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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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36 </head>
37
38 <body>
39 <div class="head">
40 <table>
41 <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>
42 </table>
43
44 <h1>Vocabularies in the Virtual Observatory, v@VERSION@</h1>
45 <h2>IVOA Working Draft, @RELEASEDATE@<!-- [Revision $Revision$]--></h2>
46 <!-- $Revision$ $Date$ -->
47
48 <dl>
49 <dt>Working Group</dt>
50 <dd><em><a href="http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics">Semantics</a></em></dd>
51
52 <dt>This version</dt>
53 <dd><span class='url'>http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics</span><br/>
54 <span class='url'>@BASEURI@.xhtml</span></dd>
55
56 <dt>Latest version</dt>
57 <dd><span class='url'>@DISTURI@</span><br/>
58 and <a href='@ISSUESLIST@' >issues list</a></dd>
59
60 <dt>Editors</dt>
61 <dd>Alasdair J G Gray,
62 <a href='http://nxg.me.uk/norman#norman' >Norman Gray</a>,
63 Frederic V Hessman and
64 Andrea Preite Martinez</dd>
65
66 <dt>Authors</dt>
67 <dd>
68 <span property="dc:creator">Sébastien Derriere</span>,
69 <span property="dc:creator">Alasdair J G Gray</span>,
70 <span property="dc:creator">Norman Gray</span>,
71 <span property="dc:creator">Frederic V Hessman</span>,
72 <span property="dc:creator">Tony Linde</span>,
73 <span property="dc:creator">Andrea Preite Martinez</span>,
74 <span property="dc:creator">Rob Seaman</span> and
75 <span property="dc:creator">Brian Thomas</span>
76 </dd>
77 </dl>
78 <hr/>
79 </div>
80
81 <div class="section-nonum" id="abstract">
82 <p class="title">Abstract</p>
83
84 <div class="abstract">
85 <p>As the astronomical information processed within the <em>Virtual Observatory
86 </em> becomes more complex, there is an increasing need for a more
87 formal means of identifying quantities, concepts, and processes not
88 confined to things easily placed in a FITS image, or expressed in a
89 catalogue or a table. We proposed that the IVOA adopt a standard
90 format for vocabularies based on the W3C's <em>Resource Description
91 Framework</em> (RDF) and <em>Simple Knowledge Organisation System</em>
92 (SKOS). By adopting a standard and simple format, the IVOA will
93 permit different groups to create and maintain their own specialised
94 vocabularies while letting the rest of the astronomical community
95 access, use, and combine them. The use of current, open standards
96 ensures that VO applications will be able to tap into resources of the
97 growing semantic web. Several examples of useful astronomical
98 vocabularies are provided, including work on a common IVOA thesaurus
99 intended to provide a semantic common base for VO applications.</p>
100 </div>
101
102 </div>
103
104 <div class="section-nonum" id="status">
105 <p class="title">Status of this document</p>
106
107 <p>This is an IVOA Working
108 Draft. The first release of this document was
109 <span property="dc:date">2008 February 22</span>.</p>
110
111 <p>This document is an IVOA Working Draft for review by IVOA members
112 and other interested parties. It is a draft document and may be
113 updated, replaced, or obsoleted by other documents at any time. It is
114 inappropriate to use IVOA Working Drafts as reference materials or to
115 cite them as other than <q>work in progress</q>.</p>
116
117 <p>A list of current IVOA Recommendations and other technical
118 documents can be found at
119 <span class='url' >http://www.ivoa.net/Documents/</span>.</p>
120
121 <h3>Acknowledgments</h3>
122
123 <p>We would like to thank the members of the IVOA semantic working
124 group for many interesting ideas and fruitful discussions.</p>
125 </div>
126
127 <h2><a id="contents" name="contents">Table of Contents</a></h2>
128 <?toc?>
129
130 <hr/>
131
132 <div class="section" id="introduction">
133 <p class="title">Introduction (informative)</p>
134
135 <div class="section">
136 <p class="title">Vocabularies in astronomy</p>
137
138 <p>Astronomical information of relevance to the Virtual Observatory
139 (VO) is not confined to quantities easily expressed in a catalogue or
140 a table.
141 Fairly simple things such as position on the sky, brightness in some
142 units, times measured in some frame, redshifts, classifications or
143 other similar quantities are easily manipulated and stored in VOTables
144 and can currently be identified using IVOA Unified Content Descriptors
145 (UCDs) <span class="cite">std:ucd</span>.
146 However, astrophysical concepts and quantities use a wide variety of
147 names, identifications, classifications and associations, most of
148 which cannot be described or labelled via UCDs.</p>
149
150 <p>There are a number of basic forms of organised semantic knowledge
151 of potential use to the VO, ranging from informal <q>folksonomies</q>
152 (where users are free to choose their own labels) at one extreme, to
153 formally structured <q>vocabularies</q> (where the label is drawn from
154 a predefined set of definitions, and which can include relationships between
155 labels) and <q>ontologies</q> (where the domain is captured in a
156 formal data model) at the other.
157 More formal definitions are presented later in this document.
158 </p>
159
160 <p>An astronomical ontology is necessary if we are to have a computer
161 (appear to) <q>understand</q> something of the domain.
162 There has been some progress towards creating an ontology of
163 astronomical object types <span
164 class="cite">std:ivoa-astro-onto</span> to meet this need.
165 However there are distinct use cases for letting human users find
166 resources of interest through search and navigation of the information space.
167 The most appropriate technology to meet these use cases derives from
168 the Information Science community, that of <em>controlled
169 vocabularies, taxonomies and thesauri</em>.
170 In the present document, we do not distinguish between controlled
171 vocabularies, taxonomies and thesauri, and use the term
172 <em>vocabulary</em> to represent all three.
173 </p>
174
175 <p>One of the best examples of the need for a simple vocabulary within
176 the VO is VOEvent <span class="cite">std:voevent</span>, the VO
177 standard for supporting rapid notification of astronomical events.
178 This standard requires some formalised indication of what a published
179 event is <q>about</q>, in a formalism which can be used straightforwardly
180 by the developer of relevant services. See <span class='xref'
181 >usecases</span> for further discussion.</p>
182
183 <p>A number of astronomical vocabularies have been created, with a
184 variety of goals and intended uses. Some examples are detailed below. </p>
185
186 <ul>
187
188 <li>The <em>Second Reference Dictionary of the Nomenclature of
189 Celestial Objects</em> <span class="cite">lortet94</span>, <span
190 class="cite">lortet94a</span> contains 500 paper pages of astronomical
191 nomenclature</li>
192
193 <li>For decades professional journals have used a set of reasonably
194 compatible keywords to help classify the content of whole articles.
