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Some counrties have high transition altitudes and transition levels while other countriea have low ones.

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Transition altitudes / levels are instituted so all aircraft have the altimeter setting and will maintain vertical separation.

For aircraft that travel long distances quickly having the ability to set 29.92 (1013) makes it much easier to maintain vertical separation as they are not required to update the altimeter setting every 100 NM or so.

If terrain is a factor, the standard altimeter setting is not accurate enough to avoid conflicts with terrain.

There is a saying "FROM HIGH TO LOW LOOK OUT BELOW". If flying in an area of low pressure, the pilot who is using 29.92 will be flying closer to the ground than they should. In this instance, having the correct altimeter setting is vital to main vertical separation from terrain.

Since all aircraft close to terrain needs that new altimeter setting it will also work to maintain vertical separation from aircraft.

The United States has a high transition altitude, and it is above the highest terrain in the country. It is my opinion, that this would be the reasoning for most countries as well.

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