How to choose?
norman at astro.gla.ac.uk
Wed Sep 26 04:28:54 PDT 2007
On 2007 Sep 25, at 09:04, Rob Seaman wrote:
> On Sep 24, 2007, at 2:08 PM, Tony Linde wrote:
>> It seems that more people, of those who are knowledgeable and
>> care, prefer the SKOS approach.
I think it's not a case of `prefer SKOS in all cases', but `feel SKOS
is a closer match to what's required' (Bernard has emphasised this
quite strongly). owl:Class and skos:Concept are not replaceable for
each other, because the SKOS broader/narrower relationships are not
at all replaceable for RDFS/OWL super-/sub-class relations.
For example, in the IAU Thesaurus, `acceleration' is a narrower term
for `kinematics', but it makes no sense to say that `acceleration' is
a subclass of `kinematics', in the sense that every instance of
`acceleration' is also a `kinematics'. Thesauri are originally
`finding aids', and the relationship between a term and a narrower
term is that everything retrieved by the narrower term would also be
retrieved by the broader one.
Classification tasks (broadly interpreted) and finding tasks are
rather different things, and different tools might be appropriate for
In the case of VOEvent (though we should remember that although this
discussion is presently being driven by the VOEvent needs, it's not
restricted to that), the place where the vocabulary term would be
used is in the <why> element, indicating what the producer of the
VOEvent packet thinks the relevant object may be. What's supposed to
happen next? That's Rob's use-case, and will determine what tools
will work best (see below).
> - What happens when the ontology evolves? After all, we're talking
> about methods for discovering and characterizing brand new phenomena.
People do care and write stuff about ontology maintainance, and while
I don't believe there are any cast-iron best practices, there are a
number of suggestions.
If I say that urn:myont-v2.0#concept1 is a subclass of urn:myont-
v1.0#concept1, then you need only very lightweight reasoning to allow
me to label something as v2.0#concept1, and you to discover that
that's also a v1.0#concept1, which is the thing you understand.
> - How efficient are the tools of different kinds for applications
> with split second timing requirements?
RDFS reasoners (limited to subClassOf, subPropertyOf, domain and
range) are very fast. OWL reasoners are slower, but it depends
heavily on the implementation and on just what reasoning you
require. It might be possible to do reasoning off-line, and
'compile' material into something an application can use very rapidly
Using broader/narrower relationships I think you'd probably walk up
and down the tree of concepts using your own code -- you wouldn't
need to use a reasoner for this.
> - How robust are they against network or server outages?
There's no intrinsic need to have anything on the network. Having
things named by URIs doesn't mean that you have to dereference
anything (was that what you had in mind?).
> - What can SKOS and OWL do for simple use cases? For example:
> 1) A high signal-to-noise optical transient is detected.
> 2) A VOEvent packet is generated.
> 3) Its discovery "signature" is compatible with a wide range of
> possible underlying phenomena.
> 4) Its brightness, location, rarity, etc., make it of interest to
> several subscribers.
> 5a) How do semantic technologies aid in the efficient and reliable
> characterization of the phenomenon? (http://
> 5b) What strategy is best used by the several subscribers to work
> together compiling follow-up observations for the common good?
There are multiple things you could do here. Two fairly well-
* You could decide that you wanted to be sent any packets that appear
which are relevant to 'b stars', and you might be sent everything
labelled with one of 'b stars' narrower terms, or its broader terms,
too. This is a sort of finding application.
* If the event packet were to include information on brightness,
location and so on, then you could ask 'is this deducible to be a
member of the class X?', where that class might be defined as the set
of things which have properties A and B, intersected with the
complement of the set of things which have property C. That's quite
hard work, partly because that requires more reasoning, but mostly
because defining and labelling concepts with that much precision
takes a fair amount of people effort. The CDS folk have done a lot
of work on this with their astro ontology <http://www.ivoa.net/
Documents/latest/AstrObjectOntology.html>, and Ed's astronmy ontology
would I imagine be able to do similar work.
One of the features of RDF is that a triple store doesn't necessarily
care where its triples come from, or what format they were converted
from. This has a number of implications, but amongst them that you
can have one individual define a concept, another map it to second
vocabulary, a third relate it to other terms, and so on. You could
potentially have a vocabulary defined or refined collaboratively,
wiki-style, though whether this is a good idea is a separate question.
> - If this is not a well posed use case for differentiating between
> our semantic software choices, then what would be?
It sounds well-posed to me. Most of the uses of SKOS-type
vocabularies are for finding things, broadly conceived (ie, including
the filtering use described above), and would include browsing the
registry and data sources.
Norman Gray : http://nxg.me.uk
eurovotech.org : University of Leicester, UK
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