"Icebound" wrote in message
...

"Matt Barrow" wrote in message
...


You're bringing in a lot of irrelevant material. The original point was
"Area" altmiter settings and another point made that a station in
mountainous areas would be more accurate if it was located on the
mountain
top at higher altitude than a station at lower altitude in the same
area.

You're still not explaining how a ground station at, say, 6000' MSL
would
be have a more accurate baro reading than one down in a nearby valley
at,
say, 2000' MSL. That is the point of the thread.





There is NO issue of "altimeter-setting accuracy" nor "barometer
accuracy".

Both stations have accurate barometers, and are reading their station
pressure correctly and accurately. Both altimeter-settings are "accurate"
in so far as the settings have been properly determined according to the
rules, for each individual station's actual-air-pressure and each
individual
station's actual-measured-elevation.

But as we all know, setting our altimeter to an "accurate altimeter
setting"
does not mean that the INDICATED altitude matches the TRUE altitude. In
almost never ever does, because the real atmosphere is almost always
different from the "standard" for which altimeters are calibrated.

So there is always a discrepancy between INDICATED and TRUE altitudes. In
most cases this does not matter, because the discrepancy is the same for
everybody.

Somebody in this thread asked WHY this discrepancy was greater if using a
valley station's setting, as opposed to a hill station's setting. (He may
have used the word "accuracy", but his meaning was: "why is my INDICATED
altitude going to be closer to my TRUE altitude when I use the
hill-station's-altimeter setting as opposed to the valley's?")

No, he claimed that ATC uses mountain top reporting stations rather than
lower level ones for "area settings". That makes two fallacies in one post.


My entire tirade was to try to explain why that is so... and just to warn,
that in very cold weather, this means that you are flying much lower (TRUE
altitude) than INDICATED. If you are not paying attention to
indicated-vs-true discrepancies, you may choose an INDICATED altitude
which
may put you below a comfortable margin of terrain/obstacle clearance.

In cold weather, then, it doesn't matter much which reporting station is
used _if pilots don't make necessary corrections for True Altitude_. Same
thing for the other side of the coin, not correcting for density altitude.
Does ATC use valley reporting stations in the summer then?