195 These keywords have been analysed by Preite Martinez &amp; Lesteven
196 <span class="cite">preitemartinez07</span>, who derived a
197 set of common keywords constituting one of the potential bases for a
198 fuller VO vocabulary. The same authors also attempted to derive a set
199 of common concepts by analysing the contents of abstracts in journal
200 articles, which should comprise a list of tokens/concepts more
201 up-to-date than the old list of journal keywords. A similar but less
202 formal attempt was made by Hessman <span class='cite'>hessman05</span>
203 for the VOEvent working group, resulting in a similar list.</li>
204
205 <li>Astronomical databases generally use simple sets of keywords
206 – sometimes hierarchically organised – to help users make queries.
207 Two examples from very
208 different contexts are the list of object types used in the <a
209 href="http://simbad.u-strasbg.fr">Simbad</a> database and the search
210 keywords used in the educational Hands-On Universe image database
211 portal.</li>
212
213 <li>The Astronomical Outreach Imagery (AOI) working group has created
214 a simple taxonomy for helping to classify images used for educational
215 or public relations <span class="cite">std:aoim</span>. See
216 <span class='xref'>vocab-aoim</span>.</li>
217
218 <!--
219 <li>The Hands-On Universe project (see <span class='url'
220 >http://sunra.lbl.gov/telescope2/index.html</span>) has maintained a
221 public database of images for use by the general public since the
222 1990s. The images are very heterogeneous, since they are gathered from
223 a variety of professional, semi-professional, amateur, and school
224 observatories, so a simple taxonomy is used to facilitate browsing
225 by the users of the database.</li>
226 -->
227
228 <li>In 1993, Shobbrook and Shobbrook published an Astronomy Thesaurus
229 endorsed by the IAU <span class='cite' >shobbrook92</span>. This
230 collection of nearly 3000 terms, in five languages, is a valuable
231 resource, but has seen little use in recent years. Its very size,
232 which gives it expressive power, is a disadvantage to the extent that
233 it is consequently hard to use. See <span class='xref'>vocab-iau93</span>.</li>
234
235 <li>The VO's Unified Content Descriptors <span class='cite'
236 >std:ucd</span> (UCD) constitute the main controlled vocabulary of the
237 IVOA and contain some taxonomic information. However, UCD has some
238 features which supports its goals, but which make it difficult to use
239 beyond the present applications of labelling VOTables: firstly, there
240 is no standard means of identifying and processing the contents of the
241 text-based reference document; secondly, the content cannot be openly
242 extended beyond that set by a formal IVOA committee without going
243 through a laborious and time-consuming negotiation process of
244 extending the primary vocabulary itself; and thirdly, the UCD
245 vocabulary is primarily concerned with data types and their
246 processing, and only peripherally with astronomical objects (for
247 example, it defines formal labels for RA, flux, and bandpass, but does
248 not mention the Sun). See <span class='xref'>vocab-ucd1</span>.</li>
249
250 </ul>
251 </div>
252
253 <div class='section' id='usecases'>
254 <p class='title'>Use-cases, and the motivation for formalised vocabularies</p>
255
256 <p>The most immediate high-level motivation for this work is the
257 requirement of the VOEvent standard <span class='cite'
258 >std:voevent</span> for a controlled vocabulary usable in the
259 VOEvent's <code>&lt;Why/&gt;</code> and <code>&lt;What/&gt;</code>
260 elements, which describe what
261 sort of object the VOEvent packet is describing, in some broadly
262 intelligible way. For example a <q>burst</q> might be a gamma-ray burst
263 due to the collapse of a star in a distant galaxy, a solar flare, or
264 the brightening of a stellar or AGN accretion disk, and having an
265 explicit list of vocabulary terms can help guide the event publisher
266 into using a term which will be usefully precise for the event's
267 consumers. A free-text label can help here (which brings us into the
268 domain sometimes referred to as folksonomies), but the astronomical
269 community, with a culture sympathetic to international agreement, can
270 do better.</p>
271
272 <p>The purpose of this proposal is to establish a set of conventions for
273 the creation, publication, use, and manipulation of
274 astronomical vocabularies within the Virtual Observatory, based upon
275 the W3C's SKOS standard. We include as appendices to this proposal
276 formalised versions of a number of existing vocabularies, encoded as
277 SKOS vocabularies <span class="cite">std:skosref</span>.</p>
278
279 <p>Specific use-cases include the following.</p>
280 <ul>
281 <li>A user wishes to process all events concerning supernovae, which
282 means that an event concerning a type 1a supernova must be understood to be
283 relevant. [This supports a system working autonomously, filtering
284 incoming information]</li>
285
286 <li>A user is searching an archive of VOEvents for microlensing
287 events, and retrieves a large number of them; the search interface may
288 then prompt her to narrow her search using one of a set of terms
289 including, say, binary lens events. [This supports so-called <q>semantic
290 search</q>, providing semantic support to an interface which is in turn
291 supporting a user]</li>
292
293 <li>A user wishes to search for resources based on the
294 journal-supported keywords in a paper; they might either initiate this by
295 hand, or have this done on their behalf by a tool which can extract
296 the keywords from a PDF. The keywords are in the A&amp;A vocabulary,
297 and mappings have been defined between this vocabulary and others,
298 which means that the query keywords are translated automatically
299 into those appropriate for a search of an outreach image database
300 (everyone likes pretty pictures), the VO Registry, a set of Simbad
301 object types, and one or more concepts in more formal ontologies. The
302 search interface is then able to support the user browsing up and down
303 the AOIM vocabulary, and a specialised Simbad tool is able to take
304 over the search, now it has an appropriate starting place. [This
305 supports interoperability, building on the investments which
306 institutions and users have made in existing vocabularies]</li>
307
308 <li>A user receives a VOTable of results from a VO application – for
309 example a catalogue of objects or observations – and wants to search a
310 database of old FITS files for potential matches. Because the UCDs
311 labeling the columns of the tables are expressed in well-documented
312 SKOS, both the official descriptions of the UCDs and their semantic
313 matches to a variety of other plain-text vocabularies (such as the IAU
314 or AOIM thesauri) are available to the VO application, providing a basis
315 for massive searches for all kinds of FITS keyword values.</li>
316
317 </ul>
318
319 <p>It is not a goal of this standard, as it is not a goal of SKOS, to
320 produce knowledge-engineering artefacts which can support elaborate
321 machine reasoning – such artefacts would be very valuable, but require
322 much more expensive work on ontologies. As the supernova use-case
323 above illustrates, even simple vocabularies can support useful machine
324 reasoning.</p>
325
326 <p>It is also not a goal of this standard to produce new vocabularies,
327 or substantially alter existing ones; instead, the vocabularies
328 included below in section <span class='xref'>distvocab</span> are directly
329 derived from existing vocabularies (the exceptions are the IVOAT
330 vocabulary, which is ultimately intended to be a significant update to
331 the IAU-93 original, and the constellations vocabulary, which is
332 intended to be purely didactic). It therefore follows that the ambiguities,
333 redundancies and incompleteness of the source vocabularies are
334 faithfully represented in the distributed SKOS vocabularies. We hope
335 that this formalisation process will create greater visibility and
336 broader use for the various vocabularies, and that this will guide the
337 maintenance efforts of the curating groups.</p>
338
339 <p>The reason for both of these limitations is that vocabularies are
340 extremely expensive to produce, maintain and deploy, and we must
341 therefore rely on such vocabularies as have been developed, and
342 attached as metadata to resources, by others. Such vocabularies are
343 less rich or less coherent than we might prefer, but widely enough
344 deployed to be useful.</p>
345
346 <p>The purpose of this document is not to produce new vocabularies but
347 to show how vocabularies can be easily expressed in a internationally
348 standardized, computer-manipulable format. Four example vocabularies
349 that have previously been expressed using non-standardized formats –
350 namely the A&amp;A keyword list, the IAU and AOIM thesauri, and UCD1 –
351 are included below as an illustration of how simple it is to publish
352 them in SKOS. In all cases, one can easily express all of the
353 information of the original source vocabularies. It is true, that
354 complex vocabularies can be expensive to produce, maintain, and
355 deploy, and it will take a while until VO applications will be able to
356 use them to full advantage. However, the example vocabularies should
357 make it fairly easy to boot-strap the process by providing useful
358 vocabularies that can be used out of the box.</p>
359
360 </div>
361
362 <div class="section">
363 <p class="title">Formalising and managing multiple vocabularies</p>
364
365 <p>We find ourselves in the situation where there are multiple
366 vocabularies in use, describing a broad range of resources of interest
367 to professional and amateur astronomers, and members of the public.
