units of measurement on altimeters

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

But I think you forget where you came into this, Jukka. The thread is
entitled "units of measurement on altimeters". The quantities that need to
be expressed are in the approximate range of 970 to 1040 hPa, with a
precision of 1 hPa. The hPa is the right unit for that job.

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.

In context, the need for a thousands separator is not great, is it?

Julian

"Julian Scarfe" wrote:

But I think you forget where you came into this, Jukka. The thread
is entitled "units of measurement on altimeters".

It's part of the very idea of the SI system that a single unit is used
for each physical quantity, in a unified manner, not varying the system
by application, country, or phase of the moon. It is clear that the
system is not always optimal when judged from a narrow perspective of a
specialized field, but if we go that way, we'll end up with expressing
quantities in incompatible ways - there's _always_ at least some reason
to deviate from a system.

The pascal is a very small unit in many areas of everyday life,
technology, and science. This is handled, as usual in the SI system,
using a systematic set of multipliers that correspond to powers of
1000, so that the numeric values can be scaled to a reasonable range,
[0.1, 1000). In some situations it might be, at least due to historical
reasons, marginally more convenient to use 100 or 42 as a multiplier.
But that's not a good approach. (It is true that some additional
multipliers exist in the SI system. But this is due to historical
reasons and discouraged in many standards, and tends to create
confusion because prefixes like h or da are not widely known outside
some specific areas of application, like the hectare.)

The quantities
that need to be expressed are in the approximate range of 970 to
1040 hPa, with a precision of 1 hPa.

It's against the principles of the SI system to select units according
to the range and precision that you have in some special situation.
We don't invent new units every time we encounter a new situation.
That was the old way.

Quantities in the range 97 kPa to 104 kPa can easily be expressed to
any precision you need or the current technology permits. Surely people
who work with such things can be expected to be able to work with
numbers with a decimal part.

(If it becomes relevant to work with a precision of 50 Pa, would you
insist on inventing a unit that equals 50 Pa, so that you can keep
using integers only? What about 42 Pa?)

The hPa is the right unit for that job.

No, the hPa is not a unit in the SI system, any more than 100 Pa is.

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

It's part of the very idea of the SI system that a single unit is used
for each physical quantity, in a unified manner


This is fine and well while you're sitting in an armchair. But in the real
world there are sometimes compelling reasons to do something different from
the way a machine might handle things. In the case where

1: Not much interfacing with other units is involved
2: Rapid and accurate organic processing of the numbers is essential,
sometimes in adverse conditions.
3: Communications is suboptimal
4: A narrow range of values is involved

I'd say that it makes sense to use whatever units are most convenient in that
case, not whatever would make some world standards body twinkle its toes.
Altimeter settings are such a case.

Jose



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