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Michael Day wrote:
>
> > - Allow (and encourage!) diversity of physical expression.
> > - that is, create instance documents in a style
> > which meets your needs/desires.
>
> This is a very good idea. However, can it be fully realised within the
> constraints imposed by using RDF/XML?
>
> In the example that you give, of SLR vs. Camera and focal-length vs.
> size, there were some constraints that could not be changed,
> regardless of the ontology use.
Hi Michael. Excellent question! You correctly note that an OWL
ontology defines the fundamental (logical) relationships of the data,
such as:
"SLR is a type of Camera"
and
"focal-length is synonymous with (lens) size"
My point is that a physical expression (i.e., an XML instance document)
can take any form, provided that it does not violate the fundamental
relationships that are declaratively documented in the OWL ontology.
Thus, this physical expression is not allowed ...
<Camera rdf:ID="Olympus-OM-10">
<optics>
<Lens>
<shutter-speed>...</shutter-speed>
</Lens>
</optics>
</Camera>
... since the Camera OWL ontology states that:
"The domain of shutter-speed is (camera) Body"
Thus, the physical expression violates the fundamental relationships of
the data and is not allowed.
> For example, both examples use:
>
> <optics>
> <Lens>
> ...
> </Lens>
> </optics>
>
> While they may *rename* the Lens element, it is not possible for them to
> *omit* the Lens element and place focal-length and f-stop directly under
> optics, as this would break the striping convention of RDF/XML.
>
> So my question is, do you intend your interoperability through ontology
> scheme to stop at element renaming?
No. Other ways: An OWL ontology does not mandate "ordering" of data, so
physical expressions are free to order the data in any way desired,
e.g.,
Physical Expression #1:
<Camera>
<optics>...</optics>
<carriage>...</carriage>
</Camera>
Physical Expression #2:
<SLR>
<carriage>...</carriage>
<optics>...</optics>
</SLR>
An OWL ontology may not mandate cardinality, so physical expressions may
differ in the occurrence of elements, e.g,
Physical Expression #1:
<Camera>
<optics>...</optics>
<carriage>...</carriage>
</Camera>
Physical Expression #2:
<SLR>
<carriage>...</carriage>
<optics>...</optics>
<cost>...</cost>
</SLR>
Note <cost> in the second but not the first.
I am sure that there are other variations allowed on the physical
expression besides synonyms, subclassing, ordering, and occurrence.
I'll have to think a bit on it.
Provided the physical expressions do not violate the statements in the
OWL ontology then they are perfectly acceptable for exchange with a
trading partner.
To give a better feel for the possible diversity, here are two physical
expressions which are both acceptable according to the Camera
Ontology[1]:
Physical Expression #1:
<?xml version="1.0" encoding="UTF-8"?>
<SLR xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns="http://www.camera.org";>
<optics>
<Lens>
<focal-length>75-300mm zoom</focal-length>
<f-stop>4.5-5.6</f-stop>
</Lens>
</optics>
<carriage>
<Body>
<shutter-speed rdf:parseType="Resource">
<min>0.002</min>
<max>1.0</max>
<units>seconds</units>
</shutter-speed>
</Body>
</carriage>
<cost rdf:parseType="Resource">
<rdf:value>325</rdf:value>
<currency>USD</currency>
</cost>
</SLR>
Physical Expression #2:
<?xml version="1.0" encoding="UTF-8"?>
<Camera xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
xmlns="http://www.camera.org";>
<viewFinder rdf:resource="ThroughTheLens"/>
<carriage>
<Body>
<shutter-speed rdf:parseType="Resource">
<min>0.002</min>
<max>1.0</max>
<units>seconds</units>
</shutter-speed>
</Body>
</carriage>
<optics>
<Lens>
<aperture>4.5-5.6</aperture>
<size>75-300mm zoom</size>
</Lens>
</optics>
</Camera>
In fact, both physical expressions are talking about SLRs. The
viewFinder property in Physical Expression #2 identifies that it is a
SLR Camera (only SLR's have a view finder that is through the lens).
Note the differences between the two physical expressions:
SLR versus {Camera, viewFinder}
<SLR>
<optics>...</optics>
<carriage>...</carriage>
<cost>...</cost>
</SLR>
versus>
<Camera>
<viewFinder rdf:resource="ThroughTheLens"/>
<carriage>...</carriage>
<optics>...</optics>
</Camera>
focal-length versus (lens) size
f-stop versus aperture
<Lens>
<focal-length>75-300mm zoom</focal-length>
<f-stop>4.5-5.6</f-stop>
</Lens>
versus
<Lens>
<aperture>4.5-5.6</aperture>
<size>75-300mm zoom</size>
</Lens>
Quite a few differences in the two physical expressions. Lots of
flexibility of expression, I'd say! Thanks for the question Michael!
/Roger
[1] http://www.xfront.com/camera/sld001.htm
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