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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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39 </head>
40
41 <body>
42 <div class="head">
43 <table>
44 <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>
45 </table>
46
47 <h1>Vocabularies in the Virtual Observatory, v@VERSION@</h1>
48 <h2>IVOA Working Draft, @RELEASEDATE@ [DRAFT $Revision$]</h2>
49 <!-- $Revision$ $Date$ -->
50
51 <dl>
52 <dt>Working Group</dt>
53 <dd><em><a href="http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics">Semantics</a></em></dd>
54
55 <dt>This version</dt>
56 <dd><span class='url'>http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics</span><br/>
57 <span class='url'>@BASEURI@.xhtml</span></dd>
58
59 <dt>Latest version</dt>
60 <dd><span class='url'>@DISTURI@</span><br/>
61 and <a href='@DISTURI@/issues' >issues list</a></dd>
62
63 <dt>Editors</dt>
64 <dd>Alasdair J G Gray,
65 <a href='http://nxg.me.uk/norman#norman' >Norman Gray</a>,
66 Frederic V Hessman and
67 Andrea Preite Martinez</dd>
68
69 <dt>Authors</dt>
70 <dd>
71 <span property="dc:creator">Sébastien Derriere</span>,
72 <span property="dc:creator">Alasdair J G Gray</span>,
73 <span property="dc:creator">Norman Gray</span>,
74 <span property="dc:creator">Frederic V Hessman</span>,
75 <span property="dc:creator">Tony Linde</span>,
76 <span property="dc:creator">Andrea Preite Martinez</span>,
77 <span property="dc:creator">Rob Seaman</span> and
78 <span property="dc:creator">Brian Thomas</span>
79 </dd>
80 </dl>
81 <hr/>
82 </div>
83
84 <div class="section-nonum" id="abstract">
85 <p class="title">Abstract</p>
86
87 <div class="abstract">
88 <p>As the astronomical information processed within the <em>Virtual Observatory
89 </em> becomes more complex, there is an increasing need for a more
90 formal means of identifying quantities, concepts, and processes not
91 confined to things easily placed in a FITS image, or expressed in a
92 catalogue or a table. We proposed that the IVOA adopt a standard
93 format for vocabularies based on the W3C's <em>Resource Description
94 Framework</em> (RDF) and <em>Simple Knowledge Organisation System</em>
95 (SKOS). By adopting a standard and simple format, the IVOA will
96 permit different groups to create and maintain their own specialised
97 vocabularies while letting the rest of the astronomical community
98 access, use, and combined them. The use of current, open standards
99 ensures that VO applications will be able to tap into resources of the
100 growing semantic web. Several examples of useful astronomical
101 vocabularies are provided, including work on a common IVOA thesaurus
102 intended to provide a semantic common base for VO applications.</p>
103 </div>
104
105 </div>
106
107 <div class="section-nonum" id="status">
108 <p class="title">Status of this document</p>
109
110 <p>This is (<strong>an internal draft of</strong>) an IVOA Working
111 Draft. The first release of this document was
112 <span property="dc:date">@RELEASEDATE@</span>.</p>
113
114 <p>This document is an IVOA Working Draft for review by IVOA members
115 and other interested parties. It is a draft document and may be
116 updated, replaced, or obsoleted by other documents at any time. It is
117 inappropriate to use IVOA Working Drafts as reference materials or to
118 cite them as other than <q>work in progress</q>.</p>
119
120 <p>A list of current IVOA Recommendations and other technical
121 documents can be found at
122 <span class='url' >http://www.ivoa.net/Documents/</span>.</p>
123
124 <h3>Acknowledgments</h3>
125
126 <p>We would like to thank the members of the IVOA semantic working
127 group for many interesting ideas and fruitful discussions.</p>
128 </div>
129
130 <h2><a id="contents" name="contents">Table of Contents</a></h2>
131 <?toc?>
132
133 <hr/>
134
135 <div class="section" id="introduction">
136 <p class="title">Introduction (informative)</p>
137
138 <div class="section">
139 <p class="title">Vocabularies in astronomy</p>
140
141 <p>Astronomical information of relevance to the Virtual Observatory
142 (VO) is not confined to quantities easily expressed in a catalogue or
143 a table.
144 Fairly simple things such as position on the sky, brightness in some
145 units, times measured in some frame, redshifts, classifications or
146 other similar quantities are easily manipulated and stored in VOTables
147 and can currently be identified using IVOA Unified Content Descriptors
148 (UCDs) <span class="cite">std:ucd</span>.
149 However, astrophysical concepts and quantities use a wide variety of
150 names, identifications, classifications and associations, most of
151 which cannot be described or labelled via UCDs.</p>
152
153 <p>There are a number of basic forms of organised semantic knowledge
154 of potential use to the VO, ranging from informal <q>folksonomies</q>
155 (where users are free to choose their own labels) at one extreme, to
156 formally structured <q>vocabularies</q> (where the label is drawn from
157 a predefined set of definitions, and which can include relationships between
158 labels) and <q>ontologies</q> (where the domain is captured in a
159 formal data model) at the other.
160 More formal definitions are presented later in this document.
161 </p>
162
163 <p>An astronomical ontology is necessary if we are to have a computer
164 (appear to) `understand' something of the domain.
165 There has been some progress towards creating an ontology of
166 astronomical object types <span
167 class="cite">std:ivoa-astro-onto</span> to meet this need.
168 However there are distinct use cases for letting human users find
169 resources of interest through search and navigation of the information space.
170 The most appropriate technology to meet these use cases derives from
171 the Information Science community, that of <em>controlled
172 vocabularies, taxonomies and thesauri</em>.
173 In the present document, we do not distinguish between controlled
174 vocabularies, taxonomies and thesauri, and use the term
175 <em>vocabulary</em> to represent all three.
176 </p>
177
178 <p>One of the best examples of the need for a simple vocabulary within
179 the VO is VOEvent <span class="cite">std:voevent</span>, the VO
180 standard for supporting rapid notification of astronomical events.
181 This standard requires some formalised indication of what a published
182 event is `about', in a formalism which can be used straightforwardly
183 by the developer of relevant services. See <span class='xref'
184 >usecases</span> for further discussion.</p>
185
186 <p>A number of astronomical vocabularies have been created, with a
187 variety of goals and intended uses. Some examples are detailed below. </p>
188
189 <ul>
190
191 <li>The <em>Second Reference Dictionary of the Nomenclature of
192 Celestial Objects</em> <span class="cite">lortet94</span>, <span
193 class="cite">lortet94a</span> contains 500 paper pages of astronomical
194 nomenclature</li>
195
196 <li>For decades professional journals have used a set of reasonably
197 compatible keywords to help classify the content of whole articles.
