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<title>Vocabularies in the Virtual Observatory</title> |
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<tr><td><a href="http://www.ivoa.net/"><img alt="IVOA logo" src="http://ivoa.net/icons/ivoa_logo_small.jpg" border="0"/></a></td></tr> |
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<h1>Vocabularies in the Virtual Observatory, v@VERSION@</h1> |
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<h2>IVOA Working Draft, @RELEASEDATE@</h2> |
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<!-- $Revision$ $Date$ --> |
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<dl> |
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<dt>Working Group</dt> |
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<dd><em><a href="http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics">Semantics</a></em></dd> |
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|
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<dt>This version</dt> |
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<dd>@BASEURI@</dd> <!-- XXX adjust current/latest URI from Makefile --> |
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|
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<dt>Latest version</dt> |
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<dd>@BASEURI@</dd> |
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|
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<dt>Editors</dt> |
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<dd>TBD</dd> |
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|
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<dt>Authors</dt> |
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<dd> |
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<!-- The following are the folk that I'm aware have contributed text or code to this document: add others as appropriate --> |
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<span property="dc:creator">Alasdair J G Gray</span>, |
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<span property="dc:creator">Norman Gray</span>, |
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<span property="dc:creator">Frederick V Hessman</span> and |
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<span property="dc:creator">Andrea Preite Martinez</span> |
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</dd> |
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</dl> |
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<hr/> |
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</div> |
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<div class="section-nonum" id="abstract"> |
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<p class="title">Abstract</p> |
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<div class="abstract"> |
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<p>Use SKOS.</p> |
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</div> |
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</div> |
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<div class="section-nonum" id="status"> |
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<p class="title">Status of this document</p> |
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<p>This is an IVOA Working Draft. The first release of this document was |
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<span property="dc:date">@RELEASEDATE@</span>.</p> |
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<p>This document is an IVOA Working Draft for review by IVOA members |
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and other interested parties. It is a draft document and may be |
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updated, replaced, or obsoleted by other documents at any time. It is |
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inappropriate to use IVOA Working Drafts as reference materials or to |
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cite them as other than <q>work in progress</q>.</p> |
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|
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<p>A list of current IVOA Recommendations and other technical |
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documents can be found at |
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<a href="http://www.ivoa.net/Documents/"><code>http://www.ivoa.net/Documents/</code></a>.</p> |
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|
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<h3>Acknowledgments</h3> |
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|
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<p>None so far.</p> |
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</div> |
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<h2><a id="contents" name="contents">Table of Contents</a></h2> |
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<?toc?> |
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<hr/> |
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<div class="section" id="introduction"> |
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<p class="title">Introduction</p> |
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<div class="section"> |
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<p class="title">Vocabularies in astronomy</p> |
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|
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<p>Astronomical information of relevance to the Virtual Observatory |
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(VO) is not confined to quantities easily expressed in a catalogue or |
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a table. Fairly simple things like position on the sky, brightness |
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in some units, times measured in some frame, redshits, classifications |
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or other similar quantities are easily manipulated and stored in |
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VOTables and can now be identified using IVOA UCDs <span class="cite">std:ucd</span>. However, astrophysical concepts and |
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quantities consist of a wide variety of names, identifications, |
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classifications and associations, most of which cannot be described or |
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labelled via UCDs.</p> |
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|
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<p>There has been some progress towards creating an ontology of |
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astronomical object types <span class="cite">std:ivoa-astro-onto</span> (an ontology is a systematic formal |
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description of a set of concepts and their relations with each other), |
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such a formal approach may not be necessary, and may be |
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counterproductive [AG Not sure counterproductive is the right argument here. Ontologies do not meet all of the navigation and retrieval use cases.]. An ontology is necessary if we are to have a |
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computer (appear to) `understand' something of a domain, but in the |
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present case, we are more concerned with the related but distinct |
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problem of letting human users find resources of interest, and so the |
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most appropriate technology derives from the Information Science |
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community, that of <em>controlled vocabularies, taxonomies and |
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thesauri</em>.</p> |
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|
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<p>One of the best examples of the need for a simple vocabulary within |
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the VO is VOEvent <span class="cite">std:voevent</span>, the VO |
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standard for handling astronomical events: if someone broadcasts, or |
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`publishes', the occurrence of an event, the implication is that |
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someone else is going to want to respond to it, but no institution is |
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interested in all possible events, so some standardised information |
