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9 <head>
10 <title>Vocabularies in the Virtual Observatory</title>
11 <link rev="made" href="http://nxg.me.uk/norman/#norman" title="Norman Gray"/>
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13 <meta name="DC.subject" content="IVOA, Virtual Observatory, Vocabulary"/>
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38 <body>
39 <div class="head">
40 <a href="http://www.ivoa.net/"><img alt="IVOA" src="http://www.ivoa.net/pub/images/IVOA_wb_300.jpg" width="300" height="169"/></a>
41
42 <h1>Vocabularies in the Virtual Observatory<br/>Version @VERSION@</h1>
43 <h2>IVOA Working Draft, @RELEASEDATE@ [Editors' draft, $Revision$]</h2>
44 <!-- $Revision$ $Date$ -->
45
46 <dl>
47
48 <dt>This version</dt>
49 <dd><span class='url'>http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics</span><br/>
50 <span class='url'>@BASEURI@/</span></dd>
51
52 <dt>Latest version</dt>
53 <dd><span class='url'>http://www.ivoa.net/Document/latest/Vocabularies.html</span><br/>
54 and <a href='@ISSUESLIST@' >issues list</a></dd>
55
56 <dt>Working Group</dt>
57 <dd><em><a href="http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics">Semantics</a></em></dd>
58
59 <dt>Editors</dt>
60 <dd>Alasdair J G Gray,
61 <a href='http://nxg.me.uk/norman#norman' >Norman Gray</a>,
62 Frederic V Hessman and
63 Andrea Preite Martinez</dd>
64
65 <dt>Authors</dt>
66 <dd>
67 <span property="dc:creator">Sébastien Derriere</span>,
68 <span property="dc:creator">Alasdair J G Gray</span>,
69 <span property="dc:creator">Norman Gray</span>,
70 <span property="dc:creator">Frederic V Hessman</span>,
71 <span property="dc:creator">Tony Linde</span>,
72 <span property="dc:creator">Andrea Preite Martinez</span>,
73 <span property="dc:creator">Rob Seaman</span> and
74 <span property="dc:creator">Brian Thomas</span>
75 </dd>
76 </dl>
77 <hr/>
78 </div>
79
80 <div class="section-nonum" id="abstract">
81 <p class="title">Abstract</p>
82
83 <div class="abstract">
84 <p>As the astronomical information processed within the <em>Virtual Observatory
85 </em> becomes more complex, there is an increasing need for a more
86 formal means of identifying quantities, concepts, and processes not
87 confined to things easily placed in a FITS image, or expressed in a
88 catalogue or a table. We proposed that the IVOA adopt a standard
89 format for vocabularies based on the W3C's <em>Resource Description
90 Framework</em> (RDF) and <em>Simple Knowledge Organisation System</em>
91 (SKOS). By adopting a standard and simple format, the IVOA will
92 permit different groups to create and maintain their own specialised
93 vocabularies while letting the rest of the astronomical community
94 access, use, and combine them. The use of current, open standards
95 ensures that VO applications will be able to tap into resources of the
96 growing semantic web. Several examples of useful astronomical
97 vocabularies are provided, including work on a common IVOA thesaurus
98 intended to provide a semantic common base for VO applications.</p>
99 </div>
100
101 </div>
102
103 <div class="section-nonum" id="status">
104 <p class="title">Status of this document</p>
105
106 <p>This is an IVOA Working
107 Draft. The first release of this document was
108 <span property="dc:date">2008 February 22</span>.</p>
109
110 <p>This document is an IVOA Working Draft for review by IVOA members
111 and other interested parties. It is a draft document and may be
112 updated, replaced, or obsoleted by other documents at any time. It is
113 inappropriate to use IVOA Working Drafts as reference materials or to
114 cite them as other than <q>work in progress</q>.</p>
115
116 <p>A list of current IVOA Recommendations and other technical
117 documents can be found at
118 <span class='url' >http://www.ivoa.net/Documents/</span>.</p>
119
120 <h3>Acknowledgments</h3>
121
122 <p>We would like to thank the members of the IVOA semantic working
123 group for many interesting ideas and fruitful discussions.</p>
124 </div>
125
126 <h2><a id="contents" name="contents">Table of Contents</a></h2>
127 <?toc?>
128
129 <hr/>
130
131 <div class="section" id="introduction">
132 <p class="title">Introduction (informative)</p>
133
134 <div class="section" id='astrovocab'>
135 <p class="title">Vocabularies in astronomy</p>
136
137 <p>Astronomical information of relevance to the Virtual Observatory
138 (VO) is not confined to quantities easily expressed in a catalogue or
139 a table.
140 Fairly simple things such as position on the sky, brightness in some
141 units, times measured in some frame, redshifts, classifications or
142 other similar quantities are easily manipulated and stored in VOTables
143 and can currently be identified using IVOA Unified Content Descriptors
144 (UCDs) <span class="cite">std:ucd</span>.
145 However, astrophysical concepts and quantities use a wide variety of
146 names, identifications, classifications and associations, most of
147 which cannot be described or labelled via UCDs.</p>
148
149 <p>There are a number of basic forms of organised semantic knowledge
150 of potential use to the VO, ranging from informal <q>folksonomies</q>
151 (where users are free to choose their own labels) at one extreme, to
152 formally structured <q>vocabularies</q> (where the label is drawn from
153 a predefined set of definitions, and which can include relationships between
154 labels) and <q>ontologies</q> (where the domain is captured in a
155 formal data model) at the other.
156 More formal definitions are presented later in this document.
157 </p>
158
159 <p>An astronomical ontology is necessary if we are to have a computer
160 (appear to) <q>understand</q> something of the domain.
161 There has been some progress towards creating an ontology of
162 astronomical object types <span
163 class="cite">std:ivoa-astro-onto</span> to meet this need.
164 However there are distinct use cases for letting human users find
165 resources of interest through search and navigation of the information space.
166 The most appropriate technology to meet these use cases derives from
167 the Information Science community, that of <em>controlled
168 vocabularies, taxonomies and thesauri</em>.
169 In the present document, we do not distinguish between controlled
170 vocabularies, taxonomies and thesauri, and use the term
171 <em>vocabulary</em> to represent all three.
172 </p>
173
174 <p>One of the best examples of the need for a simple vocabulary within
175 the VO is VOEvent <span class="cite">std:voevent</span>, the VO
176 standard for supporting rapid notification of astronomical events.
177 This standard requires some formalised indication of what a published
178 event is <q>about</q>, in a formalism which can be used straightforwardly
179 by the developer of relevant services. See <span class='xref'
180 >usecases</span> for further discussion.</p>
181
182 <p>A number of astronomical vocabularies have been created, with a
183 variety of goals and intended uses. Some examples are detailed below. </p>
184
185 <ul>
186
187 <li>The <em>Second Reference Dictionary of the Nomenclature of
188 Celestial Objects</em> <span class="cite">lortet94</span>, <span
189 class="cite">lortet94a</span> contains 500 paper pages of astronomical
190 nomenclature</li>
191
192 <li>For decades professional journals have used a set of reasonably
193 compatible keywords to help classify the content of whole articles.