198 These keywords have been analysed by Preite Martinez &amp; Lesteven
199 <span class="cite">preitemartinez07</span>, who derived a
200 set of common keywords constituting one of the potential bases for a
201 fuller VO vocabulary. The same authors also attempted to derive a set
202 of common concepts by analysing the contents of abstracts in journal
203 articles, which should comprise a list of tokens/concepts more
204 up-to-date than the old list of journal keywords. A similar but less
205 formal attempt was made by Hessman <span class='cite'>hessman05</span>
206 for the VOEvent working group, resulting in a similar list <span
207 class="todo">[TODO] Check differences from the A&amp;A
208 list</span>.</li>
209
210 <li>Astronomical databases generally use simple sets of keywords
211 – sometimes hierarchically organised – to help users make queries.
212 Two examples from very
213 different contexts are the list of object types used in the <a
214 href="http://simbad.u-strasbg.fr">Simbad</a> database and the search
215 keywords used in the educational Hands-On Universe image database
216 portal.</li>
217
218 <li>The Astronomical Outreach Imagery (AOI) working group has created
219 a simple taxonomy for helping to classify images used for educational
220 or public relations <span class="cite">std:aoim</span>. See
221 <span class='xref'>vocab-aoim</span>.</li>
222
223 <!--
224 <li>The Hands-On Universe project (see <span class='url'
225 >http://sunra.lbl.gov/telescope2/index.html</span>) has maintained a
226 public database of images for use by the general public since the
227 1990s. The images are very heterogeneous, since they are gathered from
228 a variety of professional, semi-professional, amateur, and school
229 observatories, so a simple taxonomy is used to facilitate browsing
230 by the users of the database.</li>
231 -->
232
233 <li>In 1993, Shobbrook and Shobbrook published an Astronomy Thesaurus
234 endorsed by the IAU <span class='cite' >shobbrook92</span>. This
235 collection of nearly 3000 terms, in five languages, is a valuable
236 resource, but has seen little use in recent years. Its very size,
237 which gives it expressive power, is a disadvantage to the extent that
238 it is consequently hard to use. See <span class='xref'>vocab-iau93</span>.</li>
239
240 <li>The VO's Unified Content Descriptors <span class='cite'
241 >std:ucd</span> (UCD) constitute the main controlled vocabulary of the
242 IVOA and contain some taxonomic information. However, UCD has some
243 features which supports its goals, but which make it difficult to use
244 beyond the present applications of labelling VOTables: firstly, there
245 is no standard means of identifying and processing the contents of the
246 text-based reference document; secondly, the content cannot be openly
247 extended beyond that set by a formal IVOA committee without going
248 through a laborious and time-consuming negotiation process of
249 extending the primary vocabulary itself; and thirdly, the UCD
250 vocabulary is primarily concerned with data types and their
251 processing, and only peripherally with astronomical objects (for
252 example, it defines formal labels for RA, flux, and bandpass, but does
253 not mention the Sun). See <span class='xref'>vocab-ucd1</span>.</li>
254
255 </ul>
256 </div>
257
258 <div class='section' id='usecases'>
259 <p class='title'>Use-cases, and the motivation for formalised vocabularies</p>
260
261 <p>The most immediate high-level motivation for this work is the
262 requirement of the VOEvent standard <span class='cite'
263 >std:voevent</span> for a controlled vocabulary usable in the
264 VOEvent's <code>&lt;why/&gt;</code> and <code>&lt;what/&gt;</code>
265 elements, which describe what
266 sort of object the VOEvent packet is describing, in some broadly
267 intelligible way. For example a `burst' might be a gamma-ray burst
268 due to the collapse of a star in a distant galaxy, a solar flare, or
269 the brightening of a stellar or AGN accretion disk, and having an
270 explicit list of vocabulary terms can help guide the event publisher
271 into using a term which will be usefully precise for the event's
272 consumers. A free-text label can help here (which brings us into the
273 area sometimes referred to as `folksonomies'), but the astronomical
274 community, with a culture sympathetic to international agreement, can
275 do better.</p>
276
277 <p>The purpose of this proposal is to establish a set of conventions for
278 the creation, publication, use, and manipulation of
279 astronomical vocabularies within the Virtual Observatory, based upon
280 the W3C's SKOS standard. We include as appendices to this proposal
281 formalised versions of a number of existing vocabularies, encoded as
282 SKOS vocabularies <span class="cite">std:skosref</span>.</p>
283
284 <p>Specific use-cases include the following.</p>
285 <ul>
286 <li>A user wishes to process all events concerning supernovae, which
287 means that an event concerning a type 1a supernova must be understood to be
288 relevant. [This supports a system working autonomously, filtering
289 incoming information]</li>
290
291 <li>A user is searching an archive of VOEvents for microlensing
292 events, and retrieves a large number of them; the search interface may
293 then prompt her to narrow her search using one of a set of terms
294 including, say, binary lens events. [This supports so-called `semantic
295 search', providing semantic support to an interface which is in turn
296 supporting a user]</li>
297
298 <li>A user wishes to search for resources based on the
299 journal-supported keywords in a paper; they might either initiate this by
300 hand, or have this done on their behalf by a tool which can extract
301 the keywords from a PDF. The keywords are in the A&amp;A vocabulary,
302 and mappings have been defined between this vocabulary and others,
303 which means that the query keywords is translated automatically
304 into those appropriate for a search of an outreach image database
305 (everyone likes pretty pictures), the VO Registry, a set of Simbad
306 object types, and one or more concepts in more formal ontologies. The
307 search interface is then able to support the user browsing up and down
308 the AOIM vocabulary, and a specialised Simbad tool is able to take
309 over the search, now it has an appropriate starting place. [This
310 supports interoperability, building on the investments which
311 institutions and users have made in existing vocabularies]</li>
312
313 </ul>
314
315 <p>It is not a goal of this standard, as it is not a goal of SKOS, to
316 produce knowledge-engineering artefacts which can support elaborate
317 machine reasoning – such artefacts would be very valuable, but require
318 much more expensive work on ontologies. As the supernova use-case
319 above illustrates, even simple vocabularies can support useful machine
320 reasoning.</p>
321
322 <p>It is also not a goal of this standard to produce new vocabularies, or
323 substantially alter existing ones; instead, the vocabularies included
324 below in <span class='xref'>distvocab</span> are directly derived from
325 existing vocabularies, with adjustments to make them structurally
326 compatible with SKOS, or to remove (in the case of the IAU-93 and
327 IVOAT pair) significant anachronisms. It therefore follows that the
328 ambiguities, redundancies and incompleteness of the source vocabularies
329 are faithfully represented in the distributed SKOS vocabularies.</p>
330
331 <p>The reason for both of these limitations is that vocabularies are
332 extremely expensive to produce, maintain and deploy, and we must
333 therefore rely on such vocabularies as have been developed, and
334 attached as metadata to resources, by others. Such vocabularies are
335 less rich or less coherent than we might prefer, but widely enough
336 deployed to be useful.</p>
337
338 </div>
339
340 <div class="section">
341 <p class="title">Formalising and managing multiple vocabularies</p>
342
343 <p>We find ourselves in the situation where there are multiple
344 vocabularies in use, describing a broad range of resources of interest
345 to professional and amateur astronomers, and members of the public.
