David Leal wrote:
> Dear Geoffrey,
>
> I think that this is well worth discussing with the metrology experts.
>
> Two meanings of "quantity"? ... (01)
Yes, the VIM explicitly says that the word 'quantity' is used in the
"general sense" meaning a category of quantities, and in the sense of
"particular quantity", meaning a physical instance. (02)
> NOTE 2 – Quantities that can be placed in order of magnitude relative to one
> another are called quantities of the same kind.
>
> In this note "quantity" means "particular quantity". (03)
Yes. (04)
> The metre, the
> astronomical unit and the width of the Thames at London Bridge are
> "particular quantities" that can be placed in order of magnitude. (05)
No, or at least it doesn't help to say this. The width of the Thames at
London Bridge is a 'particular quantity'. The length of the old
platinum-iridium metre bar in Paris is a 'particular quantity'. (06)
The metre is defined by a particular quantity, the wave-length of some
emission or other, but don't think the 'metre' IS that particular
quantity. I think it is the abstraction, the "magnitude", of that
quantity, because it was originally defined by a different particular
quantity, and the change in definition was not intended to change the
'metre', but rather to make it more precise and more reproduceable.
(This is a debatable point, and it is critical to the definition of
'measurement unit'.) (07)
And for astronomical unit, you can probably go either way. The
definition of the unit was a particular quantity, but I believe it has
since been standardized as a multiple of the metre, because of the
difficulties in reproducing accurate measurements of the phenomenon. (08)
> NOTE 3 – Quantities of the same kind may be grouped together into categories
> of quantities, for example:
> - work, heat, energy
> - thickness, circumference, wavelength
>
> In this note "quantity" means "quantity in a general sense". Work, heat and
> energy are "quantities in a general sense" that can be grouped together. (09)
I found this note to be very confusing. "work, heat, energy" are
themselves categories of particular quantities, and therefore
"quantities in a general sense", I think. But it is surely not the
intent here that the things that are "grouped together into" a 'category
of quantities' are the _members_ (instances) of that category. (010)
The VIM introduces the notion "kind of quantity", which is more specific
than "category of particular quantity", in that two quantities of the
same kind are always comparable, while quantities of different kinds are
not comparable. And what is being said here (badly, I think) is that
comparable subcategories are being unioned into single categories that
are 'quantity kinds'. So instances of "thickness, circumference,
wavelength" are all of the "length" 'kind of quantity'. (The system of
units assigns reference measurement units to each of these 'kinds', not
to arbitrary categories of particular quantity.) (011)
Bottom line: The VIM was written by physicists, not knowledge engineers
or semanticists, and their attempt to write precise English (and French)
failed. The French has exactly the same ambiguity. (012)
> Clarification
> -------------
> It would be good to confirmation that this is the intent of the VIM and the
> International Electrotechnical Vocabulary. (013)
Yes. (014)
> It would also be good to have confirmation that the terms "quantity in a
> general sense" and "particular quantity" are correct. If they are, perhaps
> they should be added to the vocabulary. (015)
In any case, it is important that we distinguish these concepts in the
ontology. (016)
By comparison, Martin Weber said:
> This is a confusion that should be equivalently confusing modelled! [i.e.
> keep all the overloaded meanings of the term 'quantity' to stick to
>BIPM-parlese]. (017)
I could not disagree more! (But I assume Martin was joking.) Our
objective is not to capture the terminological problems in the VIM. The
objective of the ontology is to capture the distinct _concepts_ in the
VIM (and related references), characterize those concepts, and associate
them with suitable terms. Where possible, the terms will be those of
the VIM, or something very close. (018)
Proper conceptualization is what makes an ontology valuable. Proper
choice of terms is what makes it useable. The term cart is worthless
without the concept horse. (019)
David Leal added:
> p.s. The use of one term for objects at two different meta-levels is
> familiar to those working with ISO 10303. In ISO 10303-41, the entity type
> *product* is defined as follows: "A *product* represents a product or a type
> of product." (This is not a tautology - the entity type *product* is defined
> using the defined term "product".)
>
> Ontologically this is not very nice, and as ISO 10303 evolves towards being
> an ontology, it will be good to separate out the two uses. (020)
What (I hope) David means is "not very nice" is equating the same model
element with two importantly different concepts: a thing (product) and
a classification of things (type of product). And no, we don't want to
make that mistake with "quantity". (021)
But there is a second problem that motivated David's parenthetical
addition. What is being defined in Part 41 is the meaning of the symbol
"product" in the data model. That symbol is a "term" in the EXPRESS
language, and it is being defined in the English language. Some English
term used in the definition may have the same spelling, but the terms
are in different languages. [I thought this should be obvious, until I
watched an ISO committee become completely confused about "circular
definition".] The same sort of thing will occur in writing the
ontology: we will introduce vocabulary symbols in the (e.g.) OWL
language and describe them in the English language. But unlike EXPRESS,
we may also be able to _define_ them in the OWL language! From a
reasoning point of view, those that have only English language
descriptions are undefined terms. (022)
--
Edward J. Barkmeyer Email: edbark@xxxxxxxx
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8263 Tel: +1 301-975-3528
Gaithersburg, MD 20899-8263 FAX: +1 301-975-4694 (023)
"The opinions expressed above do not reflect consensus of NIST,
and have not been reviewed by any Government authority." (024)
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