368 These different vocabularies use different terms and different
369 relationships to support the different constituencies they cater for.
370 For example, <q>delta Sct</q> and <q>RR Lyr</q> are terms one would
371 find in a vocabulary aimed at professional astronomers, associated
372 with the notion of <q>variable star</q>; however one would
373 <em>not</em> find such technical terms in a vocabulary intended to
374 support outreach activities.</p>
375
376 <p>One approach to this problem is to create a single consensus
377 vocabulary, which draws terms from the various existing vocabularies
378 to create a new vocabulary which is able to express anything its users
379 might desire. The problem with this is that such an effort would be
380 very expensive, both in terms of time and effort on the part of those
381 creating it, and to the potential users, who have to learn
382 to navigate around it, recognise the new terms, and who have to be
383 supported in using the new terms correctly (or, more often,
384 incorrectly).</p>
385
386 <p>The alternative approach to the problem is to evade it, and this is
387 the approach taken in this document. Rather than deprecating the
388 existence of multiple overlapping vocabularies, we embrace it,
389 help interest groups formalise as many of them as are appropriate, and
390 standardise the process of formally declaring the relationships between
391 them. This means that:</p>
392 <ul>
393 <li>The various vocabularies are allowed to evolve separately, on
394 their own timescales, managed either by the IVOA, individual working
395 groups within the IVOA, or by third parties;</li>
396
397 <li>Specialised vocabularies can be developed and maintained by the
398 community with the most knowledge about a specific topic, ensuring
399 that the vocabulary will have the most appropriate breadth, depth, and
400 precision;</li>
401
402 <li>Users can choose the vocabulary or combination of vocabularies most
403 appropriate to their situation, either when annotating resources, or
404 when querying them; and</li>
405
406 <li>We can retain the previous investments made in vocabularies by
407 users and resource owners.</li>
408
409 </ul>
410
411
412 </div>
413
414 </div>
415
416 <div class='section'>
417 <p class='title'>SKOS-based vocabularies (informative)</p>
418
419 <p>In this section, we introduce the concepts of SKOS-based
420 vocabularies, and the technology of mapping between them. We describe
421 some additional requirements for IVOA vocabularies in the next
422 section, <span class='xref' >publishing</span>.</p>
423
424 <div class="section" id='vocab'>
425 <p class="title">Selection of the vocabulary format</p>
426
427 <p>After extensive online and face-to-face discussions, the authors have
428 brokered a consensus within the IVOA community that
429 formalised vocabularies should be published at least in SKOS (Simple Knowledge
430 Organisation System) format, a W3C draft standard application of RDF to the
431 field of knowledge organisation <span
432 class="cite">std:skosref</span>. SKOS draws on long experience
433 within the Library and Information Science community, to address a
434 well-defined set of problems to do with the indexing and retrieval of
435 information and resources; as such, it is a close match to the problem
436 this document is addressing.</p>
437
438 <p>ISO 5964 <span class='cite' >std:iso5964</span> defines a number of
439 the relevant terms (ISO 5964:1985=BS 6723:1985; see also <span
440 class='cite' >std:bs8723-1</span> and <span class='cite'
441 >std:z39.19</span>), and some of the (lightweight) theoretical
442 background. The only technical distinction relevant to this document
443 is that between vocabulary and thesaurus: BS-8723-1 defines a
444 thesaurus as a</p>
445 <blockquote>
446 Controlled vocabulary in which concepts are represented by preferred
447 terms, formally organized so that paradigmatic relationships between
448 the concepts are made explicit, and the preferred terms are
449 accompanied by lead-in entries for synonyms or quasi-synonyms.
450 <!-- NOTE:
451 The purpose of a thesaurus is to guide both the indexer and the
452 searcher to select the same preferred term or combination of preferred
453 terms to represent a given subject. -->
454 (BS-8723-1, sect. 2.39)
455 </blockquote>
456 <p>with a similar definition in ISO-5964 sect. 3.16. The paradigmatic
457 relationships in question are those relating a term to a <q>broader</q>,
458 <q>narrower</q> or more generically <q>related</q> term. These
459 notions have an operational definition: any resource
460 retrieved as a result of a search on a given term will also be
461 retrievable through a search on that term's <q>broader term</q>
462 (<q>narrower</q> is a simple inverse, so that for any pair of terms,
463 if <code>A skos:broader B</code>, then <code>B skos:narrower A</code>;
464 a term may have multiple narrower and broader terms).