346 These different vocabularies use different terms and different
347 relationships to support the different constituencies they cater for.
348 For example, <q>delta Sct</q> and <q>RR Lyr</q> are terms one would
349 find in a vocabulary aimed at professional astronomers, associated
350 with the notion of <q>variable star</q>; however one would
351 <em>not</em> find such technical terms in a vocabulary intended to
352 support outreach activities.</p>
353
354 <p>One approach to this problem is to create a single consensus
355 vocabulary, which draws terms from the various existing vocabularies
356 to create a new vocabulary which is able to express anything its users
357 might desire. The problem with this is that such an effort would be
358 very expensive, both in terms of time and effort on the part of those
359 creating it, and to the potential users, who have to learn
360 to navigate around it, recognise the new terms, and who have to be
361 supported in using the new terms correctly (or, more often,
362 incorrectly).</p>
363
364 <p>The alternative approach to the problem is to evade it, and this is
365 the approach taken in this document. Rather than deprecating the
366 existence of multiple overlapping vocabularies, we embrace it,
367 help interest groups formalise as many of them as are appropriate, and
368 standardise the process of formally declaring the relationships between
369 them. This means that:</p>
370 <ul>
371 <li>The various vocabularies are allowed to evolve separately, on
372 their own timescales, managed either by the IVOA, individual working
373 groups within the IVOA, or by third parties;</li>
374
375 <li>Specialised vocabularies can be developed and maintained by the
376 community with the most knowledge about a specific topic, ensuring
377 that the vocabulary will have the most appropriate breadth, depth, and
378 precision;</li>
379
380 <li>Users can choose the vocabulary or combination of vocabularies most
381 appropriate to their situation, either when annotating resources, or
382 when querying them; and</li>
383
384 <li>We can retain the previous investments made in vocabularies by
385 users and resource owners.</li>
386
387 </ul>
388
389
390 </div>
391
392 </div>
393
394 <div class='section'>
395 <p class='title'>SKOS-based vocabularies (informative)</p>
396
397 <p>In this section, we introduce the concepts of SKOS-based
398 vocabularies, and the technology of mapping between them. We describe
399 some additional requirements for IVOA vocabularies in the next
400 section, <span class='xref' >publishing</span>.</p>
401
402 <div class="section" id='vocab'>
403 <p class="title">Selection of the vocabulary format</p>
404
405 <p>After extensive online and face-to-face discussions, the authors have
406 brokered a consensus within the IVOA community that
407 formalised vocabularies should be published at least in SKOS (Simple Knowledge
408 Organisation System) format, a W3C draft standard application of RDF to the
409 field of knowledge organisation <span
410 class="cite">std:skosref</span>. SKOS draws on long experience
411 within the Library and Information Science community, to address a
412 well-defined set of problems to do with the indexing and retrieval of
413 information and resources; as such, it is a close match to the problem
414 this document is addressing.</p>
415
416 <p>ISO 5964 <span class='cite' >std:iso5964</span> defines a number of
417 the relevant terms (ISO 5964:1985=BS 6723:1985; see also <span
418 class='cite' >std:bs8723-1</span> and <span class='cite'
419 >std:z39.19</span>), and some of the (lightweight) theoretical
420 background. The only technical distinction relevant to this document
421 is that between `vocabulary' and `thesaurus': BS-8723-1 defines a
422 thesaurus as a</p>
423 <blockquote>
424 Controlled vocabulary in which concepts are represented by preferred
425 terms, formally organized so that paradigmatic relationships between
426 the concepts are made explicit, and the preferred terms are
427 accompanied by lead-in entries for synonyms or quasi-synonyms.
428 <!-- NOTE:
429 The purpose of a thesaurus is to guide both the indexer and the
430 searcher to select the same preferred term or combination of preferred
431 terms to represent a given subject. -->
432 (BS-8723-1, sect. 2.39)
433 </blockquote>
434 <p>with a similar definition in ISO-5964 sect. 3.16. The paradigmatic
435 relationships in question are those relating a term to a <q>broader</q>,
436 <q>narrower</q> or more generically <q>related</q> term. These
437 notions have an operational definition: any resource
438 retrieved as a result of a search on a given term will also be
439 retrievable through a search on that term's <q>broader term</q>
440 (<q>narrower</q> is a simple inverse, so that for any pair of terms,
441 if <code>A skos:broader B</code>, then <code>B skos:narrower A</code>;
442 a term may have multiple narrower and broader terms).
443 This is not a subsumption relationship, as there is no implication
444 that the concept referred to by a narrower term is of the same
445 <em>type</em> as a broader term.</p>
446
447 <p>Thus <strong>a vocabulary (SKOS or otherwise) is not an
448 ontology</strong>. It has lighter and looser semantics than an
449 ontology, and is specialised for the restricted case of resource
450 retrieval. Those interested in ontological analyses can easily
451 transfer the vocabulary relationship information from SKOS to a formal
452 ontological format such as OWL <span class='cite' >std:owl</span>.</p>
453
454 <p>The purpose of a thesaurus is to help users find resources they
455 might be interested in, be they library books, image archives, or VOEvent
456 packets.</p>
457
458 </div>
459
460 <div class='section'>
461 <p class='title'>Content and format of a SKOS vocabulary</p>
462
463 <p>A published vocabulary in SKOS format consists of a set of
464 <q>concepts</q> – an example concept capturing the
465 vocabulary information about spiral galaxies is provided in the <a
466 href='#figexample' >Figure below</a>, with the RDF shown in both
467 RDF/XML <span class='cite' >std:rdfxml</span> and Turtle notation <span
468 class='cite' >std:turtle</span> (Turtle is similar to the more
469 informal N3 notation). The elements of a concept are detailed
470 below.</p>
471
472 <center>
473 <p><a name='figexample' >Figure: examples of SKOS vocabularies</a></p>
474 <table>
475 <tr>
476 <th bgcolor="#eecccc">XML Syntax</th>
477 <th width="10"/>
478 <th bgcolor="#cceecc">Turtle Syntax</th>
479 </tr>
480 <tr><td/></tr>
481 <tr>
482 <td bgcolor="#eecccc">
483 <pre>
484 &lt;skos:Concept rdf:about="#spiralGalaxy"&gt;
485 &lt;skos:prefLabel lang="en"&gt;
486 spiral galaxy
487 &lt;/prefLabel&gt;
488 &lt;skos:prefLabel lang="de"&gt;
489 Spiralgalaxie
490 &lt;/prefLabel&gt;
491 &lt;skos:altLabel lang="en"&gt;
492 spiral nebula
493 &lt;/skos:altLabel&gt;
494 &lt;skos:hiddenLabel lang="en"&gt;
495 spiral glaxy
496 &lt;/hiddenLabel&gt;
497 &lt;skos:definition lang="en"&gt;