194 These keywords have been analysed by Preite Martinez &amp; Lesteven
195 <span class="cite">preitemartinez07</span>, who derived a
196 set of common keywords constituting one of the potential bases for a
197 fuller VO vocabulary. The same authors also attempted to derive a set
198 of common concepts by analysing the contents of abstracts in journal
199 articles, which should comprise a list of tokens/concepts more
200 up-to-date than the old list of journal keywords. A similar but less
201 formal attempt was made by Hessman <span class='cite'>hessman05</span>
202 for the VOEvent working group, resulting in a similar list.</li>
203
204 <li>Astronomical databases generally use simple sets of keywords
205 – sometimes hierarchically organised – to help users make queries.
206 Two examples from very
207 different contexts are the list of object types used in the <a
208 href="http://simbad.u-strasbg.fr">Simbad</a> database and the search
209 keywords used in the educational Hands-On Universe image database
210 portal.</li>
211
212 <li>The Astronomical Outreach Imagery (AOI) working group has created
213 a simple taxonomy for helping to classify images used for educational
214 or public relations <span class="cite">std:aoim</span>. See
215 <span class='xref'>vocab-aoim</span>.</li>
216
217 <!--
218 <li>The Hands-On Universe project (see <span class='url'
219 >http://sunra.lbl.gov/telescope2/index.html</span>) has maintained a
220 public database of images for use by the general public since the
221 1990s. The images are very heterogeneous, since they are gathered from
222 a variety of professional, semi-professional, amateur, and school
223 observatories, so a simple taxonomy is used to facilitate browsing
224 by the users of the database.</li>
225 -->
226
227 <li>In 1993, Shobbrook and Shobbrook published an Astronomy Thesaurus
228 endorsed by the IAU <span class='cite' >shobbrook92</span>. This
229 collection of nearly 3000 terms, in five languages, is a valuable
230 resource, but has seen little use in recent years. Its very size,
231 which gives it expressive power, is a disadvantage to the extent that
232 it is consequently hard to use. See <span class='xref'>vocab-iau93</span>.</li>
233
234 <li>The VO's Unified Content Descriptors <span class='cite'
235 >std:ucd</span> (UCD) constitute the main controlled vocabulary of the
236 IVOA and contain some taxonomic information. However, UCD has some
237 features which supports its goals, but which make it difficult to use
238 beyond the present applications of labelling VOTables: firstly, there
239 is no standard means of identifying and processing the contents of the
240 text-based reference document; secondly, the content cannot be openly
241 extended beyond that set by a formal IVOA committee without going
242 through a laborious and time-consuming negotiation process of
243 extending the primary vocabulary itself; and thirdly, the UCD
244 vocabulary is primarily concerned with data types and their
245 processing, and only peripherally with astronomical objects (for
246 example, it defines formal labels for RA, flux, and bandpass, but does
247 not mention the Sun). See <span class='xref'>vocab-ucd1</span>.</li>
248
249 </ul>
250 </div>
251
252 <div class='section' id='usecases'>
253 <p class='title'>Use-cases, and the motivation for formalised vocabularies</p>
254
255 <p>The most immediate high-level motivation for this work is the
256 requirement of the VOEvent standard <span class='cite'
257 >std:voevent</span> for a controlled vocabulary usable in the
258 VOEvent's <code>&lt;Why/&gt;</code> and <code>&lt;What/&gt;</code>
259 elements, which describe what
260 sort of object the VOEvent packet is describing, in some broadly
261 intelligible way. For example a <q>burst</q> might be a gamma-ray burst
262 due to the collapse of a star in a distant galaxy, a solar flare, or
263 the brightening of a stellar or AGN accretion disk, and having an
264 explicit list of vocabulary terms can help guide the event publisher
265 into using a term which will be usefully precise for the event's
266 consumers. A free-text label can help here (which brings us into the
267 domain sometimes referred to as folksonomies), but the astronomical
268 community, with a culture sympathetic to international agreement, can
269 do better.</p>
270
271 <p>The purpose of this proposal is to establish a set of conventions for
272 the creation, publication, use, and manipulation of
273 astronomical vocabularies within the Virtual Observatory, based upon
274 the W3C's SKOS standard. We include as appendices to this proposal
275 formalised versions of a number of existing vocabularies, encoded as
276 SKOS vocabularies <span class="cite">std:skosref</span>.</p>
277
278 <p>Specific use-cases include the following.</p>
279 <ul>
280 <li>A user wishes to process all events concerning supernovae, which
281 means that an event concerning a type 1a supernova must be understood to be
282 relevant. [This supports a system working autonomously, filtering
283 incoming information]</li>
284
285 <li>A user is searching an archive of VOEvents for microlensing
286 events, and retrieves a large number of them; the search interface may
287 then prompt her to narrow her search using one of a set of terms
288 including, say, binary lens events. [This supports so-called <q>semantic
289 search</q>, providing semantic support to an interface which is in turn
290 supporting a user]</li>
291
292 <li>A user wishes to search for resources based on the
293 journal-supported keywords in a paper; they might either initiate this by
294 hand, or have this done on their behalf by a tool which can extract
295 the keywords from a PDF. The keywords are in the A&amp;A vocabulary,
296 and mappings have been defined between this vocabulary and others,
297 which means that the query keywords are translated automatically
298 into those appropriate for a search of an outreach image database
299 (everyone likes pretty pictures), the VO Registry, a set of Simbad
300 object types, and one or more concepts in more formal ontologies. The
301 search interface is then able to support the user browsing up and down
302 the AOIM vocabulary, and a specialised Simbad tool is able to take
303 over the search, now it has an appropriate starting place. [This
304 supports interoperability, building on the investments which
305 institutions and users have made in existing vocabularies]</li>
306
307 <li>A user receives a VOTable of results from a VO application – for
308 example a catalogue of objects or observations – and wants to search a
309 database of old FITS files for potential matches. Because the UCDs
310 labeling the columns of the tables are expressed in well-documented
311 SKOS, both the official descriptions of the UCDs and their semantic
312 matches to a variety of other plain-text vocabularies (such as the IAU
313 or AOIM thesauri) are available to the VO application, providing a basis
314 for massive searches for all kinds of FITS keyword values.</li>
315
316 </ul>
317
318 <p>It is not a goal of this standard, as it is not a goal of SKOS, to
319 produce knowledge-engineering artefacts which can support elaborate
320 machine reasoning – such artefacts would be very valuable, but require
321 much more expensive work on ontologies. As the supernova use-case
322 above illustrates, even simple vocabularies can support useful machine
323 reasoning.</p>
324