465 This is not a subsumption relationship, as there is no implication
466 that the concept referred to by a narrower term is of the same
467 <em>type</em> as a broader term.</p>
468
469 <p>Thus <strong>a vocabulary (SKOS or otherwise) is not an
470 ontology</strong>. It has lighter and looser semantics than an
471 ontology, and is specialised for the restricted case of resource
472 retrieval. Those interested in ontological analyses can easily
473 transfer the vocabulary relationship information from SKOS to a formal
474 ontological format such as OWL <span class='cite' >std:owl</span>.</p>
475
476 <p>The purpose of a thesaurus is to help users find resources they
477 might be interested in, be they library books, image archives, or VOEvent
478 packets.</p>
479
480 </div>
481
482 <div class='section'>
483 <p class='title'>Content and format of a SKOS vocabulary</p>
484
485 <p>A published vocabulary in SKOS format consists of a set of
486 <q>concepts</q> – an example concept capturing the
487 vocabulary information about spiral galaxies is provided in the <a
488 href='#figexample' >Figure below</a>, with the RDF shown in both
489 RDF/XML <span class='cite' >std:rdfxml</span> and Turtle notation <span
490 class='cite' >std:turtle</span> (Turtle is similar to the more
491 informal N3 notation). The elements of a concept are detailed
492 below.</p>
493
494 <center>
495 <p><a name='figexample' >Figure: examples of SKOS vocabularies</a></p>
496 <table>
497 <tr>
498 <th bgcolor="#eecccc">XML Syntax</th>
499 <th width="10"/>
500 <th bgcolor="#cceecc">Turtle Syntax</th>
501 </tr>
502 <tr><td/></tr>
503 <tr>
504 <td bgcolor="#eecccc">
505 <pre>
506 &lt;skos:Concept rdf:about="#spiralGalaxy"&gt;
507 &lt;skos:prefLabel lang="en"&gt;
508 spiral galaxy
509 &lt;/prefLabel&gt;
510 &lt;skos:prefLabel lang="de"&gt;
511 Spiralgalaxie
512 &lt;/prefLabel&gt;
513 &lt;skos:altLabel lang="en"&gt;
514 spiral nebula
515 &lt;/skos:altLabel&gt;
516 &lt;skos:hiddenLabel lang="en"&gt;
517 spiral glaxy
518 &lt;/hiddenLabel&gt;
519 &lt;skos:definition lang="en"&gt;
520 A galaxy having a spiral structure.
521 &lt;/skos:definition&gt;
522 &lt;skos:scopeNote lang="en"&gt;
523 Spiral galaxies fall into one of
524 three catagories: Sa, Sc, and Sd.
525 &lt;/skos:scopeNote&gt;
526 &lt;skos:narrower
527 rdf:resource="#barredSpiralGalaxy"/&gt;
528 &lt;skos:broader
529 rdf:resource="#galaxy"/&gt;
530 &lt;skos:related
531 rdf:resource="#spiralArm"/&gt;
532 &lt;/skos:Concept&gt;
533 </pre>
534 </td>
535 <td/>
536 <td bgcolor="#cceecc">
537 <pre>
538 &lt;#spiralGalaxy&gt; a skos:Concept;
539 skos:prefLabel
540 "spiral galaxy"@en,
541 "Spiralgalaxie"@de;
542 skos:altLabel "spiral nebula"@en;
543 skos:hiddenLabel "spiral glaxy"@en;
544 skos:definition """A galaxy having a
545 spiral structure."""@en;
546 skos:scopeNote """Spiral galaxies fall
547 into one of three categories:
548 Sa, Sc, and Sd"""@en;
549 skos:narrower &lt;#barredSpiralGalaxy&gt;;
550 skos:broader &lt;#galaxy&gt;;
551 skos:related &lt;#spiralArm&gt; .
552 </pre>
553 </td>
554 </tr>
555 </table>
556 </center>
557
558 <p>A SKOS vocabulary includes the following features.</p>
559
560 <ul>
561
562 <li>A single URI representing the concept, mainly for use by computers.
563 <!--
564 <code>&lt;#spiralGalaxy&gt; a skos:Concept</code>.
565 <code>&lt;skos:Concept rdf:about="#spiralGalaxy"&gt;</code>
566 -->
567 </li>
568
569 <li>A single prefered label in each supported language of the
570 vocabulary, for use by humans.
571 <!--
572 <code>skos:prefLabel "spiral galaxy"@en, "Spiralgalaxie"@de</code>.
573 <code>&lt;skos:prefLabel&gt;spiral galaxy&lt;/skos:prefLabel&gt;</code>
574 -->
575 </li>
576
577 <li>Optional alternative labels which applications may encounter or in
578 common use, whether simple synonyms or commonly-used aliases such as
579 <q>GRB</q> for "gamma-ray burst", or <q>Spiral nebula</q> for
580 spiral galaxies.
581 <!--
582 <code>skos:altLabel "GRB"@en</code>
583 <code>&lt;skos:altLabel lang="de"&gt;Spiralgalaxie&lt;/skos:altLabel&gt;</code>
584 -->
585 </li>
586
587 <li>Optional hidden labels which capture terms which are sometimes
588 used for the corresponding concept, but which are deprecated in some
589 sense. This might include common misspellings for
590 either the preferred or alternate labels, for example <q>glaxy</q> for
591 <q>galaxy</q>.
592 </li>
593
594 <li>A definition for the concept, where one exists in the original
595 vocabulary, to clarify the meaning of the term.
596 <!--
597 <code>skos:definition "A galaxy having a spiral structure."@en</code>
598 <code>&lt;skos:definition lang="en"&gt;<br/>A galaxy having a spiral structure.<br/>&lt;/skos:definition&gt;</code>
599 -->
600 </li>
601
602 <li>A scope note to further clarify a definition, or the usage of the
603 concept.
604 <!--
605 <code>skos:scopeNote "Spiral galaxies fall into one of three categories: Sa, Sc, and Sd"@en</code>
606 <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>
607 -->
608 </li>
609
610 <li>Optionally, a concept may be involved in any number of relationships
611 to other concepts. The types of relationships are
612 <ul>
613 <li>Narrower or more specific concepts, for example a link to the concept
614 representing a <q>barred spiral galaxy</q>.
615 <!--
616 <code>skos:narrower &lt;#barredSpiralGalaxy&gt;</code>.
617 <code>&lt;skos:narrower rdf:resource="#barredSpiralGalaxy"&gt;</code>
618 -->
619 </li>
620 <li>Broader or more general concepts, for example a link to the token
621 representing galaxies in general.
622 <!--
623 <code>skos:broader &lt;#galaxy&gt;</code>.
