ATC Altimeter Settings

"Matt Barrow" wrote in message
...

"Frank Ch. Eigler" wrote in message
...

"Matt Barrow" writes:

"Icebound" wrote:
[...]
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. [...]

Mind telling me how that variation (of that magnitude) could come
about?

If you run through the full "true altitude" calculation discussed
during early ground school, you'll see that there is a term that
relates to the elevation of the measurement station. The effect
is that the lower you are AGL, the closer the calibrated & true
altitudes tend to become, because deviations from the standard
atmosphere become less significant within less tall columns of air.

Ground stations are all AT GROUND LEVEL, whether the station is at 2000
MSL
or 6000 MSL.

Ground stations are all more or less at ground level. But they report
their
altimeter settings in terms of Mean Sea Level, in order to provide a common
basis of comparison. This means that the air pressure read from the
station's
barometer must be *corrected* to sea level. The correction is in terms of
the conditions of the ICAO standard atmosphere, taking account the
difference
in elevation from MSL only: i.e. temperature, humidity etc. or other local
differences in the composition of the atmosphere are not considered in this
correction.

An altimeter can be expected to indicate the correct altitude (+/- 75') only
on the ground at the given ground station, and elsewhere, only where ICAO
standard conditions prevail. For any other position or condition the
altimeter
can be expected to be in error (and the atmosphere can be expected to
always
differ from the ICAO standard conditions).

For positions directly above or below the given station, the amount of
error can be expected to be roughly proportional to the difference between
the actual altitude and the station elevation.

Thus, for a flight at 5000 feet, the altimeter setting for a (nearby) ground
station at 5000 feet elevation can be expected to indicate closer to true
altitude than when using the altimieter setting for a station at sea level.
For a flight nearer sea level, the setting from the (nearby) sea level
station
will indicate closer to the true altitude than the altimeter setting for the
5000'
station.

In either case, there can be expected to be other errors due to the lateral
distance to the station as well, and the difference in atmospheric
conditions
that this entails.

The desire to minimize these effects is why we generally choose the
altimeter
setting from the nearest ground station, or otherwise nearest to our route
of
flight. And why, in the point under discussion, a nearby ground station
located
nearer to our actual altitude is better than one at a different elevation.