Dear Ed, (01)
This is good progress. (02)
You say:
>But what is important is that there are 4 distinct concepts:
> - particular quantity = a physical instance to be quantified
> - kind of quantity = a category of comparable particular quantities
> - magnitude of quantity = an abstract quantification of particular quantities
> - quantity value = the expression of a magnitude as a number and a
measurement unit (where the number is the ratio of the magnitude to the unit) (03)
Probably we need to look at two "kinds of quantity":
- categories such as that which includes Ed Barkmeyer's height, width of the
Thames at London Bridge, the diameter of the earth's orbit;
- categories such as that which includes ultimate tensile strength, yield
strength in tension, yield strength in compression (all are stresses). (04)
Best regards,
David (05)
p.s. That makes 5 - "nobody expects the Spanish Inquisition". (06)
At 13:30 14/07/2009 -0400, you wrote:
>Geoff,
>
>Thanks for this. It came at exactly the right time to prevent me from
>further misleading David Leal and others, in a private exchange that
>touched on this.
>
>Geoff Williams wrote:
>> David Leal says
>>
>> <Aside>
>> Evan uses the term "quantity value" which is defined within the VIM. I have
>> read the definition many times: "number and reference together expressing
>> magnitude of a quantity". I think that this phrase should be read as follows:
>>
>> number_and_reference_together - expressing_magnitude_of - a_quantity
>>
>> but it could be read as:
>>
>> number_and_reference_together - expressing - magnitude_of_a_quantity
>>
>> Hence is the thing identified by (or "expressed by") 2.54 cm a "quantity" or
>> a "magnitude_of_a_quantity"? Somebody must know, or is there a deliberate
>> ambiguity. :)
>> </Aside> >>>>>>>>>>>
>>
>>
>> [GW] ...
>> I am certain in my own mind that the term means "magnitude_of_a_quantity"
>> not "quantity"
>
>I have come to the same conclusion, from the same sources.
>
>When the VIM speaks of 'particular quantity', it means a physical
>instance -- the quantity evidenced by a specific phenomenon, not the
>abstraction of that quantity. The VIM thinks of particular quantities
>as the instances of 'quantity', whereas we commonly think of the
>abstractions that are expressed by quantity values as the instances of
>quantity.
>
>I (now) think that the abstraction is what the VIM means by the
>'magnitude of a quantity'. Carefully stated, we want to assert that '10
>metres' refers to an instance of quantity. The VIM doesn't. It says
>that the distance from my door to the stairs is an instance of quantity
>that has a magnitude that is expressed as '10 metres'. That is, the VIM
>has three related concepts:
> quantity -- magnitude of quantity -- quantity value
> many 1 many
>
>One instance of abstract 'magnitude of quantity' is the abstraction of
>many instances of quantity. E.g., '10 metres' applies to many
>particular instances of distance, including the distance from my door to
>the stairs, and the height of the top diving platform at the local
>aquatic club.
>
>One instance of abstract 'magnitude of quantity' can be expressed as
>many different instances of quantity value, each of which is
>distinguished by the 'measurement unit' it uses. E.g., the magnitude
>that is expressed as '10 metres' can also be expressed as '1000
>centimetres', or as '32.8... feet' in English measure.
>
>> In David's example the quantity is a length and the magnitude is 2.54 cm
>> ie expressed as a number and reference (unit).
>
>This is a bit garbled. From what Geoff says above:
>"2.54 cm" expresses a 'magnitude of a quantity'. David did not in fact
>identify any particular instance of 'quantity'. Every (particular)
>quantity that has that 'magnitude' must be an instance of 'length',
>which is a 'kind of quantity' (a subclass of 'quantity'). We know this,
>because the system-of-units being used (SI) assigns the 'measurement
>unit' denoted by "cm" to 'length'.
>
>And 'measurement unit' is then a subclass of what the VIM calls
>'magnitude of quantity'. It is an abstract reference amount that is
>defined by a particular quantity -- a reference phenomenon.
>
>> I can seek clarification of this interpretation from JCGM/WG 2 if this is
>> a contentious issue
>
>Please do. (And I will consult our NIST expert as well.)
>To be fair, I think the VIM is a bit ambiguous in its use of the term
>'quantity'. But what is important is that there are 4 distinct concepts:
> - particular quantity = a physical instance to be quantified
> - kind of quantity = a category of comparable particular quantities
> - magnitude of quantity = an abstract quantification of particular
>quantities
> - quantity value = the expression of a magnitude as a number and a
>measurement unit (where the number is the ratio of the magnitude to the
>unit)
>
>And we can argue about which of those concepts gets the term 'quantity'
>in our ontology.
>
>But we are hardly the first to tread this path, and I strongly suggest
>we begin by looking at the reference ontologies, including the OUM
>ontology to which Hajo Rijgersberg pointed us (his work).
>
>-Ed
>
>
>--
>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
>
>"The opinions expressed above do not reflect consensus of NIST,
> and have not been reviewed by any Government authority."
>
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> (07)
============================================================
David Leal
CAESAR Systems Limited
registered office: 29 Somertrees Avenue, Lee, London SE12 0BS
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