units of measurement on altimeters

Does anybody use feet for altitude?

Everyone except the Former USSR and China, as near as I can figure.

all the best -- Dan Ford
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In misc.metric-system Jukka K. Korpela wrote:
"Julian Scarfe" wrote:

How can you possibly suggest that it would be more "practical" to
use kPa?

Because one digit less is needed, and conversions are easier when
powers of 1,000 are used as normally.


No, you need the same number of digits, and a decimal point in
addition. A pressure difference of 1 hPa corresponds to an altitude
difference of 8 m at sea level. That is just enough precision, but 80
m (corresponding to 1 kPa) would be intolerable. Pilots are usually
required to keep an assigned altitude to within +- 15 m (50 feet).

--
Klaus Wacker
Experimentelle Physik V http://www.physik.uni-dortmund.de/~wacker
Universitaet Dortmund Tel.: +49 231 755 3587
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"Jukka K. Korpela" wrote in message
. ..

Describing them as hPa makes it clear what the unit is for someone
familiar with the SI, without risking accidents through unit
confusion.

Would it be clearer to use a non-recommended prefix than a recommended
prefix? Besides, your argument indicates a fundamental confusion. There
is only one SI unit of pressure, the pascal (Pa). That's part of the
beauty and practicality of the system. All the rest that is used to
express pressures relates just the way of expressing the numerical
value. For convenience, we can use multiplier prefixes of _the_ unit if
we like, or a multiplier of the number, consisting of a power of ten.

I'm not sure where you believe the "confusion" lies. Describing the unit as
hPa rather than mbar makes it clear that the unit is Pa and the prefix,
which is a standard SI prefix, gives the multiplier.

The preference to use powers of 1000 is just a preference because
practicality and pragmatism is sometimes more important than an
arbitrary recommendation. This is a perfect example of where
pragmatism should (and does) win.

The reason for preferring powers of 1,000, explicitly expressed in
several recommendations and standards, is its practicality, based on
the use of the system as a whole. If you take arbitrary special
aspects, you can always find arguments in favor of using non-SI units
or non-recommended SI expressions - but then you lose all the benefits
of a unified system.

Do you really believe that you lose *all* the benefits of a unified system
by using a prefix described (without deprecation, BTW) in the SI Brochure?

Using hPA is a half-hearted "solution" that
combines the trouble of transition

One man's half-hearted solution is another's essential compromise. :-)

Julian Scarfe

"Julian Scarfe" wrote:

How can you possibly suggest that it would be more "practical" to
use kPa?

"Jukka K. Korpela" wrote in message
. ..

Because one digit less is needed, and conversions are easier when
powers of 1,000 are used as normally.

I forgot to mention in my response, BTW, that the same number of digits *is*
required. Aviation applications require a precision of 100 Pa in measured
pressures. Your choice is between 1013 hPa or 101.3 kPa. By adding the
"daycimal", you simply make it more difficult for pilots to say.

Julian Scarfe

"Gene Nygaard" wrote in message
...

The preference to use powers of 1000 is just a preference because
practicality and pragmatism is sometimes more important than an arbitrary
recommendation. This is a perfect example of where pragmatism should
(and
does) win.

Oh, good grief.

Does anybody use feet for altitude?

Do all of the analog instruments show all the digits in those feet?

Pilots have become accustomed to using feet for altitude. Can you imagine
the confusion that would arise if the unit suddenly shifted by a factor of
10?

"Descend and maintain 300 decafeet"

Any room for confusion there?

We already have trouble enough with pilots screwing up inHg to mbar
conversions. What does an altimeter setting of "nine ninety two" mean?

As a physicist and a pilot, I'd rather live with the inconvenience of using
a hecto prefix for stuff that comes across my desk than risking confusion in
the cockpit. So would ICAO.

Julian Scarfe

"Julian Scarfe" wrote:

I forgot to mention in my response, BTW, that the same number of
digits *is* required.

It depends on the quantities. I was referring to the most common
quantities that people see expressed. When tagging isobars in weather
maps, the trailing zero is just a nuisance. And when more accuracy is
needed, it is natural to accept that fractions might be needed.

Your choice is between 1013 hPa or
101.3 kPa.

You just gave one more reason to favor kPa. The numeric value 1013 is
not in the recommended range, and it raises the question of a thousands
separator, which is language dependent, so that some cultures would use
1 013 (and would need a no-break space to prevent undesired line
breaks, and an en space to avoid too wide a gap, and cannot get both)
while some would use 1'013 or 1.013 or 1,013. Situations where the
quantity will be taken as a thousand times too small would be quite
rare, but the damage could be serious, so why take the risk.

--
Yucca, http://www.cs.tut.fi/~jkorpela/

"Julian Scarfe" wrote:

We already have trouble enough with pilots screwing up inHg to mbar
conversions.

So while you take the trouble, wouldn't it be best to move to something
that lasts, due to being part of a system that is meant to be applied
in all areas of life when expressing physical quantities? It's better
to move directly to the SI system as recommended in general, rather
than first moving, say, to a partly decimal-based variant of the Anglo-
Saxon system, or - to take an example about different quantities -
first move from the use of different gallons to a Unified Gallon, then
to hectoliters, later to what the SI system really recommends.

--
Yucca, http://www.cs.tut.fi/~jkorpela/

The numeric value 1013 is
not in the recommended range, and it raises the question of a thousands
separator, which is language dependent, so that some cultures would use
1 013 (and would need a no-break space to prevent undesired line
breaks, and an en space to avoid too wide a gap, and cannot get both)
while some would use 1'013 or 1.013 or 1,013.


I think this depends on the context of usage. In aviation, I think the
thousands separator would be omitted most of the time, it's primarily a
convenience when you have lots of digits, and four isn't "lots". As for units,
it depends on what you are integrating with. In aviation, you are integrating
with nothing, so you could measure in quattloos for all it matters. It is in
engineering, where many conversions and calculations are taking place, that the
units need to fit into a system and kPa would be preferred.

I live with meters, millimeters, and centimeters just fine. And
(interestingly) in aviation, I live with hundreds of feet and thousands of feet
just fine too. (I flight plan in thousands, such as 4.5K for 4500 feet, but
weather comes in hundreds, as in 45 for a cloud layer at the same alititude. I
kinda wish it were more consistant, but only kinda. Each system has its place.

Jose

--
(for Email, make the obvious changes in my address)

In misc.metric-system Julian Scarfe wrote:
Pilots have become accustomed to using feet for altitude. Can you imagine

As a european pilot I would prefer meters.
But how to rearrange separation standards, any ideas?

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Peter Hermann wrote:

As a european pilot I would prefer meters.
But how to rearrange separation standards, any ideas?

Use 300 meters for every 1,000'. That's a little less separation, but it wouldn't
be too hard for a pilot to deal with the arithmetic.

George Patterson
Battle, n; A method of untying with the teeth a political knot that would
not yield to the tongue.