624 <code>&lt;skos:broader rdf:resource="#galaxy"&gt;</code>
625 -->
626 </li>
627 <li>Related concepts, for example a link to the token representing spiral
628 arms of galaxies
629 <!--
630 <code>skos:related &lt;#spiralArm&gt;</code>
631 <code>&lt;skos:related rdf:resource="#spiralArm"&gt;</code>
632 -->
633 <br/>
634 (note this relationship does not say that spiral galaxies have spiral
635 arms – that would be ontological information of a higher order which
636 is beyond the requirements for information stored in a vocabulary).</li>
637 </ul>
638 </li>
639 </ul>
640
641 <p>In addition to the information about a single concept, a vocabulary
642 can contain information to help users navigate its structure and
643 contents:</p>
644 <ul>
645 <li>The <q>top concepts</q> of the vocabulary, i.e. those that occur
646 at the top of the vocabulary hierarchy defined by the broader/narrower
647 relationships, can be explicitly stated to make it easier to navigate
648 the vocabulary.</li>
649
650 <li>Concepts that form a natural group can be defined as being members
651 of a <q>collection</q>.</li>
652
653 <li>Versioning information can be added using change notes.</li>
654
655 <li>Additional metadata about the vocabulary, for example indicating the publisher, may
656 be documented using the Dublin Core metadata set <span class='cite'
657 >std:dublincore</span>.</li>
658 </ul>
659 </div>
660
661
662 <div class='section'>
663 <p class='title'>Relationships Between Vocabularies</p>
664
665 <p>
666 There already exist several vocabularies in the domain of astronomy.
667 Instead of attempting to replace all these existing vocabularies,
668 which have been developed to achieve different aims and user groups,
669 we embrace them.
670 This requires a mechanism to relate the concepts in the different
671 vocabularies.
672 </p>
673
674 <p>
675 Part of the SKOS standard <span class='cite'>std:skosref</span>
676 allows a concept in one vocabulary to be related to a concept in
677 another vocabulary.
678
679 There are four types of relationship provided to capture the
680 relationships between concepts in vocabularies, which are similar to
681 those defined for relationships between concepts within a single
682 vocabulary.
683 The types of mapping relationships are:
684 </p>
685
686 <ul>
687
688 <li>
689 Equivalence between concepts, i.e. the concepts in the different
690 vocabularies refer to the same real world entity.
691 This is captured with the RDF statement
692 <blockquote>
693 <code>AAkeys:#Cosmology skos:exactMatch aoim:#Cosmology</code>
694 </blockquote>
695 which states that the cosmology concept in the A&amp;A Keywords is the
696 same as the cosmology concept in the AOIM.
697 (Note the use of an external namespaces <code>AAkeys</code> and
698 <code>aoim</code> which must be defined within the document.)
699 </li>
700
701 <li>
702 Broader concept, i.e. there is not an equivalent concept but there is
703 a more general one.
704 This is captured with the RDF statement
705 <blockquote>
706 <code>AAkeys:#Moon skos:broadMatch aoim:PlanetSatellite</code>
707 </blockquote>
708 which states that the AOIM concept Planet Satellite is a more general
709 term than the A&amp;A Keywords concept Moon.
710 </li>
711
712 <li>
713 Narrower concept, i.e. there is not an equivalent concept but there is
714 a more specific one.
715 This is captured with the RDF statement
716 <blockquote>
717 <code>AAkeys:#IsmClouds skos:narrowMatch
718 aoim:#NebulaAppearanceDarkMolecularCloud</code>
719 </blockquote>
720 which states that the AOIM concept Nebula Appearance Dark Molecular
721 Cloud is more specific than the A&amp;A Keywords concept ISM Clouds.
722 </li>
723
724 <li>
725 Related concept, i.e. there is some form of relationship.
726 This is captured with the RDF statement
727 <blockquote>
728 <code>AAkeys:#BlackHolePhysics skos:relatedMatch
729 aoim:#StarEvolutionaryStageBlackHole</code>
730 </blockquote>
731 which states that the A&amp;A Keywords concept Black Hole Physics has
732 an association with the AOIM concept Star Evolutionary Stage Black Hole.
733 </li>
734
735 </ul>
736
737 <p>The semantic mapping relationships have certain properties.
738 The broadMatch relationship has the narrowMatch relationship as its
739 inverse and the exactMatch and relatedMatch relationships are
740 symmetrical.
741 The consequence of these properties is that if you have a mapping from
742 concept <code>A</code> in one vocabulary to concept <code>B</code> in
743 another vocabulary then you can infer a mapping from concept
744 <code>B</code> to concept <code>A</code>.
745 </p>
746
747 </div>
748
749 </div>
750
751 <div class='section' id='publishing'>
752 <p class='title'>Publishing vocabularies (normative)</p>
753
754 <div class='section' id='pubreq'>
755 <p class='title'>Requirements</p>
756
757 <p>A vocabulary which conforms to this IVOA standard has the following
758 features. In this section, the keywords
759 <span class='rfc2119' >must</span>,
760 <span class='rfc2119' >should</span>
761 and so on, are to be interpreted as described in <span
762 class='cite'>std:rfc2119</span>.</p>
763
764 <div class='section'>
765 <p class='title'>Dereferenceable namespace</p>
766
767 <p>The namespace of the
768 vocabulary <span class='rfc2119'>must</span> be dereferenceable on the
769 web. That is, typing the namespace URL into a web browser will
770 produce human-readable documentation about the vocabulary. In
771 addition, the namespace URL <span class='rfc2119' >should</span>
772 return the RDF version of the vocabulary if it is retrieved with an
773 HTTP Accept header of <code>application/rdf+xml</code>.</p>
774
775 <p><em>Rationale: These prescriptions are intended to be compatible
776 with the patterns described in <span class='cite'>berrueta08</span>
777 and <span class='cite'>sauermann07</span>, and vocabulary distributors
778 <span class='rfc2119' >should</span> follow these patterns where
779 possible.</em></p>
780 </div>
781
782 <div class='section'>
783 <p class='title'>Long-term availability</p>
784
785 <p>The files defining a
786 vocabulary, including those of superseded versions, should remain
787 permanently available. There is no requirement that the namespace
788 URL be at any particular location, although the IVOA web pages, or the
789 online sections of the A&amp;A journal would likely be suitable
790 archival locations.</p>
791 </div>
792
793 <div class='section'>
794 <p class='title'>Distribution format</p>
795
796 <p>Vocabularies <span class='rfc2119'>must</span> be made available
797 for distribution as SKOS RDF files, in either RDF/XML <span
798 class='cite'>std:rdfxml</span> or Turtle <span
799 class='cite'>std:turtle</span> format; vocabularies <span
800 class='rfc2119'>should</span> be made available in both formats. See
801 issue <a href='@ISSUESLIST@#distformat-2'>[distformat-2]</a>.</p>
802
803 <p>A publisher <span class='rfc2119'>may</span> make available
804 documentation and supporting files in other formats.</p>
805
806 <p><em>Rationale: this does imply that the vocabulary source files can only
807 realistically be parsed using an RDF parser. An alternative is to
808 require that vocabularies be distributed using a subset of RDF/XML
809 which can also be naively handled as traditional XML; however as well
810 as creating an extra standardisation requirement, this would make it