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about what the event `is about' is necessary, in a form which |
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ensures that the parties can communicate effectively. If a `burst' is |
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announced, is it a Gamma Ray Burst due to the collapse of a star in a |
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distant galaxy, a solar flare, or the brightening of a stellar or AGN |
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accretion disk? If a publisher doesn't use the label one might have |
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expected, how is one to guess what other equivalent labels might have |
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been used?</p> |
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|
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<p>There have been a number of attempts to create astronomical |
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vocabularies (in the present note we will not need to distinguish |
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vocabularies, taxonomies and thesauri, and will use the term |
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`vocabulary' for all three cases).</p> |
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<ul> |
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<li>The <em>Second Reference Dictionary of the Nomenclature of |
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Celestial Objects</em> <span class="cite">lortet94</span>, <span class="cite">lortet94a</span> contains 500 paper pages of |
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astronomical nomenclature</li> |
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|
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<li>For decades professional journals have used a set of reasonably |
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compatible keywords to help classify the content of whole articles. |
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These keywords have been analysed by Preite Martinez & Lesteven |
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<span class="cite">preitemartinez07</span>, from which they derived a set |
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of common keywords constituting one of the potential bases for a |
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fuller VO vocabulary. The same authors also attempted to derive a set |
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of common concepts by analyzing the contents of abstracts in journal |
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articles, the list of which should contain more up-to-date |
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tokens/concepts than the old list of journal keywords. A similar but |
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less formal attempt was made by Hessman for the VOEvent working group, |
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resulting in a similar list <span class="todo">Find Hessman05 |
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reference, and check differences from the A&A list</span>.</li> |
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|
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<li>Astronomical databases generally use simple sets of keywords – |
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sometimes hierarchically organized – to aid the users in the querying |
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of the databases. Two examples from totally different contexts are the |
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list of object types used in the <a href="http://simbad.u-strasbg.fr">Simbad</a> database and the search keywords used in the educational |
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Hands-On Universe image database portal.</li> |
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|
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<li>The Astronomical Outreach Imagery (AOI) working group has created a simple |
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taxonomy for helping to classify images used for educational or public |
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relations. See <span class='url'>http://ivoa.net/Documents/latest/AOIMetadata.html</span></li> |
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|
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<li>The Hands-On Universe project (see <span class='url' |
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>http://sunra.lbl.gov/telescope2/index.html</span> has maintained a |
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public database of images for use by the general public since the |
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1990s. The images are very heterogeneous, since they are gathered from |
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a variety of professional, semi-professional, amateur, and school |
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observatories, so a simple taxonomy is used to facilitate the browsing |
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by the users of the database.</li> |
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|
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<li>Remote Telescope Markup Language <span |
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class="cite">std:rtml</span>, a document definition for the transfer |
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of observing requests that has been adopted by the Heterogeneous |
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Telescope Network (HTN) Consortium and is indirectly supported by the |
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VOEvent protocol, currently contains several telescope and |
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observation-related taxonomies of terms (e.g. for devices, filters, |
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objects).<span class='todo'>Confirm status: does this need to be |
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converted to SKOS? [AG]. Possibly: chase with Rick? [NG]</span></li> |
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|
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<li>In 1993, Shobbrook and Shobbrook published an Astronomy Thesaurus |
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endoresed by the IAU (see <span class='url' |
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>http://www.aao.gov.au/lib/thesaurus.html</span> <span class='todo' |
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>What's the correct citation for this?</span>. This collection of |
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just short of 3000 terms, in four languages, is a valuable resource, |
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but has been unfortunately little used in recent years. Its very |
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size, which gives it expressive power, is a disadvantage to the extent |
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that it is therefore hard to use.</li> |
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</ul> |
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</div> |
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<div class="section"> |
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<p class="title">Formalising and managing multiple vocabularies</p> |
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<p>We find ourselves in the situation where there are multiple |
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vocabularies in use, describing a broad range of resources of interest |
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to professional and amateur astronomers, and members of the public. |
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These different vocabularies use different terms and different |
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relationships to support the different constituencies they cater for. |
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For example, `delta Sct' and `RR Lyr' are terms one would hope to find |
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in a vocabulary aimed at professional astronomers, associated with the |
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notion of `variable star'; one would hope <em>not</em> to find such |
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technical terms in a vocabulary intended to support outreach |
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activities.</p> |
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|
