"Matt Barrow" wrote in message
...

"John R. Copeland" wrote in message
...

In the Rockies, there are ground stations at widely varying elevations.
I've guessed that ATC might not use settings from airports in deep
valleys
for enroute settings, wherever more representative values are available.

How would a station be more representative if it was/wasn't in a valley or
on a hill top?



This is an obstruction-clearance issue in very cold weather.

If the altimeter setting came from a station in the valley 5000 feet in true
height below the aircraft, the indicated height could differ from true
height by as much as 1500 feet feet.

If it came from a station on the hill only 1000 feet in true height from the
aircraft, the difference is likely less than 300 feet. (Source: Canadian
AIP)

If you are choosing flight altitudes without much margin for obstacle
clearance, you may want to ask for somewhat higher altitudes if you will be
using settings from valley stations.