811 effectively infeasible to write out the distribution version of the
812 vocabulary using an RDF or general SKOS tool.</em></p>
813 </div>
814
815 <div class='section'>
816 <p class='title'>Clearly versioned vocabulary</p>
817
818 <p><span class='todo' >Open issue. There are interactions with the requirements for
819 long-term availability and a dereferenceable namespace, since this
820 implies that the vocabulary version should be manifestly encoded in
821 the namespace URI.</span> See issue <a
822 href='@ISSUESLIST@#versioning-3' >[versioning-3]</a>.</p>
823
824 </div>
825
826 <div class='section'>
827 <p class='title'>No restrictions on source files</p>
828
829 <p>This standard does not place any restrictions on the format of the
830 files managed by the maintenance process, as long as the distributed
831 files are as specified above. See issue
832 <a href='@ISSUESLIST@#masterformat-1' >[masterformat-1]</a>.</p>
833 </div>
834
835 </div>
836
837 <div class='section' id='practices'>
838 <p class='title'>Suggested good practices</p>
839
840 <p>This standard imposes a number of requirements on conformant
841 vocabularies (see <span class='xref' >publishing</span>). In
842 this section we list a number of good practices that IVOA vocabularies
843 <span class='rfc2119'>should</span> abide by. Some of the
844 prescriptions below are more specific than good-practice guidelines
845 for vocabularies in general.</p>
846
847 <p>The adoption of the following guidelines will make it easier to use
848 vocabularies in generic VO applications. However, VO applications
849 <span class='rfc2119'>should</span> be able to accept any vocabulary
850 that complies with the latest SKOS standard
851 <span class="cite">std:skosref</span> (this does not imply, of
852 course, that an application will necessarily understand the terms in
853 an alien vocabulary, although the presence of mappings to a known
854 vocabulary should allow it to derive some benefit).</p>
855
856 <ol>
857
858 <li>Concept identifiers <span class='rfc2119'>should</span> consist
859 only of the letters a-z, A-Z, and numbers 0-9, i.e. no spaces, no
860 exotic letters (for example umlauts), and no characters which would make a
861 token inexpressible as part of a URI; since tokens are for use by
862 computers only, this is not a big restriction, since the exotic
863 letters can be used within the labels and documentation if
864 appropriate.</li>
865
866 <li>The concept identifiers <span class='rfc2119'>should</span> be
867 kept in human-readable form, directly reflect the implied meaning, and
868 not be semi-random identifiers only (for example, use
869 <code>spiralGalaxy</code>, not <code>t1234567</code>); tokens <span
870 class='rfc2119'>should</span> preferably be created via a direct
871 conversion from the preferred label via removable/translation of
872 non-token characters (see above) and sub-token separation via
873 capitalisation of the first sub-token character (for example the label <code>My
874 favourite idea-label #42</code> is converted into
875 <code>MyFavouriteIdeaLabel42</code>).</li>
876
877 <li>Labels <span class='rfc2119'>should</span> be in the form of the source vocabulary. When
878 developing a new vocabulary the singular form <span
879 class='rfc2119'>should</span> be preferred,
880 for example <code>spiral galaxy</code>, not <code>spiral galaxies</code>.
881 <span class='todo'><a href="http://code.google.com/p/volute/issues/detail?id=1">Open issue</a></span></li>
882
883 <li>Each concept <span class='rfc2119'>should</span> have a definition
884 (<code>skos:definition</code>) that constitutes a short description of
885 the concept which could be adopted by an application using the
886 vocabulary. Each concept <span class='rfc2119'>should</span> have
887 additional documentation using SKOS Notes or
888 Dublin Core terms as appropriate
889 (see <span class='cite'>std:skosref</span>)</li>
890
891 <li>The language localisation <span class='rfc2119'>should</span> be
892 declared where appropriate, in preferred labels, alternate labels,
893 definitions, and the like.</li>
894
895 <li>Relationships (<q>broader</q>, <q>narrower</q>, <q>related</q>)
896 between concepts <span class='rfc2119'>should</span> be present, but
897 are not required; if used, they <span class='rfc2119'>should</span> be
898 complete (thus all <q>broader</q> links have corresponding
899 <q>narrower</q> links in the referenced entries and <q>related</q>
900 entries link each other).</li>
901
902 <li><q>TopConcept</q> entries (see above) <span
903 class='rfc2119'>should</span> be declared and normally consist of
904 those concepts that do not have any <q>broader</q> relationships
905 (i.e. not at a sub-ordinate position in the hierarchy).</li>
906
907 <li>The SKOS standard describes some good practices for vocabulary
908 maintenance, such as using <code>&lt;skos:changeNote&gt;</code> and
909 the like. Publishers <span class='rfc2119'>should</span> respect such
910 good maintenance practices are are available.</li>
911
912 <li>Publishers <span class='rfc2119'>should</span> publish
913 <q>mappings</q> between their vocabularies and other commonly used
914 vocabularies. These <span class='rfc2119'>should</span> be external to
915 the defining vocabulary document so that the vocabulary can be used
916 independently of the publisher's mappings. <span class='todo' ><a
917 href='http://code.google.com/p/volute/issues/detail?id=8' >Open
918 issue</a></span>.</li>
919 </ol>
920
921 <!--
922 <p>These suggestions are by no means trivial – there was
923 considerable discussion within the semantic working group on many of
924 these topics, particularly about token formats (some wanted lower-case
925 only), and singular versus plural forms of the labels (different
926 traditions exist within the international library science
927 community). Obviously, no publisher of an astronomical vocabulary has
928 to adopt these rules, but the adoption of these rules will make it
929 easier to use the vocabularly in external generic VO
930 applications. However, VO applications should be developed to accept
931 any vocabulary that complies with the latest SKOS standard <span
932 class="cite">std:skosref</span>.</p>
933 -->
934 </div>
935
936 </div>
937
938
939 <div class="section" id='distvocab'>
940 <p class="title">Example vocabularies</p>
941
942 <p>The intent of having the IVOA adopt SKOS as the preferred format for
943 astronomical vocabularies is to encourage the creation and management
944 of diverse vocabularies by competent astronomical groups, so that
945 users of the VO and related resources can benefit directly and
946 dynamically without the intervention of the IAU or IVOA. However, we
947 felt it important to provide several examples of vocabularies in the
948 SKOS format as part of the proposal, to illustrate their simplicity
949 and power, and to provide an immediate vocabulary basis for VO
950 applications.</p>
951
952 <p>The vocabularies described below are included, as SKOS files, in
953 the distributed version of this standard. These vocabularies have
954 stable URLs <span class='todo' >Format TBD, see <a
955 href='@ISSUESLIST@#versioning-3' >[versioning-3]</a></span>, and
956 may be cited and used indefinitely. These vocabularies will not,
957 however, be developed as part of the maintenance of this standard.