498 A galaxy having a spiral structure.
499 &lt;/skos:definition&gt;
500 &lt;skos:scopeNote lang="en"&gt;
501 Spiral galaxies fall into one of
502 three catagories: Sa, Sc, and Sd.
503 &lt;/skos:scopeNote&gt;
504 &lt;skos:narrower
505 rdf:resource="#barredSpiralGalaxy"/&gt;
506 &lt;skos:broader
507 rdf:resource="#galaxy"/&gt;
508 &lt;skos:related
509 rdf:resource="#spiralArm"/&gt;
510 &lt;/skos:Concept&gt;
511 </pre>
512 </td>
513 <td/>
514 <td bgcolor="#cceecc">
515 <pre>
516 &lt;#spiralGalaxy&gt; a skos:Concept;
517 skos:prefLabel
518 "spiral galaxy"@en,
519 "Spiralgalaxie"@de;
520 skos:altLabel "spiral nebula"@en;
521 skos:hiddenLabel "spiral glaxy"@en;
522 skos:definition """A galaxy having a
523 spiral structure."""@en;
524 skos:scopeNote """Spiral galaxies fall
525 into one of three categories:
526 Sa, Sc, and Sd"""@en;
527 skos:narrower &lt;#barredSpiralGalaxy&gt;;
528 skos:broader &lt;#galaxy&gt;;
529 skos:related &lt;#spiralArm&gt; .
530 </pre>
531 </td>
532 </tr>
533 </table>
534 </center>
535
536 <p>A SKOS vocabulary includes the following features.</p>
537
538 <ul>
539
540 <li>A single URI representing the concept, mainly for use by computers.
541 <!--
542 <code>&lt;#spiralGalaxy&gt; a skos:Concept</code>.
543 <code>&lt;skos:Concept rdf:about="#spiralGalaxy"&gt;</code>
544 -->
545 </li>
546
547 <li>A single prefered label in each supported language of the
548 vocabulary, for use by humans.
549 <!--
550 <code>skos:prefLabel "spiral galaxy"@en, "Spiralgalaxie"@de</code>.
551 <code>&lt;skos:prefLabel&gt;spiral galaxy&lt;/skos:prefLabel&gt;</code>
552 -->
553 </li>
554
555 <li>Optional alternative labels which applications may encounter or in
556 common use, whether simple synonyms or commonly-used aliases,
557 e.g. <q>GRB</q> for "gamma-ray burst", or <q>Spiral nebula</q> for
558 spiral galaxies.
559 <!--
560 <code>skos:altLabel "GRB"@en</code>
561 <code>&lt;skos:altLabel lang="de"&gt;Spiralgalaxie&lt;/skos:altLabel&gt;</code>
562 -->
563 </li>
564
565 <li>Optional hidden labels which capture terms which are sometimes
566 used for the corresponding concept, but which are deprecated in some
567 sense. This might include common misspellings for
568 either the preferred or alternate labels, for example <q>glaxy</q> for
569 <q>galaxy</q>.
570 </li>
571
572 <li>A definition for the concept, where one exists in the original
573 vocabulary, to clarify the meaning of the term.
574 <!--
575 <code>skos:definition "A galaxy having a spiral structure."@en</code>
576 <code>&lt;skos:definition lang="en"&gt;<br/>A galaxy having a spiral structure.<br/>&lt;/skos:definition&gt;</code>
577 -->
578 </li>
579
580 <li>A scope note to further clarify a definition, or the usage of the
581 concept.
582 <!--
583 <code>skos:scopeNote "Spiral galaxies fall into one of three categories: Sa, Sc, and Sd"@en</code>
584 <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>
585 -->
586 </li>
587
588 <li>Optionally, a concept may be involved in any number of relationships
589 to other concepts. The types of relationships are
590 <ul>
591 <li>Narrower or more specific concepts, for example a link to the concept
592 representing a <q>barred spiral galaxy</q>.
593 <!--
594 <code>skos:narrower &lt;#barredSpiralGalaxy&gt;</code>.
595 <code>&lt;skos:narrower rdf:resource="#barredSpiralGalaxy"&gt;</code>
596 -->
597 </li>
598 <li>Broader or more general concepts, for example a link to the token
599 representing galaxies in general.
600 <!--
601 <code>skos:broader &lt;#galaxy&gt;</code>.
602 <code>&lt;skos:broader rdf:resource="#galaxy"&gt;</code>
603 -->
604 </li>
605 <li>Related concepts, for example a link to the token representing spiral
606 arms of galaxies
607 <!--
608 <code>skos:related &lt;#spiralArm&gt;</code>
609 <code>&lt;skos:related rdf:resource="#spiralArm"&gt;</code>
610 -->
611 <br/>
612 (note this relationship does not say that spiral galaxies have spiral
613 arms – that would be ontological information of a higher order which
614 is beyond the requirements for information stored in a vocabulary).</li>
615 </ul>
616 </li>
617 </ul>
618
619 <p>In addition to the information about a single concept, a vocabulary
620 can contain information to help users navigate its structure and
621 contents:</p>
622 <ul>
623 <li>The <q>top concepts</q> of the vocabulary, i.e. those that occur
624 at the top of the vocabulary hierarchy defined by the broader/narrower
625 relationships, can be explicitly stated to make it easier to navigate
626 the vocabulary.</li>
627
628 <li>Concepts that form a natural group can be defined as being members
629 of a <q>collection</q>.</li>
630
631 <li>Versioning information can be added using change notes.</li>
632
633 <li>Additional metadata about the vocabulary, e.g. the publisher, may
634 be documented using the Dublin Core metadata set <span class='cite'
635 >std:dublincore</span>.</li>
636 </ul>
637 </div>
638
639
640 <div class='section'>
641 <p class='title'>Relationships Between Vocabularies</p>
642
643 <p>
644 There already exist several vocabularies in the domain of astronomy.
645 Instead of attempting to replace all these existing vocabularies,
646 which have been developed to achieve different aims and user groups,
647 we embrace them.