325 <p>It is also not a goal of this standard to produce new vocabularies,
326 or substantially alter existing ones; instead, the vocabularies
327 included below in section <span class='xref'>distvocab</span> are directly
328 derived from existing vocabularies (the exceptions are the IVOAT
329 vocabulary, which is ultimately intended to be a significant update to
330 the IAU-93 original, and the constellations vocabulary, which is
331 intended to be purely didactic). It therefore follows that the ambiguities,
332 redundancies and incompleteness of the source vocabularies are
333 faithfully represented in the distributed SKOS vocabularies. We hope
334 that this formalisation process will create greater visibility and
335 broader use for the various vocabularies, and that this will guide the
336 maintenance efforts of the curating groups.</p>
337
338 <p>The reason for both of these limitations is that vocabularies are
339 extremely expensive to produce, maintain and deploy, and we must
340 therefore rely on such vocabularies as have been developed, and
341 attached as metadata to resources, by others. Such vocabularies are
342 less rich or less coherent than we might prefer, but widely enough
343 deployed to be useful.</p>
344
345 <p>The purpose of this document is not to produce new vocabularies but
346 to show how vocabularies can be easily expressed in a internationally
347 standardized, computer-manipulable format. Four example vocabularies
348 that have previously been expressed using non-standardized formats –
349 namely the A&amp;A keyword list, the IAU and AOIM thesauri, and UCD1 –
350 are included below as an illustration of how simple it is to publish
351 them in SKOS. In all cases, one can easily express all of the
352 information of the original source vocabularies. It is true, that
353 complex vocabularies can be expensive to produce, maintain, and
354 deploy, and it will take a while until VO applications will be able to
355 use them to full advantage. However, the example vocabularies should
356 make it fairly easy to boot-strap the process by providing useful
357 vocabularies that can be used out of the box.</p>
358
359 </div>
360
361 <div class="section" id='formalising'>
362 <p class="title">Formalising and managing multiple vocabularies</p>
363
364 <p>We find ourselves in the situation where there are multiple
365 vocabularies in use, describing a broad range of resources of interest
366 to professional and amateur astronomers, and members of the public.
367 These different vocabularies use different terms and different
368 relationships to support the different constituencies they cater for.
369 For example, <q>delta Sct</q> and <q>RR Lyr</q> are terms one would
370 find in a vocabulary aimed at professional astronomers, associated
371 with the notion of <q>variable star</q>; however one would
372 <em>not</em> find such technical terms in a vocabulary intended to
373 support outreach activities.</p>
374
375 <p>One approach to this problem is to create a single consensus
376 vocabulary, which draws terms from the various existing vocabularies
377 to create a new vocabulary which is able to express anything its users
378 might desire. The problem with this is that such an effort would be
379 very expensive, both in terms of time and effort on the part of those
380 creating it, and to the potential users, who have to learn
381 to navigate around it, recognise the new terms, and who have to be
382 supported in using the new terms correctly (or, more often,
383 incorrectly).</p>
384
385 <p>The alternative approach to the problem is to evade it, and this is
386 the approach taken in this document. Rather than deprecating the
387 existence of multiple overlapping vocabularies, we embrace it,
388 help interest groups formalise as many of them as are appropriate, and
389 standardise the process of formally declaring the relationships between
390 them. This means that:</p>
391 <ul>
392 <li>The various vocabularies are allowed to evolve separately, on
393 their own timescales, managed either by the IVOA, individual working
394 groups within the IVOA, or by third parties;</li>
395
396 <li>Specialised vocabularies can be developed and maintained by the
397 community with the most knowledge about a specific topic, ensuring
398 that the vocabulary will have the most appropriate breadth, depth, and
399 precision;</li>
400
401 <li>Users can choose the vocabulary or combination of vocabularies most
402 appropriate to their situation, either when annotating resources, or
403 when querying them; and</li>
404
405 <li>We can retain the previous investments made in vocabularies by
406 users and resource owners.</li>
407
408 </ul>
409
410
411 </div>
412
413 </div>
414
415 <div class='section' id='skos'>
416 <p class='title'>SKOS-based vocabularies (informative)</p>
417
418 <p>In this section, we introduce the concepts of SKOS-based
419 vocabularies, and the technology of mapping between them. We describe
420 some additional requirements for IVOA vocabularies in the next
421 section, <span class='xref' >publishing</span>.</p>
422
423 <div class="section" id='vocab'>
424 <p class="title">Selection of the vocabulary format</p>
425
426 <p>After extensive online and face-to-face discussions, the authors have
427 brokered a consensus within the IVOA community that
428 formalised vocabularies should be published at least in SKOS (Simple Knowledge
429 Organisation System) format, a W3C draft standard application of RDF to the
430 field of knowledge organisation <span
431 class="cite">std:skosref</span>. SKOS draws on long experience
432 within the Library and Information Science community, to address a
433 well-defined set of problems to do with the indexing and retrieval of
434 information and resources; as such, it is a close match to the problem
435 this document is addressing.</p>
436
437 <p>ISO 5964 <span class='cite' >std:iso5964</span> defines a number of
438 the relevant terms (ISO 5964:1985=BS 6723:1985; see also <span
439 class='cite' >std:bs8723-1</span> and <span class='cite'
440 >std:z39.19</span>), and some of the (lightweight) theoretical
441 background. The only technical distinction relevant to this document
442 is that between vocabulary and thesaurus: BS-8723-1 defines a
443 thesaurus as a</p>
444 <blockquote>
445 Controlled vocabulary in which concepts are represented by preferred
446 terms, formally organized so that paradigmatic relationships between
447 the concepts are made explicit, and the preferred terms are
448 accompanied by lead-in entries for synonyms or quasi-synonyms.
449 <!-- NOTE:
450 The purpose of a thesaurus is to guide both the indexer and the
451 searcher to select the same preferred term or combination of preferred
452 terms to represent a given subject. -->
453 (BS-8723-1, sect. 2.39)
454 </blockquote>
455 <p>with a similar definition in ISO-5964 sect. 3.16. The paradigmatic
456 relationships in question are those relating a term to a <q>broader</q>,
457 <q>narrower</q> or more generically <q>related</q> term. These
458 notions have an operational definition: any resource
459 retrieved as a result of a search on a given term will also be
460 retrievable through a search on that term's <q>broader term</q>
461 (<q>narrower</q> is a simple inverse, so that for any pair of terms,
462 if <code>A skos:broader B</code>, then <code>B skos:narrower A</code>;
463 a term may have multiple narrower and broader terms).