958 Interested groups, within and outwith the IVOA, are encouraged to take
959 these as a starting point and absorb them within existing
960 processes.</p>
961
962 <p>The exceptions to this rule are the constellation vocabulary,
963 provided here mainly for didactic purposes, and the proposed IVOA
964 Thesaurus, which is being developed as a separate project and whose
965 aim is to provide a corrected, more user-friendly, more complete, and
966 updated version of the 1993 IAU thesaurus. Although work on the IVOA
967 Thesaurus is on-going, the fact that it is largely based on the IAU
968 thesaurus means that it is already a very useful resource, so a usable
969 snapshot of this vocabulary will be published with the other
970 examples.</p>
971
972 <p>We provide a set of SKOS files representing the vocabularies which
973 have been developed, and mappings between them. These can be
974 downloaded at the URL</p>
975 <blockquote>
976 <span class='url'>@BASEURI@/@DISTNAME@.tar.gz</span>
977 </blockquote>
978
979 <!--
980 <p class='todo'>Not yet: instead go to
981 <span class='url'>http://code.google.com/p/volute/downloads/list</span></p>-->
982
983 <div class='section' id='vocab-constellation'>
984 <p class='title'>A Constellation Name Vocabulary</p>
985
986 <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>
987
988 <p>The &lt;skos:ConceptScheme&gt; contains a single &lt;skos:TopConcept&gt;, <q>constellation</q></p>
989 <br/><br/><center>
990 <table>
991 <tr><th bgcolor="#eecccc">XML Syntax</th>
992 <th width="10"/><th bgcolor="#cceecc">Turtle Syntax</th></tr>
993 <tr><td/></tr>
994 <tr>
995 <td bgcolor="#eecccc">
996 <pre>
997 &lt;skos:Concept rdf:about="#constellation"&gt;
998 &lt;skos:inScheme rdf:resource=""/&gt;
999 &lt;skos:prefLabel&gt;
1000 constellation
1001 &lt;/skos:prefLabel&gt;
1002 &lt;skos:definition&gt;
1003 IAU-sanctioned constellation names
1004 &lt;/skos:definition&gt;
1005 &lt;skos:narrower rdf:resource="#Andromeda"/&gt;
1006 ...
1007 &lt;skos:narrower rdf:resource="#Vulpecula"/&gt;
1008 &lt;/skos:Concept&gt;
1009 </pre>
1010 </td>
1011 <td/>
1012 <td bgcolor="#cceecc">
1013 <pre>
1014 &lt;#constellation&gt; a :Concept;
1015 :inScheme &lt;&gt;;
1016 :prefLabel "constellation";
1017 :definition "IAU-sanctioned constellation names";
1018 :narrower &lt;#Andromeda&gt;;
1019 ...
1020 :narrower &lt;#Vulpecula&gt;.
1021 </pre>
1022 </td></tr>
1023 </table></center>
1024 <p>and the entry for <q>Cygnus</q> is</p>
1025 <center><table><tr>
1026 <td bgcolor="#eecccc">
1027 <pre>
1028 &lt;skos:Concept rdf:about="#Cygnus"&gt;
1029 &lt;skos:inScheme rdf:resource=""/&gt;
1030 &lt;skos:prefLabel&gt;Cygnus&lt;/skos:prefLabel&gt;
1031 &lt;skos:definition&gt;Cygnus&lt;/skos:definition&gt;
1032 &lt;skos:altLabel&gt;Cygni&lt;/skos:altLabel&gt;
1033 &lt;skos:altLabel&gt;Cyg&lt;/skos:altLabel&gt;
1034 &lt;skos:broader rdf:resource="#constellation"/&gt;
1035 &lt;skos:scopeNote&gt;
1036 Cygnus is nominative form; the alternative
1037 labels are the genitive and short forms
1038 &lt;/skos:scopeNote&gt;
1039 &lt;/skos:Concept&gt;
1040 </pre>
1041 </td>
1042 <td width="10"/>
1043 <td bgcolor="#cceecc">
1044 <pre>
1045 &lt;#Cygnus&gt; a :Concept;
1046 :inScheme &lt;&gt;;
1047 :prefLabel "Cygnus";
1048 :definition "Cygnus";
1049 :altLabel "Cygni";
1050 :altLabel "Cyg";
1051 :broader &lt;#constellation&gt;;
1052 :scopeNote """Cygnus is nominative form;
1053 the alternative labels are the genitive and
1054 short forms""" .
1055 </pre>
1056 </td>
1057 </tr></table></center>
1058
1059 <p>Note that SKOS alone does not permit the distinct differentiation
1060 of genitive forms and abbreviations, but the use of alternate labels
1061 is more than adequate enough for processing by VO applications where
1062 the difference between <q>SS Cygni</q>, <q>SS Cyg</q>, and the incorrect form
1063 <q>SS Cygnus</q> is probably irrelevant.</p>
1064 </div>
1065
1066 <div class='section' id='vocab-aa'>
1067 <p class='title'>The Astronomy &amp; Astrophysics Keyword List</p>
1068
1069 <p>
1070 This vocabulary is a set of keywords made available on a web page by
1071 the publisher of the journal.
1072 The intended usage of the vocabulary is to tag articles with
1073 descriptive keywords to aid searching for articles on a particular
1074 topic.
1075 </p>
1076
1077 <p>
1078 The keywords are organised into categories which have been modelled as
1079 hierarchical relationships.
1080 Additionally, some of the keywords are grouped into collections which
1081 has been mirrored in the SKOS version.
1082 The vocabulary contains no definitions or related links as these are
1083 not provided in the original keyword list, and only a handful of
1084 alternative labels and scope notes that are present in the original
1085 keyword list.
1086 </p>
1087
1088 </div>
1089
1090 <div class='section' id='vocab-aoim'>
1091 <p class='title'>The AOIM Taxonomy</p>
1092
1093 <p>
1094 This vocabulary is published by the IVOA to allow images to be tagged
1095 with keywords that are relevant for the public.
1096 It consists of a set of keywords organised into an enumerated
1097 hierarchical structure.
1098 Each term consists of a taxonomic number and a label.
1099 There are no definitions, scope notes, or cross references.
1100 </p>
1101
1102 <p>When converting the AOIM into SKOS, it was decided to model the
1103 taxonomic number as an alternative label.
1104 Since there are duplication of terms, the token for a term consists of
1105 the full hierarchical location of the term.
