on the location - in the US it is mostly 18000, but in other regions this can differ substantially.

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