464 This is not a subsumption relationship, as there is no implication
465 that the concept referred to by a narrower term is of the same
466 <em>type</em> as a broader term.</p>
467
468 <p>Thus <strong>a vocabulary (SKOS or otherwise) is not an
469 ontology</strong>. It has lighter and looser semantics than an
470 ontology, and is specialised for the restricted case of resource
471 retrieval. Those interested in ontological analyses can easily
472 transfer the vocabulary relationship information from SKOS to a formal
473 ontological format such as OWL <span class='cite' >std:owl</span>.</p>
474
475 <p>The purpose of a thesaurus is to help users find resources they
476 might be interested in, be they library books, image archives, or VOEvent
477 packets.</p>
478
479 </div>
480
481 <div class='section' id='skos-format'>
482 <p class='title'>Content and format of a SKOS vocabulary</p>
483
484 <p>A published vocabulary in SKOS format consists of a set of
485 <q>concepts</q> – an example concept capturing the
486 vocabulary information about spiral galaxies is provided in the <a
487 href='#figexample' >Figure below</a>, with the RDF shown in both
488 RDF/XML <span class='cite' >std:rdfxml</span> and Turtle notation <span
489 class='cite' >std:turtle</span> (Turtle is similar to the more
490 informal N3 notation). The elements of a concept are detailed
491 below.</p>
492
493 <center>
494 <p><a name='figexample' >Figure: examples of SKOS vocabularies</a></p>
495 <table>
496 <tr>
497 <th bgcolor="#eecccc">XML Syntax</th>
498 <th width="10"/>
499 <th bgcolor="#cceecc">Turtle Syntax</th>
500 </tr>
501 <tr><td/></tr>
502 <tr>
503 <td bgcolor="#eecccc">
504 <pre>
505 &lt;skos:Concept rdf:about="#spiralGalaxy"&gt;
506 &lt;skos:prefLabel lang="en"&gt;
507 spiral galaxy
508 &lt;/prefLabel&gt;
509 &lt;skos:prefLabel lang="de"&gt;
510 Spiralgalaxie
511 &lt;/prefLabel&gt;
512 &lt;skos:altLabel lang="en"&gt;
513 spiral nebula
514 &lt;/skos:altLabel&gt;
515 &lt;skos:hiddenLabel lang="en"&gt;
516 spiral glaxy
517 &lt;/hiddenLabel&gt;
518 &lt;skos:definition lang="en"&gt;
519 A galaxy having a spiral structure.
520 &lt;/skos:definition&gt;
521 &lt;skos:scopeNote lang="en"&gt;
522 Spiral galaxies fall into one of
523 three catagories: Sa, Sc, and Sd.
524 &lt;/skos:scopeNote&gt;
525 &lt;skos:narrower
526 rdf:resource="#barredSpiralGalaxy"/&gt;
527 &lt;skos:broader
528 rdf:resource="#galaxy"/&gt;
529 &lt;skos:related
530 rdf:resource="#spiralArm"/&gt;
531 &lt;/skos:Concept&gt;
532 </pre>
533 </td>
534 <td/>
535 <td bgcolor="#cceecc">
536 <pre>
537 &lt;#spiralGalaxy&gt; a skos:Concept;
538 skos:prefLabel
539 "spiral galaxy"@en,
540 "Spiralgalaxie"@de;
541 skos:altLabel "spiral nebula"@en;
542 skos:hiddenLabel "spiral glaxy"@en;
543 skos:definition """A galaxy having a
544 spiral structure."""@en;
545 skos:scopeNote """Spiral galaxies fall
546 into one of three categories:
547 Sa, Sc, and Sd"""@en;
548 skos:narrower &lt;#barredSpiralGalaxy&gt;;
549 skos:broader &lt;#galaxy&gt;;
550 skos:related &lt;#spiralArm&gt; .
551 </pre>
552 </td>
553 </tr>
554 </table>
555 </center>
556
557 <p>A SKOS vocabulary includes the following features.</p>
558
559 <ul>
560
561 <li>A single URI representing the concept, mainly for use by computers.
562 <!--
563 <code>&lt;#spiralGalaxy&gt; a skos:Concept</code>.
564 <code>&lt;skos:Concept rdf:about="#spiralGalaxy"&gt;</code>
565 -->
566 </li>
567
568 <li>A single prefered label in each supported language of the
569 vocabulary, for use by humans.
570 <!--
571 <code>skos:prefLabel "spiral galaxy"@en, "Spiralgalaxie"@de</code>.
572 <code>&lt;skos:prefLabel&gt;spiral galaxy&lt;/skos:prefLabel&gt;</code>
573 -->
574 </li>
575
576 <li>Optional alternative labels which applications may encounter or in
577 common use, whether simple synonyms or commonly-used aliases such as
578 <q>GRB</q> for "gamma-ray burst", or <q>Spiral nebula</q> for
579 spiral galaxies.
580 <!--
581 <code>skos:altLabel "GRB"@en</code>
582 <code>&lt;skos:altLabel lang="de"&gt;Spiralgalaxie&lt;/skos:altLabel&gt;</code>
583 -->
584 </li>
585
586 <li>Optional hidden labels which capture terms which are sometimes
587 used for the corresponding concept, but which are deprecated in some
588 sense. This might include common misspellings for
589 either the preferred or alternate labels, for example <q>glaxy</q> for
590 <q>galaxy</q>.
591 </li>
592
593 <li>A definition for the concept, where one exists in the original
594 vocabulary, to clarify the meaning of the term.
595 <!--
596 <code>skos:definition "A galaxy having a spiral structure."@en</code>
597 <code>&lt;skos:definition lang="en"&gt;<br/>A galaxy having a spiral structure.<br/>&lt;/skos:definition&gt;</code>
598 -->
599 </li>
600
601 <li>A scope note to further clarify a definition, or the usage of the
602 concept.
603 <!--
604 <code>skos:scopeNote "Spiral galaxies fall into one of three categories: Sa, Sc, and Sd"@en</code>
605 <code>&lt;skos:scopeNote lang="en"&gt;<br/>Spiral galaxies fall into one of three catagories: Sa, Sc, and Sd.<br/>&lt;/skos:scopeNote&gt;</code>
606 -->
607 </li>
608
609 <li>Optionally, a concept may be involved in any number of relationships
610 to other concepts. The types of relationships are
611 <ul>
612 <li>Narrower or more specific concepts, for example a link to the concept
613 representing a <q>barred spiral galaxy</q>.
614 <!--
615 <code>skos:narrower &lt;#barredSpiralGalaxy&gt;</code>.
616 <code>&lt;skos:narrower rdf:resource="#barredSpiralGalaxy"&gt;</code>
617 -->
618 </li>
619 <li>Broader or more general concepts, for example a link to the token
620 representing galaxies in general.
621 <!--
622 <code>skos:broader &lt;#galaxy&gt;</code>.