1106 Thus, it is possible to distinguish between</p>
1107 <pre>
1108 Planet -> Feature -> Surface -> Canyon
1109 </pre>
1110 <p>and</p>
1111 <pre>
1112 Planet -> Satellite -> Feature -> Surface -> Canyon
1113 </pre>
1114 <p>which have the tokens <code>PlanetFeatureSurfaceCanyon</code> and
1115 <code>PlanetSatelliteFeatureSurfaceCanyon</code> respectively.
1116 </p>
1117
1118 </div>
1119
1120 <div class='section' id='vocab-ucd1'>
1121 <p class='title'>The UCD1+ Vocabulary</p>
1122
1123 <p>The UCD standard is an officially sanctioned and managed vocabulary
1124 of the IVOA. The normative document is a simple text file containing
1125 entries consisting of tokens (for example <code>em.IR</code>), a short
1126 description, and usage information (<q>syntax codes</q> which permit
1127 UCD tokens to be concatenated). The form of the tokens implies a
1128 natural hierarchy: <code>em.IR.8-15um</code> is obviously a narrower
1129 term than <code>em.IR</code>, which in turn is narrower than
1130 <code>em</code>.</p>
1131
1132 <p>Given the structure of the UCD1+ vocabulary, the natural
1133 translation to SKOS consists of preferred labels equal to the original
1134 tokens (the UCD1 words include dashes and periods), vocabulary tokens
1135 created using guidelines in <span class='xref'
1136 >practices</span> (for example, "emIR815Um" for
1137 <code>em.IR.8-15um</code>), direct use of the definitions, and the syntax codes
1138 placed in usage documentation: <code>&lt;skos:scopeNote&gt;UCD syntax code: P&lt;/skos:scopeNote&gt;</code></p>
1139
1140 <p>Note that the SKOS document containing the UCD1+ vocabulary does
1141 NOT consistute the official version: the normative document is still
1142 the text list. However, on the long term, the IVOA may decide to make
1143 the SKOS version normative, since the SKOS version contains all of the
1144 information contained in the original text document but has the
1145 advantage of being in a standard format easily read and used by any
1146 application on the semantic web whilst still being usable in the
1147 current ways.</p>
1148
1149 </div>
1150
1151 <div class='section' id='vocab-iau93'>
1152 <p class='title'>The 1993 IAU Thesaurus</p>
1153
1154 <p>The IAU Thesaurus consists of concepts with mostly capitalised
1155 labels and a rich set of thesaurus relationships (<q>BT</q> for
1156 "broader term", <q>NT</q> for <q>narrower term</q>, and <q>RT</q> for
1157 <q>related term</q>). The thesaurus also contains <q>U</q> (for
1158 <q>use</q>) and <q>UF</q> (<q>use for</q>) relationships. In a SKOS
1159 model of a vocabulary these are captured as alternative labels. A
1160 separate document contains translations of the vocabulary terms in
1161 five languages: English, French, German, Italian, and
1162 Spanish. Enumerable concepts are plural (for example <q>SPIRAL
1163 GALAXIES</q>) and non-enumerable concepts are singular
1164 (for example <q>STABILITY</q>). Finally, there are some usage hints like
1165 <q>combine with other</q>, which have been modelled as scope notes.</p>
1166
1167 <p>In converting the IAU Thesaurus to SKOS, we have been as faithful
1168 as possible to the original format of the thesaurus. Thus, preferred
1169 labels have been kept in their uppercase format.</p>
1170
1171 <p>The IAU Thesaurus has been unmaintained since its initial production in
1172 1993; it is therefore significantly out of date in places. This
1173 vocabulary is published for the sake of completeness, and to make the
1174 link between the evolving vocabulary work and any uses of the 1993
1175 vocabulary which come to light. We do not expect to make any future
1176 maintenance changes to this vocabulary, and would expect the IVOAT
1177 vocabulary, based on this one, to be used instead (see <span class='xref'>vocab-ivoat</span>).</p>
1178
1179 </div>
1180
1181 <div class='section' id='vocab-ivoat'>
1182 <p class='title'>Towards an IVOA Thesaurus</p>
1183
1184 <p>While it is true that the adoption of SKOS will make it easy to
1185 publish and access different astronomical vocabularies, the fact is
1186 that there is no vocabulary which makes it easy to jump-start the
1187 use of vocabularies in generic astrophysical VO applications: each of
1188 the previously developed vocabularies has their own limits and
1189 biases. For example, the IAU Thesaurus provides a large number of
1190 entries, copious relationships, and translations to four other languages,
1191 but there are no definitions, many concepts are now only useful for
1192 historical purposes (for example many photographic or historical instrument
1193 entries), some of the relationships are false or outdated, and many
1194 important or newer concepts and their common abbreviations are
1195 missing.</p>
1196
1197 <p>Despite its faults, the IAU Thesaurus constitutes a very extensive
1198 vocabulary which could easily serve as the basis vocabulary once
1199 we have removed its most egregious faults and extended it to cover the
1200 most obvious semantic holes. To this end, a heavily revised IAU
1201 thesaurus is in preparation for use within the IVOA and other
1202 astronomical contexts. The goal is to provide a general vocabulary
1203 foundation to which other, more specialised, vocabularies can be added
1204 as needed, and to provide a good <q>lingua franca</q> for the creation of
1205 vocabulary mappings.</p>
1206 </div>
1207 </div> <!-- End: Example vocabularies -->
1208
1209 <div class='section' id='distmappings'>
1210 <p class='title'>Example Mapping</p>
1211
1212 <p>To show how mappings can be expressed between two vocabularies, we
1213 have provided one example mapping document which maps the concepts in
1214 the A&amp;A Keywords vocabulary to the concepts in the AOIM
1215 vocabulary.
1216 All four types of mappings were required.
1217 Since all the mapping relationships have inverse relationships
1218 defined, the mapping document can also be used to infer the set of
1219 mappings from the AOIM vocabulary to the A&amp;A keywords.</p>
1220
1221 <p>To provide provenence information about the set of mappings in a
1222 document, Dublin Core metadata is included in the mapping
1223 document.</p>
1224
1225 <p class='todo'>Mappings to appear: see issue <a href='@ISSUESLIST@#mappings-6' >[mappings-6]</a>.</p>
1226
1227 </div>
1228
1229 <div class="appendices">
1230
1231 <!-- <p><span class='todo'>To come</span></p>-->
1232
1233
1234 <div class="section-nonum" id="bibliography">
1235 <p class="title">References</p>
1236 <?bibliography rm-refs ?>
1237 </div>
1238
1239 <p style="text-align: right; font-size: x-small; color: #888;">
1240 $Revision$ $Date$
1241 </p>
1242
1243 </div>
1244
1245 </body>
1246 </html>

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