648 This requires a mechanism to relate the concepts in the different
649 vocabularies.
650 </p>
651
652 <p>
653 Part of the SKOS standard <span class='cite'>std:skosref</span>
654 allows a concept in one vocabulary to be related to a concept in
655 another vocabulary.
656
657 There are four types of relationship provided to capture the
658 relationships between concepts in vocabularies, which are similar to
659 those defined for relationships between concepts within a single
660 vocabulary.
661 The types of mapping relationships are:
662 </p>
663
664 <ul>
665
666 <li>
667 Equivalence between concepts, i.e. the concepts in the different
668 vocabularies refer to the same real world entity.
669 This is captured with the RDF statement
670 <blockquote>
671 <code>AAkeys:#Cosmology skos:exactMatch aoim:#Cosmology</code>
672 </blockquote>
673 which states that the cosmology concept in the A&amp;A Keywords is the
674 same as the cosmology concept in the AOIM.
675 (Note the use of an external namespaces <code>AAkeys</code> and
676 <code>aoim</code> which must be defined within the document.)
677 </li>
678
679 <li>
680 Broader concept, i.e. there is not an equivalent concept but there is
681 a more general one.
682 This is captured with the RDF statement
683 <blockquote>
684 <code>AAkeys:#Moon skos:broadMatch aoim:PlanetSatellite</code>
685 </blockquote>
686 which states that the AOIM concept Planet Satellite is a more general
687 term than the A&amp;A Keywords concept Moon.
688 </li>
689
690 <li>
691 Narrower concept, i.e. there is not an equivalent concept but there is
692 a more specific one.
693 This is captured with the RDF statement
694 <blockquote>
695 <code>AAkeys:#IsmClouds skos:narrowMatch
696 aoim:#NebulaAppearanceDarkMolecularCloud</code>
697 </blockquote>
698 which states that the AOIM concept Nebula Appearance Dark Molecular
699 Cloud is more specific than the A&amp;A Keywords concept ISM Clouds.
700 </li>
701
702 <li>
703 Related concept, i.e. there is some form of relationship.
704 This is captured with the RDF statement
705 <blockquote>
706 <code>AAkeys:#BlackHolePhysics skos:relatedMatch
707 aoim:#StarEvolutionaryStageBlackHole</code>
708 </blockquote>
709 which states that the A&amp;A Keywords concept Black Hole Physics has
710 an association with the AOIM concept Star Evolutionary Stage Black Hole.
711 </li>
712
713 </ul>
714
715 <p>
716 <!-- <span class='todo'>[TODO:] Enter text regarding the resolution of <a
717 href="http://code.google.com/p/volute/issues/detail?id=7">Issue
718 7</a>.</span> -->
719
720 The semantic mapping relationships have certain properties.
721 The broadMatch relationship has the narrowMatch relationship as its
722 inverse and the exactMatch and relatedMatch relationships are
723 symmetrical.
724 The consequence of these properties is that if you have a mapping from
725 concept <code>A</code> in one vocabulary to concept <code>B</code> in
726 another vocabulary then you can infer a mapping from concept
727 <code>B</code> to concept <code>A</code>.
728 </p>
729
730 </div>
731
732 </div>
733
734 <div class='section' id='publishing'>
735 <p class='title'>Publishing vocabularies (normative)</p>
736
737 <div class='section' id='pubreq'>
738 <p class='title'>Requirements</p>
739
740 <p>A vocabulary which conforms to this IVOA standard has the following
741 features. In this section, the keywords
742 <span class='rfc2119' >must</span>,
743 <span class='rfc2119' >should</span>
744 and so on, are to be interpreted as described in <span
745 class='cite'>std:rfc2119</span>.</p>
746
747 <div class='section'>
748 <p class='title'>Dereferenceable namespace</p>
749
750 <p>The namespace of the
751 vocabulary <span class='rfc2119'>must</span> be dereferenceable on the
752 web. That is, typing the namespace URL into a web browser will
753 produce human-readable documentation about the vocabulary. In
754 addition, the namespace URL <span class='rfc2119' >should</span>
755 return the RDF version of the vocabulary if it is retrieved with an
756 HTTP Accept header of <code>application/rdf+xml</code>.</p>
757
758 <p><em>Rationale: These prescriptions are intended to be compatible
759 with the patterns described in <span class='cite'>berrueta08</span>
760 and <span class='cite'>sauermann07</span>, and vocabulary distributors
761 <span class='rfc2119' >should</span> follow these patterns where
762 possible.</em></p>
763 </div>
764
765 <div class='section'>
766 <p class='title'>Long-term availability</p>
767
768 <p>The files defining a
769 vocabulary, including those of superseded versions, should remain
770 permanently available. There is no requirement that the namespace
771 URL be at any particular location, although the IVOA web pages, or the
772 online sections of the A&amp;A journal would likely be suitable
773 archival locations.</p>
774 </div>
775
776 <div class='section'>
777 <p class='title'>Distribution format</p>
778
779 <p>Vocabularies <span class='rfc2119'>must</span> be made available
780 for distribution as SKOS RDF files, in either RDF/XML <span
781 class='cite'>std:rdfxml</span> or Turtle <span
782 class='cite'>std:turtle</span> format; vocabularies <span
783 class='rfc2119'>should</span> be made available in both formats. See
784 issue <a href='@DISTURI@/issues#distformat-2'>[distformat-2]</a>.</p>
785
786 <p>A publisher <span class='rfc2119'>may</span> make available
787 documentation and supporting files in other formats.</p>
788
789 <p><em>Rationale: this does imply that the vocabulary source files can only
790 realistically be parsed using an RDF parser. An alternative is to
791 require that vocabularies be distributed using a subset of RDF/XML
792 which can also be naively handled as traditional XML; however as well
793 as creating an extra standardisation requirement, this would make it
794 effectively infeasible to write out the distribution version of the
795 vocabulary using an RDF or general SKOS tool.</em></p>
796 </div>
797
798 <div class='section'>
799 <p class='title'>Clearly versioned vocabulary</p>
800
801 <p><span class='todo' >To be decided. There are interactions with
802 'long-term availability' and 'dereferenceable namespace', since this
803 implies that the vocabulary version should be manifestly encoded in
804 the namespace URI.</span> See issue <a
805 href='@DISTURI@/issues#versioning-3' >[versioning-3]</a>.</p>
806
807 </div>
808
809 <div class='section'>
810 <p class='title'>No restrictions on source files</p>
811
812 <p>This standard does not place any restrictions on the format of the