623 <code>&lt;skos:broader rdf:resource="#galaxy"&gt;</code>
624 -->
625 </li>
626 <li>Related concepts, for example a link to the token representing spiral
627 arms of galaxies
628 <!--
629 <code>skos:related &lt;#spiralArm&gt;</code>
630 <code>&lt;skos:related rdf:resource="#spiralArm"&gt;</code>
631 -->
632 <br/>
633 (note this relationship does not say that spiral galaxies have spiral
634 arms – that would be ontological information of a higher order which
635 is beyond the requirements for information stored in a vocabulary).</li>
636 </ul>
637 </li>
638 </ul>
639
640 <p>In addition to the information about a single concept, a vocabulary
641 can contain information to help users navigate its structure and
642 contents:</p>
643 <ul>
644 <li>The <q>top concepts</q> of the vocabulary, i.e. those that occur
645 at the top of the vocabulary hierarchy defined by the broader/narrower
646 relationships, can be explicitly stated to make it easier to navigate
647 the vocabulary.</li>
648
649 <li>Concepts that form a natural group can be defined as being members
650 of a <q>collection</q>.</li>
651
652 <li>Versioning information can be added using change notes.</li>
653
654 <li>Additional metadata about the vocabulary, for example indicating the publisher, may
655 be documented using the Dublin Core metadata set <span class='cite'
656 >std:dublincore</span>.</li>
657 </ul>
658 </div>
659
660
661 <div class='section' id='skos-relationships'>
662 <p class='title'>Relationships Between Vocabularies</p>
663
664 <p>
665 There already exist several vocabularies in the domain of astronomy.
666 Instead of attempting to replace all these existing vocabularies,
667 which have been developed to achieve different aims and user groups,
668 we embrace them.
669 This requires a mechanism to relate the concepts in the different
670 vocabularies.
671 </p>
672
673 <p>
674 Part of the SKOS standard <span class='cite'>std:skosref</span>
675 allows a concept in one vocabulary to be related to a concept in
676 another vocabulary.
677
678 There are four types of relationship provided to capture the
679 relationships between concepts in vocabularies, which are similar to
680 those defined for relationships between concepts within a single
681 vocabulary.
682 The types of mapping relationships are:
683 </p>
684
685 <ul>
686
687 <li>
688 Equivalence between concepts, i.e. the concepts in the different
689 vocabularies refer to the same real world entity.
690 This is captured with the RDF statement
691 <blockquote>
692 <code>AAkeys:#Cosmology skos:exactMatch aoim:#Cosmology</code>
693 </blockquote>
694 which states that the cosmology concept in the A&amp;A Keywords is the
695 same as the cosmology concept in the AOIM.
696 (Note the use of an external namespaces <code>AAkeys</code> and
697 <code>aoim</code> which must be defined within the document.)
698 </li>
699
700 <li>
701 Broader concept, i.e. there is not an equivalent concept but there is
702 a more general one.
703 This is captured with the RDF statement
704 <blockquote>
705 <code>AAkeys:#Moon skos:broadMatch aoim:PlanetSatellite</code>
706 </blockquote>
707 which states that the AOIM concept Planet Satellite is a more general
708 term than the A&amp;A Keywords concept Moon.
709 </li>
710
711 <li>
712 Narrower concept, i.e. there is not an equivalent concept but there is
713 a more specific one.
714 This is captured with the RDF statement
715 <blockquote>
716 <code>AAkeys:#IsmClouds skos:narrowMatch
717 aoim:#NebulaAppearanceDarkMolecularCloud</code>
718 </blockquote>
719 which states that the AOIM concept Nebula Appearance Dark Molecular
720 Cloud is more specific than the A&amp;A Keywords concept ISM Clouds.
721 </li>
722
723 <li>
724 Related concept, i.e. there is some form of relationship.
725 This is captured with the RDF statement
726 <blockquote>
727 <code>AAkeys:#BlackHolePhysics skos:relatedMatch
728 aoim:#StarEvolutionaryStageBlackHole</code>
729 </blockquote>
730 which states that the A&amp;A Keywords concept Black Hole Physics has
731 an association with the AOIM concept Star Evolutionary Stage Black Hole.
732 </li>
733
734 </ul>
735
736 <p>The semantic mapping relationships have certain properties.
737 The broadMatch relationship has the narrowMatch relationship as its
738 inverse and the exactMatch and relatedMatch relationships are
739 symmetrical.
740 The consequence of these properties is that if you have a mapping from
741 concept <code>A</code> in one vocabulary to concept <code>B</code> in
742 another vocabulary then you can infer a mapping from concept
743 <code>B</code> to concept <code>A</code>.
744 </p>
745
746 </div>
747
748 </div>
749
750 <div class='section' id='publishing'>
751 <p class='title'>Publishing vocabularies (normative)</p>
752
753 <div class='section' id='pubreq'>
754 <p class='title'>Requirements</p>
755
756 <p>A vocabulary which conforms to this IVOA standard has the following
757 features. In this section, the keywords
758 <span class='rfc2119' >must</span>,
759 <span class='rfc2119' >should</span>
760 and so on, are to be interpreted as described in <span
761 class='cite'>std:rfc2119</span>.</p>
762
763 <div class='section'>
764 <p class='title'>Dereferenceable namespace</p>
765
766 <p>The namespace of the
767 vocabulary <span class='rfc2119'>must</span> be dereferenceable on the
768 web. That is, typing the namespace URL into a web browser will
769 produce human-readable documentation about the vocabulary. In
770 addition, the namespace URL <span class='rfc2119' >should</span>
771 return the RDF version of the vocabulary if it is retrieved with an
772 HTTP Accept header of <code>application/rdf+xml</code>.</p>
773
774 <p><em>Rationale: These prescriptions are intended to be compatible
775 with the patterns described in <span class='cite'>berrueta08</span>
776 and <span class='cite'>sauermann07</span>, and vocabulary distributors
777 <span class='rfc2119' >should</span> follow these patterns where
778 possible.</em></p>
779 </div>
780
781 <div class='section'>
782 <p class='title'>Long-term availability</p>
783
784 <p>The files defining a
785 vocabulary, including those of superseded versions, should remain
786 permanently available. There is no requirement that the namespace
787 URL be at any particular location, although the IVOA web pages, or the
788 online sections of the A&amp;A journal would likely be suitable
789 archival locations.</p>
790 </div>
791
792 <div class='section'>
793 <p class='title'>Distribution format</p>
794
795 <p>Vocabularies <span class='rfc2119'>must</span> be made available
796 for distribution as SKOS RDF files, in either RDF/XML <span
797 class='cite'>std:rdfxml</span> or Turtle <span
798 class='cite'>std:turtle</span> format; vocabularies <span
799 class='rfc2119'>should</span> be made available in both formats. See
800 issue <a href='@ISSUESLIST@#distformat-2'>[distformat-2]</a>.</p>
801
802 <p>A publisher <span class='rfc2119'>may</span> make available
803 documentation and supporting files in other formats.</p>
804
805 <p><em>Rationale: this does imply that the vocabulary source files can only
806 realistically be parsed using an RDF parser. An alternative is to
807 require that vocabularies be distributed using a subset of RDF/XML
808 which can also be naively handled as traditional XML; however as well
809 as creating an extra standardisation requirement, this would make it
