Please feel fit to tell me to ......
As a very occasional enthusiasist Flyer; I understand that QNH will give the PIC his height above mean sea level in the vacinity of an airfield. ASR when set will give the lowest expected pressure QNH for the region in which the setting applies for a given time - So assisting the pilot against a drop in actual atmospheric pressure, erroding minimum safe sector Altitude.
Given my very limited understanding of things like this - FL 1013 Mb (old money) is set as you pass "transition altitude" on departure to provide aircraft on "Airways" with a setting calibrated to ensure a 1000 ft seperation between aircraft.
OK I suppose modern aircraft climb through the proposed transition altitude comparitively quickly so no problem. But are there routes where even modern jet aircraft actually fly comparitively low (say 18000 - FL180) because of fixed height restrictions at given waypoints.
Can you imagine the complexity imposed in a country the size of the UK by flights transititing ASR's within airways with differiing regional settings - not to mention turboprops or perhaps a day where the isobars are tightly packed.
I know in the USA (a big country) the transition altitude is something like 16,000 ft but I imagine their ASR regions are a lot bigger.
tThe small thoughts of an enthusiast)