813 files managed by the maintenance process, as long as the distributed
814 files are as specified above. See issue
815 <a href='@DISTURI@/issues#masterformat-1' >[masterformat-1]</a>.</p>
816 </div>
817
818 </div>
819
820 <div class='section' id='practices'>
821 <p class='title'>Suggested good practices</p>
822
823 <p>This standard imposes a number of requirements on conformant
824 vocabularies (see <span class='xref' >publishing</span>). In
825 this section we list a number of good practices that IVOA vocabularies
826 <span class='rfc2119'>should</span> abide by. Some of the
827 prescriptions below are more specific than good-practice guidelines
828 for vocabularies in general.</p>
829
830 <p>The adoption of the following guidelines will make it easier to use
831 vocabularies in generic VO applications. However, VO applications
832 <span class='rfc2119'>should</span> be able to accept any vocabulary
833 that complies with the latest SKOS standard
834 <span class="cite">std:skosref</span> (this does not imply, of
835 course, that an application will necessarily understand the terms in
836 an alien vocabulary, although the presence of mappings to a known
837 vocabulary should allow it to derive some benefit).</p>
838
839 <ol>
840
841 <li>Concept identifiers <span class='rfc2119'>should</span> consist
842 only of the letters a-z, A-Z, and numbers 0-9, i.e. no spaces, no
843 exotic letters (e.g. umlauts), and no characters which would make a
844 token inexpressible as part of a URI; since tokens are for use by
845 computers only, this is not a big restriction, since the exotic
846 letters can be used within the labels and documentation if
847 appropriate.</li>
848
849 <li>The concept identifiers <span class='rfc2119'>should</span> be
850 kept in human-readable form, directly reflect the implied meaning, and
851 not be semi-random identifiers only (for example, use
852 <q>spiralGalaxy</q>, not "t1234567"); tokens <span
853 class='rfc2119'>should</span> preferably be created via a direct
854 conversion from the preferred label via removable/translation of
855 non-token characters (see above) and sub-token separation via
856 capitalisation of the first sub-token character (e.g. the label "My
857 favourite idea-label #42" is converted into
858 "MyFavouriteIdeaLabel42").</li>
859
860 <li>Labels <span class='rfc2119'>should</span> be in the form of the source vocabulary. When
861 developing a new vocabulary the singular form <span
862 class='rfc2119'>should</span> be preferred,
863 e.g. <q>spiral galaxy</q>, not "spiral galaxies". <span
864 class='todo'><a
865 href="http://code.google.com/p/volute/issues/detail?id=1">Open
866 issue</a></span></li>
867
868 <li>Each concept <span class='rfc2119'>should</span> have a definition
869 (<code>skos:definition</code>) that constitutes a short description of
870 the concept which could be adopted by an application using the
871 vocabulary. Each concept <span class='rfc2119'>should</span> have
872 additional documentation using SKOS Notes or
873 Dublin Core terms as appropriate
874 (see <span class='cite'>std:skosref</span>)</li>
875
876 <li>The language localisation <span class='rfc2119'>should</span> be
877 declared where appropriate, in preferred labels, alternate labels,
878 definitions, and the like.</li>
879
880 <li>Relationships (<q>broader</q>, <q>narrower</q>, <q>related</q>)
881 between concepts <span class='rfc2119'>should</span> be present, but
882 are not required; if used, they <span class='rfc2119'>should</span> be
883 complete (thus all <q>broader</q> links have corresponding
884 <q>narrower</q> links in the referenced entries and <q>related</q>
885 entries link each other).</li>
886
887 <li><q>TopConcept</q> entries (see above) <span
888 class='rfc2119'>should</span> be declared and normally consist of
889 those concepts that do not have any <q>broader</q> relationships
890 (i.e. not at a sub-ordinate position in the hierarchy).</li>
891
892 <li>The SKOS standard describes some good practices for vocabulary
893 maintenance, such as using <code>&lt;skos:changeNote&gt;</code> and
894 the like. Publishers <span class='rfc2119'>should</span> respect such
895 good maintenance practices are are available.</li>
896
897 <li>Publishers <span class='rfc2119'>should</span> publish
898 <q>mappings</q> between their vocabularies and other commonly used
899 vocabularies. These <span class='rfc2119'>should</span> be external to
900 the defining vocabulary document so that the vocabulary can be used
901 independently of the publisher's mappings. <span class='todo' ><a
902 href='http://code.google.com/p/volute/issues/detail?id=8' >Open
903 issue</a></span>.</li>
904 </ol>
905
906 <!--
907 <p>These suggestions are by no means trivial – there was
908 considerable discussion within the semantic working group on many of
909 these topics, particularly about token formats (some wanted lower-case
910 only), and singular versus plural forms of the labels (different
911 traditions exist within the international library science
912 community). Obviously, no publisher of an astronomical vocabulary has
913 to adopt these rules, but the adoption of these rules will make it
914 easier to use the vocabularly in external generic VO
915 applications. However, VO applications should be developed to accept
916 any vocabulary that complies with the latest SKOS standard <span
917 class="cite">std:skosref</span>.</p>
918 -->
919 </div>
920
921 </div>
922
923
924 <div class="section" id='distvocab'>
925 <p class="title">Example vocabularies</p>
926
927 <p>The intent of having the IVOA adopt SKOS as the preferred format for
928 astronomical vocabularies is to encourage the creation and management
929 of diverse vocabularies by competent astronomical groups, so that
930 users of the VO and related resources can benefit directly and
931 dynamically without the intervention of the IAU or IVOA. However, we
932 felt it important to provide several examples of vocabularies in the
933 SKOS format as part of the proposal, to illustrate their simplicity
934 and power, and to provide an immediate vocabulary basis for VO
935 applications.</p>
936
937 <p>The vocabularies described below are included, as SKOS files, in
938 the distributed version of this standard. These vocabularies have
939 stable URLs <span class='todo' >Format TBD, see
940 <a href='@DISTURI@/issues#versioning-3' >[versioning-3]</a></span>, and
941 may be cited and used indefinitely. These vocabularies will not,
942 however, be developed as part of the maintenance of this standard.
