ATC Altimeter Settings

The way I see it, if you have two stations, one on the ground at 350 feet
MSL, and the other on the ground (on top of a mountain) at 5000 feet MSL,
and they are both "nearby", then the one that is actually at 5000 feet
MSL
will give an altimeter setting that will be more accurate for an airplane
that's flying at 5000 feet MSL.

Both stations use a corrected reading; it's the altimeter aboard the
aircraft the will need to adjust for temperature, air pressure being
constant.


--
Matt
---------------------
Matthew W. Barrow
Site-Fill Homes, LLC.
Montrose, CO

"Matt Barrow" wrote in message
...

The way I see it, if you have two stations, one on the ground at 350 feet
MSL, and the other on the ground (on top of a mountain) at 5000 feet
MSL,
and they are both "nearby", then the one that is actually at 5000 feet
MSL
will give an altimeter setting that will be more accurate for an
airplane
that's flying at 5000 feet MSL.

Both stations use a corrected reading; it's the altimeter aboard the
aircraft the will need to adjust for temperature, air pressure being
constant.

.... which suggests you don't.

(Air pressure being constant??? The whole point of the altimeter
is that it isn't, with altitude. What are you trying to say here??)

Though it is true that both station pressures are 'corrected'
which in fact determines their respective altimeter settings,
this is only part of the story.

It is also true that you can 'correct' the indicated altitude
for temperature according to the observed outside air
temperature (and knowing the vertical distance
above or below the ground station from which the
altimeter setting was determined).

But remember that this 'correction', too, is an approximation,
and assumes a particular temperature profile in the
atmosphere between the observation location, and the
reference location.

But there's no way to be assured that that profile is any
more valid than the ICAO Standard Atmosphere that you're
correcting for. And this is only a temperature conversion
- there's no correction available for humidity or other things
that can affect the profile, nor any way in practice to determine
them. So even if you 'correct' for temperature, you can (and
should) still expect the altimeter to be in error.

But the error can generally be expected to be proportional
to the vertical distance between the observation location
(the altimeter) and the reference location (the ground station
from which the altimeter-setting was taken). So if you
reduce this vertical distance, you reduce the error.

Which, again, is why an altimeter setting from a (nearby)
ground station with an closer to your actual altitude
can be expected to produce an indicated altitude
with less error than an altimeter setting from a (nearby)
station that is vertically more distant.

The mountain was stated to be "nearby".

Like many terms used by the FAA, "nearby" was undefined. I intended it
to be interpreted as near enough so that lateral differences in the
airmass would not be a significant factor, but not so nearby that
collision avoidance becomes dicey, or the ILS takes you underground.
And yes, I know of no ILS installations that actually would take you
underground if flown correctly.

It is possible that critera such as the above are unattainable, for
example the classic security dilemma we face after 9/11 ("greater than
six but less than four"). However in the case above ("nearby") I
believe that there does exist a range of distance that satisfies the
criteria. To pull a number out of the hat, ten miles laterally comes to
mind.

Yes, but we're discussing this in rec.aviation.ifr.

The forum in which we discuss something does not alter its truth value.
And IFR can be in VMC.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

"Ron McKinnon" wrote in message
news:nWn5e.924843$6l.234357@pd7tw2no...

"Matt Barrow" wrote in message
...

The way I see it, if you have two stations, one on the ground at 350
feet
MSL, and the other on the ground (on top of a mountain) at 5000 feet
MSL,
and they are both "nearby", then the one that is actually at 5000 feet
MSL
will give an altimeter setting that will be more accurate for an
airplane
that's flying at 5000 feet MSL.

Both stations use a corrected reading; it's the altimeter aboard the
aircraft the will need to adjust for temperature, air pressure being
constant.

... which suggests you don't.

(Air pressure being constant??? The whole point of the altimeter
is that it isn't, with altitude. What are you trying to say here??)

Sorry...wrong choice of words...SB "does not oscillate within the column of
air (i.e., it's predictable)".

Funny, after 20 or so replies, no one has answered the original question,
only reprinted the explanation of air pressure from the first week of ground
school.

The original point was that +/- "ATC uses reading front reporting stations
on hill tops, rather than from stations nearby in valleys". Gee, I have
terrain over 13000 feet high just a few miles from here, but they (ATC) use
the Montrose or Grand Junction station readings.

"Matt Barrow" writes:

[...]
Funny, after 20 or so replies, no one has answered the original question,
[...]

The original point was that +/- "ATC uses reading front reporting
stations on hill tops, rather than from stations nearby in
valleys". Gee, I have terrain over 13000 feet high just a few miles
from here, but they (ATC) use the Montrose or Grand Junction station
readings.

And what exactly is your question now? Why your local ATC does that?
Maybe there is no more better observation station higher up, even
though that would yield more accurate altitudes there.

As you seem to forget, you doubted this point, and that caused the
bulk of the thread. To quote you: "How would a station be more
representative if it was/wasn't in a valley or on a hill top?", and
"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.".

Maybe for en-route purposes, or with their weather patterns, it tends
to be close enough. Maybe they don't want to issue a separate
altimeter call for people doing approaches into the lower areas. Call
and ask them if you really need to know.


- FChE

"Jose" wrote in message
om...

Like many terms used by the FAA, "nearby" was undefined. I intended it to
be interpreted as near enough so that lateral differences in the airmass
would not be a significant factor, but not so nearby that collision
avoidance becomes dicey, or the ILS takes you underground. And yes, I know
of no ILS installations that actually would take you underground if flown
correctly.


"Nearby" means it's located a short distance away. What you intended it to
mean is irrelevant.



The forum in which we discuss something does not alter its truth value.
And IFR can be in VMC.


Minimum IFR altitudes apply to IFR operations without regard to IMC or VMC.

"Nearby" means it's located a short distance away.

.... and what is "short"? A distance which is nearby?

What you intended it to mean is irrelevant.

Unless one wants to communicate.

Minimum IFR altitudes apply to IFR operations without regard to IMC or VMC.

I believe I said that, or something consistant with that.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

"Jose" wrote in message
. com...

... and what is "short"? A distance which is nearby?


Short is having little length; not long.



Unless one wants to communicate.


Wanting to communicate is one thing, having the ability to do so is another.



I believe I said that, or something consistant with that.


You're mistaken.

Short is having little length; not long.

....and what is long? "Not short"?

Wanting to communicate is one thing, having the ability to do so is another.

One of us lacks that ability. I won't speculate on which one that is.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

"Jose" wrote in message
m...

...and what is long? "Not short"?


Long is having the greater length of two or the greatest length of several.



One of us lacks that ability.

Correct.



I won't speculate on which one that is.


It is you.