810 effectively infeasible to write out the distribution version of the
811 vocabulary using an RDF or general SKOS tool.</em></p>
812 </div>
813
814 <div class='section'>
815 <p class='title'>Clearly versioned vocabulary</p>
816
817 <p><span class='todo' >Open issue. There are interactions with the requirements for
818 long-term availability and a dereferenceable namespace, since this
819 implies that the vocabulary version should be manifestly encoded in
820 the namespace URI.</span> See issue <a
821 href='@ISSUESLIST@#versioning-3' >[versioning-3]</a>.</p>
822
823 </div>
824
825 <div class='section'>
826 <p class='title'>No restrictions on source files</p>
827
828 <p>This standard does not place any restrictions on the format of the
829 files managed by the maintenance process, as long as the distributed
830 files are as specified above. See issue
831 <a href='@ISSUESLIST@#masterformat-1' >[masterformat-1]</a>.</p>
832 </div>
833
834 </div>
835
836 <div class='section' id='practices'>
837 <p class='title'>Suggested good practices</p>
838
839 <p>This standard imposes a number of requirements on conformant
840 vocabularies (see <span class='xref' >publishing</span>). In
841 this section we list a number of good practices that IVOA vocabularies
842 <span class='rfc2119'>should</span> abide by. Some of the
843 prescriptions below are more specific than good-practice guidelines
844 for vocabularies in general.</p>
845
846 <p>The adoption of the following guidelines will make it easier to use
847 vocabularies in generic VO applications. However, VO applications
848 <span class='rfc2119'>should</span> be able to accept any vocabulary
849 that complies with the latest SKOS standard
850 <span class="cite">std:skosref</span> (this does not imply, of
851 course, that an application will necessarily understand the terms in
852 an alien vocabulary, although the presence of mappings to a known
853 vocabulary should allow it to derive some benefit).</p>
854
855 <ol>
856
857 <li>Concept identifiers <span class='rfc2119'>should</span> consist
858 only of the letters a-z, A-Z, and numbers 0-9, i.e. no spaces, no
859 exotic letters (for example umlauts), and no characters which would make a
860 token inexpressible as part of a URI; since tokens are for use by
861 computers only, this is not a big restriction, since the exotic
862 letters can be used within the labels and documentation if
863 appropriate.</li>
864
865 <li>The concept identifiers <span class='rfc2119'>should</span> be
866 kept in human-readable form, directly reflect the implied meaning, and
867 not be semi-random identifiers only (for example, use
868 <code>spiralGalaxy</code>, not <code>t1234567</code>); tokens <span
869 class='rfc2119'>should</span> preferably be created via a direct
870 conversion from the preferred label via removable/translation of
871 non-token characters (see above) and sub-token separation via
872 capitalisation of the first sub-token character (for example the label <code>My
873 favourite idea-label #42</code> is converted into
874 <code>MyFavouriteIdeaLabel42</code>).</li>
875
876 <li>Labels <span class='rfc2119'>should</span> be in the form of the source vocabulary. When
877 developing a new vocabulary the singular form <span
878 class='rfc2119'>should</span> be preferred,
879 for example <code>spiral galaxy</code>, not <code>spiral galaxies</code>.
880 <span class='todo'><a href="http://code.google.com/p/volute/issues/detail?id=1">Open issue</a></span></li>
881
882 <li>Each concept <span class='rfc2119'>should</span> have a definition
883 (<code>skos:definition</code>) that constitutes a short description of
884 the concept which could be adopted by an application using the
885 vocabulary. Each concept <span class='rfc2119'>should</span> have
886 additional documentation using SKOS Notes or
887 Dublin Core terms as appropriate
888 (see <span class='cite'>std:skosref</span>)</li>
889
890 <li>The language localisation <span class='rfc2119'>should</span> be
891 declared where appropriate, in preferred labels, alternate labels,
892 definitions, and the like.</li>
893
894 <li>Relationships (<q>broader</q>, <q>narrower</q>, <q>related</q>)
895 between concepts <span class='rfc2119'>should</span> be present, but
896 are not required; if used, they <span class='rfc2119'>should</span> be
897 complete (thus all <q>broader</q> links have corresponding
898 <q>narrower</q> links in the referenced entries and <q>related</q>
899 entries link each other).</li>
900
901 <li><q>TopConcept</q> entries (see above) <span
902 class='rfc2119'>should</span> be declared and normally consist of
903 those concepts that do not have any <q>broader</q> relationships
904 (i.e. not at a sub-ordinate position in the hierarchy).</li>
905
906 <li>The SKOS standard describes some good practices for vocabulary
907 maintenance, such as using <code>&lt;skos:changeNote&gt;</code> and
908 the like. Publishers <span class='rfc2119'>should</span> respect such
909 good maintenance practices are are available.</li>
910
911 <li>Publishers <span class='rfc2119'>should</span> publish
912 <q>mappings</q> between their vocabularies and other commonly used
913 vocabularies. These <span class='rfc2119'>should</span> be external to
914 the defining vocabulary document so that the vocabulary can be used
915 independently of the publisher's mappings. <span class='todo' ><a
916 href='http://code.google.com/p/volute/issues/detail?id=8' >Open
917 issue</a></span>.</li>
918 </ol>
919
920 <!--
921 <p>These suggestions are by no means trivial – there was
922 considerable discussion within the semantic working group on many of
923 these topics, particularly about token formats (some wanted lower-case
924 only), and singular versus plural forms of the labels (different
925 traditions exist within the international library science
926 community). Obviously, no publisher of an astronomical vocabulary has
927 to adopt these rules, but the adoption of these rules will make it
928 easier to use the vocabularly in external generic VO
929 applications. However, VO applications should be developed to accept
930 any vocabulary that complies with the latest SKOS standard <span
931 class="cite">std:skosref</span>.</p>
932 -->
933 </div>
934
935 </div>
936
937
938 <div class="section" id='distvocab'>
939 <p class="title">Example vocabularies</p>
940
941 <p>The intent of having the IVOA adopt SKOS as the preferred format for
942 astronomical vocabularies is to encourage the creation and management
943 of diverse vocabularies by competent astronomical groups, so that
944 users of the VO and related resources can benefit directly and
945 dynamically without the intervention of the IAU or IVOA. However, we
946 felt it important to provide several examples of vocabularies in the
947 SKOS format as part of the proposal, to illustrate their simplicity
948 and power, and to provide an immediate vocabulary basis for VO
949 applications.</p>
950
951 <p>The vocabularies described below are included, as SKOS files, in
952 the distributed version of this standard. These vocabularies have
953 stable URLs <span class='todo' >Format TBD, see <a
954 href='@ISSUESLIST@#versioning-3' >[versioning-3]</a></span>, and
955 may be cited and used indefinitely. These vocabularies will not,
956 however, be developed as part of the maintenance of this standard.