943 Interested groups, within and outwith the IVOA, are encouraged to take
944 these as a starting point and absorb them within an existing
945 process.</p>
946
947 <p>The exception to this is the IVOA-T (see <span class='xref'
948 >vocab-ivoat</span>), which will be developed as part of a process
949 which has already begun. <span class='todo' >Clarify wording here;
950 include reference to forthcoming IVOA-T document; ??include snapshot
951 of vocabulary in the distribution??</span>.</p>
952
953 <p>We provide a set of SKOS files representing the vocabularies which
954 have been developed, and mappings between them. These can be
955 downloaded at the URL</p>
956 <blockquote>
957 <span class='url'>@BASEURI@/@DISTNAME@.tar.gz</span>
958 </blockquote>
959 <p class='todo'>Not yet: instead go to
960 <span class='url'>http://code.google.com/p/volute/downloads/list</span></p>
961
962 <p><span class='todo' >[To be expanded:] there are no mappings at the
963 moment. Also, the vocabularies are all in a single language, though
964 translations of the IAU93 thesaurus are available. See also
965 issue <a href='@DISTURI@/issues#mappings-6'>[mappings-6]</a>.</span></p>
966
967 <div class='section' id='vocab-constellation'>
968 <p class='title'>A Constellation Name Vocabulary</p>
969
970 <p>This vocabulary is presented as a simple example of an astronomical vocabulary for a very particular purpose, e.g. 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>
971
972 <p>The &lt;skos:ConceptScheme&gt; contains a single &lt;skos:TopConcept&gt;, <q>constellation</q></p>
973 <br/><br/><center>
974 <table>
975 <tr><th bgcolor="#eecccc">XML Syntax</th>
976 <th width="10"/><th bgcolor="#cceecc">Turtle Syntax</th></tr>
977 <tr><td/></tr>
978 <tr>
979 <td bgcolor="#eecccc">
980 <pre>
981 &lt;skos:Concept rdf:about="#constellation"&gt;
982 &lt;skos:inScheme rdf:resource=""/&gt;
983 &lt;skos:prefLabel&gt;
984 constellation
985 &lt;/skos:prefLabel&gt;
986 &lt;skos:definition&gt;
987 IAU-sanctioned constellation names
988 &lt;/skos:definition&gt;
989 &lt;skos:narrower rdf:resource="#Andromeda"/&gt;
990 ...
991 &lt;skos:narrower rdf:resource="#Vulpecula"/&gt;
992 &lt;/skos:Concept&gt;
993 </pre>
994 </td>
995 <td/>
996 <td bgcolor="#cceecc">
997 <pre>
998 &lt;#constellation&gt; a :Concept;
999 :inScheme &lt;&gt;;
1000 :prefLabel "constellation";
1001 :definition "IAU-sanctioned constellation names";
1002 :narrower &lt;#Andromeda&gt;;
1003 ...
1004 :narrower &lt;#Vulpecula&gt;.
1005 </pre>
1006 </td></tr>
1007 </table></center>
1008 <p>and the entry for <q>Cygnus</q> is</p>
1009 <center><table><tr>
1010 <td bgcolor="#eecccc">
1011 <pre>
1012 &lt;skos:Concept rdf:about="#Cygnus"&gt;
1013 &lt;skos:inScheme rdf:resource=""/&gt;
1014 &lt;skos:prefLabel&gt;Cygnus&lt;/skos:prefLabel&gt;
1015 &lt;skos:definition&gt;Cygnus&lt;/skos:definition&gt;
1016 &lt;skos:altLabel&gt;Cygni&lt;/skos:altLabel&gt;
1017 &lt;skos:altLabel&gt;Cyg&lt;/skos:altLabel&gt;
1018 &lt;skos:broader rdf:resource="#constellation"/&gt;
1019 &lt;skos:scopeNote&gt;
1020 Cygnus is nominative form; the alternative
1021 labels are the genitive and short forms
1022 &lt;/skos:scopeNote&gt;
1023 &lt;/skos:Concept&gt;
1024 </pre>
1025 </td>
1026 <td width="10"/>
1027 <td bgcolor="#cceecc">
1028 <pre>
1029 &lt;#Cygnus&gt; a :Concept;
1030 :inScheme &lt;&gt;;
1031 :prefLabel "Cygnus";
1032 :definition "Cygnus";
1033 :altLabel "Cygni";
1034 :altLabel "Cyg";
1035 :broader &lt;#constellation&gt;;
1036 :scopeNote """Cygnus is nominative form;
1037 the alternative labels are the genitive and
1038 short forms""" .
1039 </pre>
1040 </td>
1041 </tr></table></center>
1042
1043 <p>Note that SKOS alone does not permit the distinct differentiation
1044 of genitive forms and abbreviations, but the use of alternate labels
1045 is more than adequate enough for processing by VO applications where
1046 the difference between <q>SS Cygni</q>, <q>SS Cyg</q>, and the incorrect form
1047 <q>SS Cygnus</q> is probably irrelevant.</p>
1048 </div>
1049
1050 <div class='section' id='vocab-aa'>
1051 <p class='title'>The Astronomy &amp; Astrophysics Keyword List</p>
1052
1053 <p>
1054 This vocabulary is a set of keywords made available on a web page by
1055 the publisher of the journal.
1056 The intended usage of the vocabulary is to tag articles with
1057 descriptive keywords to aid searching for articles on a particular
1058 topic.
1059 </p>
1060
1061 <p>
1062 The keywords are organised into categories which have been modelled as
1063 hierarchical relationships.
1064 Additionally, some of the keywords are grouped into collections which
1065 has been mirrored in the SKOS version.
1066 The vocabulary contains no definitions or related links as these are
1067 not provided in the original keyword list, and only a handful of
1068 alternative labels and scope notes that are present in the original
1069 keyword list.
1070 </p>
1071
1072 </div>
1073
1074 <div class='section' id='vocab-aoim'>
1075 <p class='title'>The AOIM Taxonomy</p>
1076
1077 <p>
1078 This vocabulary is published by the IVOA to allow images to be tagged
1079 with keywords that are relevant for the public.
1080 It consists of a set of keywords organised into an enumerated
1081 hierarchical structure.
1082 Each term consists of a taxonomic number and a label.
1083 There are no definitions, scope notes, or cross references.
1084 </p>
1085
1086 <p>When converting the AOIM into SKOS, it was decided to model the
1087 taxonomic number as an alternative label.
1088 Since there are duplication of terms, the token for a term consists of
1089 the full hierarchical location of the term.