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<p>One approach to this problem is to create a single consensus |
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vocabulary, which draws terms from the various existing vocabularies |
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to create a new vocabulary which is able to express anything its users |
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might desire. The problem with this is that such an effort would be |
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very expensive: both in terms of time and effort on the part of those |
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creating it, and to the potential users, who have to learn |
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to navigate around it, recognise the new terms, and who have to be |
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supported in using the new terms correctly (or, more often, |
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incorrectly).</p> |
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|
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<p>The alternative approach to the problem is to evade it, and this is |
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the approach taken in this Draft. Rather than deprecating the |
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existence of multiple overlapping vocabularies, we embrace it, |
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formalise all of them, and formally declare the relationships between |
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them. This means that:</p> |
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<ul> |
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<li>The various vocabularies can evolve separately, on their own |
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timescales, managed by the IVOA or by third parties;</li> |
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<li>Users can use the vocabulary most appropriate to their situation, |
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either when annotating resources, or when querying them;</li> |
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<li>We retain the investments made in vocabularies by users and |
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resource owners.</li> |
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</ul> |
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<p>To this end we present in this Draft formalised versions of a |
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number of existing vocabularies, encoded as SKOS vocabularies <span class="cite">std:skoscore</span>.</p> |
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</div> |
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</div> |
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<div class="section"> |
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<p class="title">Formalising the Vocabularies</p> |
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|
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<p>After a number of online and face-to-face discussions, the authors |
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brokered a consensus within the IVOA community that the published formats of |
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formalised vocabularies should include at least SKOS (Simple Knowledge |
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Organising Systems), a W3C draft standard application of RDF to the |
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field of knowledge organisation <span |
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class="cite">std:skoscore</span>. SKOS draws on long experience |
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within the Library and Information Science community, to address a |
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well-defined set of problems to do with the indexing and retrieval of |
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information and resources; as such, it is a close match to the problem |
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this working group is addressing.</p> |
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<p>ISO 5964 <span class='cite' >std:iso5964</span> defines a number of |
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the relevant terms (ISO 5964:1985=BS 6723:1985; see also <span |
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class='cite' >std:bs8723-1</span> and <span class='cite' |
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>std:z39.19</span>), and some of the (lightweight) theoretical |
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background. The only technical distinction relevant to this document |
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is that between `vocabulary' and `thesaurus': BS-8723-1 defines a |
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thesaurus as a</p> |
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<blockquote> |
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controlled vocabulary in which concepts are represented by preferred |
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terms, formally organized so that paradigmatic relationships between |
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the concepts are made explicit, and the preferred terms are |
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accompanied by lead-in entries for synonyms or quasi-synonyms. NOTE: |
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The purpose of a thesaurus is to guide both the indexer and the |
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searcher to select the same preferred term or combination of preferred |
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terms to represent a given subject. (BS-8723-1, sect. 2.39) |
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</blockquote> |
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<p>with a similar definition in ISO-5964 sect. 3.16. The paradigmatic |
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relationships in question are those relating a term to a `broader', |
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`narrower' or more generically `related' term, with an operational |
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definition of `broader term' which is such that a resource retrieved |
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by a given term will also be retrieved by that term's `broader term'. |
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This is not a subsumption relationship, as there is no implication |
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that the concept referred to by a narrower term is of the same |
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<em>type</em> as a broader term.</p> |
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<p>Thus a vocabulary (SKOS or otherwise) is not an ontology. It has |
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lighter and looser semantics than an ontology, and is specialised for |
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the restricted case of resource retrieval.</p> |
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<p><span class='todo' >What is to be the format of the `master' files? |
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SKOS or mildly-formatted plain text?</span></p> |
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<div class="section"> |
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<p class="title">SKOS files (normative)</p> |
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|
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<p>We provide a set of SKOS files representing the vocabularies which |
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have been developed.</p> |
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<p>To come: one SKOS file per vocabulary, defining the list of |
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concepts; at least one file per vocabulary, giving mappings to other |
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vocabularies; possibly translations. See Makefile in ../Vocabularies, |
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which produces a tarball, at present without mappings or translations.</p> |
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</div> |
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</div> |
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<div class="appendices"> |
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<div class="section-nonum" id="bibliography"> |
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<p class="title">Bibliography</p> |
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