UCDs vs ontologies?
seaman at noao.edu
Wed Jun 1 08:32:36 PDT 2005
On Jun 1, 2005, at 6:54 AM, Pierre Didelon wrote:
> But do/must UCD be the solution to (all?) natural langage parsing?
It would be premature to begin the work of the board by willy-nilly
reassigning identifiers such as "em". It seems likely that we will
need to be able to express the concepts of both "electromagnetism"
and "emission". If there is emission, there is absorption. And
sometimes we may want to express the combination of "electromagnetic
emission" at the same time.
On the other hand, we're told that the role of UCDs is distinct from
that of ontologies. An ontology is an (attempt) at expressing the
complete range of some knowledge domain. Astronomy is a big subject
- its ontology will be big. Perhaps by analogy we can view an
ontology as the unabridged dictionary for some subject, whereas UCDs
are simply one way to build a glossary for a specific purpose.
Glossaries are often small enough to be appended to a brief document.
Personally, I think the VO community will need to develop several
separate ontologies over time as well as several separate glossaries
of UCDs or UCD-like constructs. It is not obvious that a glossary of
UCDs for tabular convenience is equivalent to a glossary of UCDs for
VOEvent convenience. An ontology can afford to be large and unwieldy
to reach its goal of being complete and accurate. A UCD style
glossary, on the other hand, will eventually reach an optimum size.
Its utility will pass a point of diminishing returns. Too much
precision engenders confusion. The availability of too many options
results in overlapping shades of meaning.
I gather the current list of UCDs was generated by looking at actual
tables in the literature. This is just how the unabridged OED was
created from words sieved from millions of quotations. Just like a
dictionary, the work of maintaining the list of UCDs will continue
indefinitely as new tabular usage is coined.
I would suggest that the creation of this new list of UCD-like
entities to describe astronomical "concepts" is fundamentally a
different exercise. We may not be trying to generate a complete
ontology with all interrelationships clearly drawn between all
concepts, but we are trying to be complete in the sense of not
leaving any gaps in the web of concepts. "Star" and "galaxy" will
clearly make the final cut. "Star.white_dwarf" and "galaxy.spiral"
most likely, too. But it won't take many levels to exhaust the
utility of compiling such a list. I expect the final list to have
hundreds of entries, not tens of thousands.
One final point. The nature of this board is to participate in the
process of certifying an official list of terms. I think the true
utility of both glossaries and dictionaries will be achieved when
facilities are available for creating and maintaining *unofficial*
lists. For VOEvent, for instance, it seems likely that each project
publishing events will adopt its own glossary pertinent to its own
instrumentation and observations. We should support these activities
and provide a framework for project specific glossaries. They will
spring into existence whether or not we do so. At least if we
support the creation of project specific glossaries, we can have some
say in controlling a common semantic structure and a standardized
distribution mechanism. This might also naturally lead to the next
step of layering UCD glossaries on top of our emerging ontology
(ies). A glossary, after all, is nothing but a well chosen list of
words out of the dictionary. It is the dictionary that provides
etymology, synonyms and antonyms, classification by part of speech,
tenses and gender, pronunciation, ...
Sorry for the cross-posting. If we can't restrain ourselves from
generating all these mailing lists, I'm not sure what hope we have
for a coherent set of UCD lists :-)
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