1090 Thus, it is possible to distinguish between</p>
1091 <pre>
1092 Planet -> Feature -> Surface -> Canyon
1093 </pre>
1094 <p>and</p>
1095 <pre>
1096 Planet -> Satellite -> Feature -> Surface -> Canyon
1097 </pre>
1098 <p>which have the tokens <code>PlanetFeatureSurfaceCanyon</code> and
1099 <code>PlanetSatelliteFeatureSurfaceCanyon</code> respectively.
1100 </p>
1101
1102 </div>
1103
1104 <div class='section' id='vocab-ucd1'>
1105 <p class='title'>The UCD1+ Vocabulary</p>
1106
1107 <p>The UCD standard is an officially sanctioned and managed vocabulary
1108 of the IVOA. The normative document is a simple text file containing
1109 entries consisting of tokens (e.g. <code>em.IR</code>), a short
1110 description, and usage information (<q>syntax codes</q> which permit
1111 UCD tokens to be concatenated). The form of the tokens implies a
1112 natural hierarchy: <code>em.IR.8-15um</code> is obviously a narrower
1113 term than <code>em.IR</code>, which in turn is narrower than
1114 <code>em</code>.</p>
1115
1116 <p>Given the structure of the UCD1+ vocabulary, the natural
1117 translation to SKOS consists of preferred labels equal to the original
1118 tokens (the UCD1 words include dashes and periods), vocabulary tokens
1119 created using guidelines in <span class='xref'
1120 >practices</span> (e.g., "emIR815Um" for
1121 <code>em.IR.8-15um</code>), direct use of the definitions, and the syntax codes
1122 placed in usage documentation: <code>&lt;skos:scopeNote&gt;UCD syntax code: P&lt;/skos:scopeNote&gt;</code></p>
1123
1124 <p>Note that the SKOS document containing the UCD1+ vocabulary does
1125 NOT consistute the official version: the normative document is still
1126 the text list. However, on the long term, the IVOA may decide to make
1127 the SKOS version normative, since the SKOS version contains all of the
1128 information contained in the original text document but has the
1129 advantage of being in a standard format easily read and used by any
1130 application on the semantic web whilst still being usable in the
1131 current ways.</p>
1132
1133 </div>
1134
1135 <div class='section' id='vocab-iau93'>
1136 <p class='title'>The 1993 IAU Thesaurus</p>
1137
1138 <p>The IAU Thesaurus consists of concepts with mostly capitalised
1139 labels and a rich set of thesaurus relationships (<q>BT</q> for
1140 "broader term", <q>NT</q> for <q>narrower term</q>, and <q>RT</q> for
1141 <q>related term</q>). The thesaurus also contains <q>U</q> (for
1142 <q>use</q>) and <q>UF</q> (<q>use for</q>) relationships. In a SKOS
1143 model of a vocabulary these are captured as alternative labels. A
1144 separate document contains translations of the vocabulary terms in
1145 five languages: English, French, German, Italian, and
1146 Spanish. Enumerable concepts are plural (e.g. <q>SPIRAL
1147 GALAXIES</q>) and non-enumerable concepts are singular
1148 (e.g. <q>STABILITY</q>). Finally, there are some usage hints like
1149 <q>combine with other</q></p>
1150
1151 <p>In converting the IAU Thesaurus to SKOS, we have been as faithful
1152 as possible to the original format of the thesaurus. Thus, preferred
1153 labels have been kept in their uppercase format.</p>
1154
1155 <p>The IAU Thesaurus has been unmaintained since its initial production in
1156 1993; it is therefore significantly out of date in places. This
1157 vocabulary is published for the sake of completeness, and to make the
1158 link between the evolving vocabulary work and any uses of the 1993
1159 vocabulary which come to light. We do not expect to make any future
1160 maintenance changes to this vocabulary, and would expect the IVOAT
1161 vocabulary, based on this one, to be used instead (see <span class='xref'>vocab-ivoat</span>).</p>
1162
1163 </div>
1164
1165 <div class='section' id='vocab-ivoat'>
1166 <p class='title'>Towards an IVOA Thesaurus</p>
1167
1168 <p>While it is true that the adoption of SKOS will make it easy to
1169 publish and access different astronomical vocabularies, the fact is
1170 that there is no vocabulary which makes it easy to jump-start the
1171 use of vocabularies in generic astrophysical VO applications: each of
1172 the previously developed vocabularies has their own limits and
1173 biases. For example, the IAU Thesaurus provides a large number of
1174 entries, copious relationships, and translations to four other languages,
1175 but there are no definitions, many concepts are now only useful for
1176 historical purposes (e.g. many photographic or historical instrument
1177 entries), some of the relationships are false or outdated, and many
1178 important or newer concepts and their common abbreviations are
1179 missing.</p>
1180
1181 <p>Despite its faults, the IAU Thesaurus constitutes a very extensive
1182 vocabulary which could easily serve as the basis vocabulary once
1183 we have removed its most egregious faults and extended it to cover the
1184 most obvious semantic holes. To this end, a heavily revised IAU
1185 thesaurus is in preparation for use within the IVOA and other
1186 astronomical contexts. The goal is to provide a general vocabulary
1187 foundation to which other, more specialised, vocabularies can be added
1188 as needed, and to provide a good <q>lingua franca</q> for the creation of
1189 vocabulary mappings.</p>
1190 </div>
1191 </div> <!-- End: Example vocabularies -->
1192
1193 <div class='section' id='distmappings'>
1194 <p class='title'>Example Mapping</p>
1195
1196 <p>To show how mappings can be expressed between two vocabularies, we
1197 have provided one example mapping document which maps the concepts in
1198 the A&amp;A Keywords vocabulary to the concepts in the AOIM
1199 vocabulary.
1200 All four types of mappings were required.
1201 Since all the mapping relationships have inverse relationships
1202 defined, the mapping document can also be used to infer the set of
1203 mappings from the AOIM vocabulary to the A&amp;A keywords.</p>
1204
1205 <p>To provide provenence information about the set of mappings in a
1206 document, Dublin Core metadata is included in the mapping
1207 document.</p>
1208
1209 <p><span class='todo'>To come</span></p>
1210
1211 </div>
1212
1213 <div class="appendices">
1214
1215 <div class="section-nonum" id="bibliography">
1216 <p class="title">Bibliography</p>
1217 <?bibliography rm-refs ?>
1218 </div>
1219
1220 <p style="text-align: right; font-size: x-small; color: #888;">
1221 $Revision$ $Date$
1222 </p>
1223
1224 </div>
1225
1226 </body>
1227 </html>

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