957 Interested groups, within and outwith the IVOA, are encouraged to take
958 these as a starting point and absorb them within existing
959 processes.</p>
960
961 <p>The exceptions to this rule are the constellation vocabulary,
962 provided here mainly for didactic purposes, and the proposed IVOA
963 Thesaurus, which is being developed as a separate project and whose
964 aim is to provide a corrected, more user-friendly, more complete, and
965 updated version of the 1993 IAU thesaurus. Although work on the IVOA
966 Thesaurus is on-going, the fact that it is largely based on the IAU
967 thesaurus means that it is already a very useful resource, so a usable
968 snapshot of this vocabulary will be published with the other
969 examples.</p>
970
971 <p>We provide a set of SKOS files representing the vocabularies which
972 have been developed, and mappings between them. These can be
973 downloaded at the URL</p>
974 <blockquote>
975 <span class='url'>@BASEURI@/@DISTNAME@.tar.gz</span>
976 </blockquote>
977
978 <!--
979 <p class='todo'>Not yet: instead go to
980 <span class='url'>http://code.google.com/p/volute/downloads/list</span></p>-->
981
982 <div class='section' id='vocab-constellation'>
983 <p class='title'>A Constellation Name Vocabulary</p>
984
985 <p>This vocabulary is presented as a simple example of an astronomical vocabulary for a very particular purpose, such as handling constellation information like that commonly encountered in variable star research. For example, <q>SS Cygni</q> is a cataclysmic variable located in the constellation <q>Cygnus</q>. The name of the star uses the genitive form <q>Cygni</q>, but the alternate label <q>SS Cyg</q> uses the standard abbreviation <q>Cyg</q>. Given the constellation vocabulary, all of these forms are recorded together in a computer-manipulatable format. Various incorrect forms should probably be represented in SKOS ‘hidden labels’</p>
986
987 <p>The &lt;skos:ConceptScheme&gt; contains a single &lt;skos:TopConcept&gt;, <q>constellation</q></p>
988 <br/><br/><center>
989 <table>
990 <tr><th bgcolor="#eecccc">XML Syntax</th>
991 <th width="10"/><th bgcolor="#cceecc">Turtle Syntax</th></tr>
992 <tr><td/></tr>
993 <tr>
994 <td bgcolor="#eecccc">
995 <pre>
996 &lt;skos:Concept rdf:about="#constellation"&gt;
997 &lt;skos:inScheme rdf:resource=""/&gt;
998 &lt;skos:prefLabel&gt;
999 constellation
1000 &lt;/skos:prefLabel&gt;
1001 &lt;skos:definition&gt;
1002 IAU-sanctioned constellation names
1003 &lt;/skos:definition&gt;
1004 &lt;skos:narrower rdf:resource="#Andromeda"/&gt;
1005 ...
1006 &lt;skos:narrower rdf:resource="#Vulpecula"/&gt;
1007 &lt;/skos:Concept&gt;
1008 </pre>
1009 </td>
1010 <td/>
1011 <td bgcolor="#cceecc">
1012 <pre>
1013 &lt;#constellation&gt; a :Concept;
1014 :inScheme &lt;&gt;;
1015 :prefLabel "constellation";
1016 :definition "IAU-sanctioned constellation names";
1017 :narrower &lt;#Andromeda&gt;;
1018 ...
1019 :narrower &lt;#Vulpecula&gt;.
1020 </pre>
1021 </td></tr>
1022 </table></center>
1023 <p>and the entry for <q>Cygnus</q> is</p>
1024 <center><table><tr>
1025 <td bgcolor="#eecccc">
1026 <pre>
1027 &lt;skos:Concept rdf:about="#Cygnus"&gt;
1028 &lt;skos:inScheme rdf:resource=""/&gt;
1029 &lt;skos:prefLabel&gt;Cygnus&lt;/skos:prefLabel&gt;
1030 &lt;skos:definition&gt;Cygnus&lt;/skos:definition&gt;
1031 &lt;skos:altLabel&gt;Cygni&lt;/skos:altLabel&gt;
1032 &lt;skos:altLabel&gt;Cyg&lt;/skos:altLabel&gt;
1033 &lt;skos:broader rdf:resource="#constellation"/&gt;
1034 &lt;skos:scopeNote&gt;
1035 Cygnus is nominative form; the alternative
1036 labels are the genitive and short forms
1037 &lt;/skos:scopeNote&gt;
1038 &lt;/skos:Concept&gt;
1039 </pre>
1040 </td>
1041 <td width="10"/>
1042 <td bgcolor="#cceecc">
1043 <pre>
1044 &lt;#Cygnus&gt; a :Concept;
1045 :inScheme &lt;&gt;;
1046 :prefLabel "Cygnus";
1047 :definition "Cygnus";
1048 :altLabel "Cygni";
1049 :altLabel "Cyg";
1050 :broader &lt;#constellation&gt;;
1051 :scopeNote """Cygnus is nominative form;
1052 the alternative labels are the genitive and
1053 short forms""" .
1054 </pre>
1055 </td>
1056 </tr></table></center>
1057
1058 <p>Note that SKOS alone does not permit the distinct differentiation
1059 of genitive forms and abbreviations, but the use of alternate labels
1060 is more than adequate enough for processing by VO applications where
1061 the difference between <q>SS Cygni</q>, <q>SS Cyg</q>, and the incorrect form
1062 <q>SS Cygnus</q> is probably irrelevant.</p>
1063 </div>
1064
1065 <div class='section' id='vocab-aa'>
1066 <p class='title'>The Astronomy &amp; Astrophysics Keyword List</p>
1067
1068 <p>
1069 This vocabulary is a set of keywords made available on a web page by
1070 the publisher of the journal.
1071 The intended usage of the vocabulary is to tag articles with
1072 descriptive keywords to aid searching for articles on a particular
1073 topic.
1074 </p>
1075
1076 <p>
1077 The keywords are organised into categories which have been modelled as
1078 hierarchical relationships.
1079 Additionally, some of the keywords are grouped into collections which
1080 has been mirrored in the SKOS version.
1081 The vocabulary contains no definitions or related links as these are
1082 not provided in the original keyword list, and only a handful of
1083 alternative labels and scope notes that are present in the original
1084 keyword list.
1085 </p>
1086
1087 </div>
1088
1089 <div class='section' id='vocab-aoim'>
1090 <p class='title'>The AOIM Taxonomy</p>
1091
1092 <p>
1093 This vocabulary is published by the IVOA to allow images to be tagged
1094 with keywords that are relevant for the public.
1095 It consists of a set of keywords organised into an enumerated
1096 hierarchical structure.
1097 Each term consists of a taxonomic number and a label.
1098 There are no definitions, scope notes, or cross references.
1099 </p>
1100
1101 <p>When converting the AOIM into SKOS, it was decided to model the
1102 taxonomic number as an alternative label.
