Re: Paper on Quantities in OWL

Dear Bijan,

Language tags - a useful analogy
--------------------------------
You say that XML language tags are analogous to your proposed approach to
units and scales. I agree that their use is simple and ubiquitous, and do
not oppose this approach.

On the other hand there many requirements to do with languages which XML
language tags do not cover:
- librarians wish to specify which ink on paper or digital resources in
their libraries are in which language;
- linguists wish to record relationships between languages;
- for many purposes it is useful to record which language is spoken where.
(When you find an address in Google maps, it might be useful to be told that
the local language is French, or Gujarati. :)

For the more sophisticated applications a language ontology is required. It
would be useful to know how an XML language tag relates to objects in this
ontology.

Quantities, units and scales
----------------------------
It is probably true that 90% of the time, the only thing that you need to
know about a quantity is a single identification by a number and a unit or
scale. But, the other 10% is very important and is of very high value to
science and engineering. It includes all real measurements, and all
quantities with tolerances.

(It may be that of the other 90%, many of the quantities which appear to be
nominal dimensions or rated values are not really quantities at all. For
example, in a "2 inch" pipe fitting, there is not necessarily any dimension
that is 2 inches. Instead, "2 inch" is a classification of the pipe fitting
with respect to a standard.)

In dimensioning, it is often necessary to make statements such as:
 X less than (diameter of MyShaft) + (maximum clearance specified by
MyDesignManual)
 X greater than (diameter of MyShaft) + (minimum clearance specified by
MyDesignManual)

The diameter of MyShaft is a quantity, but it may not yet be known. It is
possible to make the statements involving an anonymous quantity, e.g.:

  <Length>
   <diameterOf rdf:resource="MyShaft"/>
  </Length>

but it may be convenient to assign an identifier to the quantity.

Politics
--------
It is necessary to address these difficult cases, because there is a
perception in some of the engineering community that "RDF/OWL is all very
well for trivial cases, but does not address what we really do". Their
response is to use homemade XML schemas or to use RDF/OWL in bizarre ways.

Personally, I would very much like to see MathML and OWL closer together.
MathML can make the statements about the relationships between quantities
that you want, but it is a closed world and cannot specify the semantics of
the quantities with respect to real world objects.

Best regards,
David

At 10:12 21/08/2008 +0100, you wrote:
>On Thu, Aug 21, 2008 at 9:32 AM, David Leal
><david.leal@caesarsystems.co.uk> wrote:
>> Dear Kendall and Bijan,
>>
>> The paper is very interesting. I am just going on holiday for a few days
>> would like to make some detailed comments when I return.
>>
>> An initial thought is that probably both an OWL-extension approach and an
>> ontology-based approach will exist side by side. The OWL-extension approach
>> has the benefit of easier computability.
>
>Actually, it also has the benefits of correctness and usability
>(inherently, from sane performance, and from non-contamination of the
>user modeling) as well.
>
>> However, the ontology based approach is also important, for two reasons:
>>
>> 1) There are circumstances in which it is necessary to identify a quantity
>> (e.g. 5.3 metres, the freezing point of gold - one of the calibration points
>> in ITS90), a quantity space (e.g. length, temperature, earthquake
>> magnitude), a unit (e.g. metre), or a scale (e.g. Celsius, Richter),  and to
>> make statements about it.
>
>You can actually say a fair bit about all of these things under our
>approach. Indeed, you can make all sorts of statements about
>particular quantities or dimensions. You can't say very much about
>units, per se, but I really don't think that's necessary. The key
>things to say about a unit is either that you want to use some unit
>for input validation or display (which we talk about) or you want to
>add a new unit.
>
>> A statement about a quantity can be its numerical value according to a unit
>> or scale, but can also be more complicated than that.
>
>Most other things, I think, are pretty irrelevant for modeling. E.g.,
>that a unit is a base unit for some complete system of measurement for
>some domain is exactly the kind of thing I don't think is worth
>supporting.
>
>> 2) W3C does not have the expertise to deal with all of quantities, units and
>> scales. Hence we need to engage the community which standardises in this
>> area. If we are going to ask them to assign URIs to these objects,
>
>I don't want uris for quantities. That just seems like a boondoggle
>for the majority of them. I prefer to think of them the way people
>think of language tags.
>
>Of course, we are engaging with outside groups. But that doesn't mean
>we don't build things in.
>
>> then
>> there needs to be a precise definition of what the objects are and how they
>> relate to each other. So we have an ontology whether we like it or not.
>
>That doesn't mean we need or want an *OWL* or *RDF* ontology. Indeed,
>I think such is not helpful at all. I spent quite a bit of time in
>UnitDim which is a *valiant* effort. It's just not worth it. We can
>describe quantities quite well outside of OWL and then *use* those
>descriptions to add a theory of quantities *to* OWL. Just as we do
>with integers.
>
>> For me the ideal solution would be an ontology for quantities, units and
>> scales, for which a special syntax is available. This would allow some users
>> to use the special syntax and ignore the ontology, and others to use the
>> ontology and ignore the special syntax.
>
>I'm curious as to what concrete uses for the ontology you imagine. I
>can think of none, really. As we point out in the paper, there are two
>reasons for quanities "in" an ontology: To help *model* quantities
>(see Gruber's work) or to model *with* quantities. The latter is the
>cruicial bit for the overwhelming majority of use, esp. for this
>group, I would imagine!
>
>As I think we show adequately in the paper, an ontology for modeling
>quantities (presuming, which is a huge presumption, that you can even
>do it correctly in OWL) is useless for modeling *with* quantities.
>
>-- 
>Cheers,
>Bijan.
>
>http://clarkparsia.com
>

============================================================
David Leal
CAESAR Systems Limited
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Lee London SE12 0BS
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Received on Monday, 25 August 2008 09:44:03 UTC