RE: SV: do we need it?

Quoting Tony Linde <Tony.Linde-at-leicester.ac.uk>:

> I'm wondering what astronomical terms would not be included in the SV?
>

The point of the draft under discussion is not to include all possible astronomical terms in the SV, but to be able to express any possible astronomical term, locally defined, in terms of standard SV tokens (or words, as we call them in UCD-ish).

Example 1. You are a data provider publishing in the VO your data on the optical V-band magnitudes of your sources. You locally call them v-mag. But v-mag is your dialect, so you register your data as providing phot.mag;em.opt.V, which is the standard VO way of saying optical magnitude in the V band.
Now, phot.mag;em.opt.V is an UCD, and UCDs are a VO standard that tell people how to form UCDs using the standard, maintained, list if UCD-words (or UCD-tokens!!). So, you are expressing your concept (your quantity) not looking for the UCD-equivalent of optical V-band magnitude in a list of UCDs (which does not exist!) but you are expressing your concept in terms of the standard UCD-words, in this example: phot.mag and em.opt.V .

Example 2: suppose you want to register (I take the example of registering your data, just because this is the first step to move from local to VO-wide) your data on circumstellar matter around B stars. This is the sort of metadata information that you cannot express in terms of UCDs. We were not yet done with UCDs that we (all) realized the need to overcome this problem. Is circumstellar matter around B stars an astronomical concept? Yes. Can you find it in the SV? No. The SV tells you how to combine the list of SV tokens in order to get something like: diffuse;location.circumstellar;star.spectralType.B

Ex. 3: Or you can publish an alert on a GRB, using in the "what" (I suppose) field the SV equivalence
time.variation.burst;ucd:em.gamma
(note here the use of the namespace ucd: to mark that em.gamma is a token in the UCD-words list) in order to be sure to be understood in the VO compliant community.

This approach is by far more flexible and practical than standardizing a list of all possible astronomical terms. I say so by personal experience. Last year I spent many weeks of tedious skimming through the way astronomers generate neologisms in astronomy. Take a look at Table 5 of the Note at
http://ivoa.net/Documents/latest/AstroKeysTN.html .

There you can find a shortened list of well semantically defined terms, that are used by astronomers to mint a large variety of semantically different terms (or sub-terms, but this is not the point for the moment). And this is only the floating part of the iceberg! I also tested for variations around the term x-ray: I found more that 700 different terms. I cannot tell you how many SV+UCD tokens would be necessary to express these 700 concepts, but I'll be happy to pay a round in Edinburgh if they were more than a few ten! We'll find a reason to drink anyway!

This said, I think the discussion going on (in particular on vocab formats) is extremely important, because the draft addresses the problem only at the, let?s say, the default, or minimal, level. Being able to define higher levels could take us to a fascinating land indeed!

Andrea

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