From: Thomas B. Passin <tpassin@comcast.net>

Date: Sun, 15 Jun 2003 11:36:20 -0400

To: www-rdf-interest@w3.org

Message-id: <004a01c33353$de71d4d0$6401a8c0@tbp1>

Date: Sun, 15 Jun 2003 11:36:20 -0400

To: www-rdf-interest@w3.org

Message-id: <004a01c33353$de71d4d0$6401a8c0@tbp1>

[Roger L. Costello] > > The more that I think about this problem, the more convinced I am that > adding something like a conversionFactor attribute is the right way to > go, e.g., > > <owl:DatatypeProperty rdf:ID="length-in"> > <owl:equivalentProperty rdf:resource="#length-cm" > owl-x:conversionFactor="length-in/value() = > length-cm/value() / 2.54"/> > <rdfs:range rdf:resourse="&xsd;decimal"/> > </owl:DatatypeProperty> > Approaches like this come from thinking that a length expressed in inches is somehow related to a length expressed in centimeters. This leads one to think of expressing it using a predicate, which is what Roger is attempting here. But under the surface, the real situation is richer. There is some physical quantity, one that obeys the laws of nature. When we write it down, we choose one or another system of units to express it. In my post a few minutes ago, I suggested modeling the conversion as a concept in its own right. I think this is an obvious approach since we have been talking about these conversions as things in themselves, so obviously they are concepts. More fundamentally, it is useful to view the expression of the actual numerical value as an __operation__ on the underlying physical quantity. Modern physics uses operators all the time. There is an operator that translates an object in time, another that translates it in space, and so on. A units conversion is, from this viewpoint, just another operation. Many business functions can be viewed as transformations of business data from one state to another using an appropriate operator. An operation has parameters and an algorithm or formula. It is not (necessarily) just a predicate. Modeling a units conversion as a resource, as I did in my last post, is a way to represent a simple, scalar operator. It would probably be better in the long run to devise a general way to represent operators that can perform non-scalar as well as scalar transformations. These would probably turn to be extremely useful in many ways, some of them not anticipated now. Cheers, Tom PReceived on Sunday, 15 June 2003 11:33:30 UTC

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