1103 Since there are duplication of terms, the token for a term consists of
1104 the full hierarchical location of the term.
1105 Thus, it is possible to distinguish between</p>
1106 <pre>
1107 Planet -> Feature -> Surface -> Canyon
1108 </pre>
1109 <p>and</p>
1110 <pre>
1111 Planet -> Satellite -> Feature -> Surface -> Canyon
1112 </pre>
1113 <p>which have the tokens <code>PlanetFeatureSurfaceCanyon</code> and
1114 <code>PlanetSatelliteFeatureSurfaceCanyon</code> respectively.
1115 </p>
1116
1117 </div>
1118
1119 <div class='section' id='vocab-ucd1'>
1120 <p class='title'>The UCD1+ Vocabulary</p>
1121
1122 <p>The UCD standard is an officially sanctioned and managed vocabulary
1123 of the IVOA. The normative document is a simple text file containing
1124 entries consisting of tokens (for example <code>em.IR</code>), a short
1125 description, and usage information (<q>syntax codes</q> which permit
1126 UCD tokens to be concatenated). The form of the tokens implies a
1127 natural hierarchy: <code>em.IR.8-15um</code> is obviously a narrower
1128 term than <code>em.IR</code>, which in turn is narrower than
1129 <code>em</code>.</p>
1130
1131 <p>Given the structure of the UCD1+ vocabulary, the natural
1132 translation to SKOS consists of preferred labels equal to the original
1133 tokens (the UCD1 words include dashes and periods), vocabulary tokens
1134 created using guidelines in <span class='xref'
1135 >practices</span> (for example, "emIR815Um" for
1136 <code>em.IR.8-15um</code>), direct use of the definitions, and the syntax codes
1137 placed in usage documentation: <code>&lt;skos:scopeNote&gt;UCD syntax code: P&lt;/skos:scopeNote&gt;</code></p>
1138
1139 <p>Note that the SKOS document containing the UCD1+ vocabulary does
1140 NOT consistute the official version: the normative document is still
1141 the text list. However, on the long term, the IVOA may decide to make
1142 the SKOS version normative, since the SKOS version contains all of the
1143 information contained in the original text document but has the
1144 advantage of being in a standard format easily read and used by any
1145 application on the semantic web whilst still being usable in the
1146 current ways.</p>
1147
1148 </div>
1149
1150 <div class='section' id='vocab-iau93'>
1151 <p class='title'>The 1993 IAU Thesaurus</p>
1152
1153 <p>The IAU Thesaurus consists of concepts with mostly capitalised
1154 labels and a rich set of thesaurus relationships (<q>BT</q> for
1155 "broader term", <q>NT</q> for <q>narrower term</q>, and <q>RT</q> for
1156 <q>related term</q>). The thesaurus also contains <q>U</q> (for
1157 <q>use</q>) and <q>UF</q> (<q>use for</q>) relationships. In a SKOS
1158 model of a vocabulary these are captured as alternative labels. A
1159 separate document contains translations of the vocabulary terms in
1160 five languages: English, French, German, Italian, and
1161 Spanish. Enumerable concepts are plural (for example <q>SPIRAL
1162 GALAXIES</q>) and non-enumerable concepts are singular
1163 (for example <q>STABILITY</q>). Finally, there are some usage hints like
1164 <q>combine with other</q>, which have been modelled as scope notes.</p>
1165
1166 <p>In converting the IAU Thesaurus to SKOS, we have been as faithful
1167 as possible to the original format of the thesaurus. Thus, preferred
1168 labels have been kept in their uppercase format.</p>
1169
1170 <p>The IAU Thesaurus has been unmaintained since its initial production in
1171 1993; it is therefore significantly out of date in places. This
1172 vocabulary is published for the sake of completeness, and to make the
1173 link between the evolving vocabulary work and any uses of the 1993
1174 vocabulary which come to light. We do not expect to make any future
1175 maintenance changes to this vocabulary, and would expect the IVOAT
1176 vocabulary, based on this one, to be used instead (see <span class='xref'>vocab-ivoat</span>).</p>
1177
1178 </div>
1179
1180 <div class='section' id='vocab-ivoat'>
1181 <p class='title'>Towards an IVOA Thesaurus</p>
1182
1183 <p>While it is true that the adoption of SKOS will make it easy to
1184 publish and access different astronomical vocabularies, the fact is
1185 that there is no vocabulary which makes it easy to jump-start the
1186 use of vocabularies in generic astrophysical VO applications: each of
1187 the previously developed vocabularies has their own limits and
1188 biases. For example, the IAU Thesaurus provides a large number of
1189 entries, copious relationships, and translations to four other languages,
1190 but there are no definitions, many concepts are now only useful for
1191 historical purposes (for example many photographic or historical instrument
1192 entries), some of the relationships are false or outdated, and many
1193 important or newer concepts and their common abbreviations are
1194 missing.</p>
1195
1196 <p>Despite its faults, the IAU Thesaurus constitutes a very extensive
1197 vocabulary which could easily serve as the basis vocabulary once
1198 we have removed its most egregious faults and extended it to cover the
1199 most obvious semantic holes. To this end, a heavily revised IAU
1200 thesaurus is in preparation for use within the IVOA and other
1201 astronomical contexts. The goal is to provide a general vocabulary
1202 foundation to which other, more specialised, vocabularies can be added
1203 as needed, and to provide a good <q>lingua franca</q> for the creation of
1204 vocabulary mappings.</p>
1205 </div>
1206 </div> <!-- End: Example vocabularies -->
1207
1208 <div class='section' id='distmappings'>
1209 <p class='title'>Example Mapping</p>
1210
1211 <p>To show how mappings can be expressed between two vocabularies, we
1212 have provided one example mapping document which maps the concepts in
1213 the A&amp;A Keywords vocabulary to the concepts in the AOIM
1214 vocabulary.
1215 All four types of mappings were required.
1216 Since all the mapping relationships have inverse relationships
1217 defined, the mapping document can also be used to infer the set of
1218 mappings from the AOIM vocabulary to the A&amp;A keywords.</p>
1219
1220 <p>To provide provenence information about the set of mappings in a
1221 document, Dublin Core metadata is included in the mapping
1222 document.</p>
1223
1224 <p class='todo'>Mappings to appear: see issue <a href='@ISSUESLIST@#mappings-6' >[mappings-6]</a>.</p>
1225
1226 </div>
1227
1228 <div class="appendices">
1229
1230 <!-- <p><span class='todo'>To come</span></p>-->
1231
1232
1233 <div class="section-nonum" id="bibliography">
1234 <p class="title">References</p>
1235 <?bibliography rm-refs ?>
1236 </div>
1237
1238 <p style="text-align: right; font-size: x-small; color: #888;">
1239 $Revision$ $Date$
1240 </p>
1241
1242 </div>
1243
1244 </body>
1245 </html>

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