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10 <title>Vocabularies in the Virtual Observatory</title>
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45 <a href="http://www.ivoa.net/"><img alt="IVOA" src="http://www.ivoa.net/pub/images/IVOA_wb_300.jpg" width="300" height="169"/></a>
46
47 <h1>Vocabularies in the Virtual Observatory<br/>Version @VERSION@</h1>
48
49 <!-- for release: uncomment this text... -->
50 <h2>IVOA Working Draft, @RELEASEDATE@</h2>
51 <!-- for editors' drafts, uncomment this text...-->
52 <!--
53 <h2>IVOA Working Draft, @RELEASEDATE@ [Editors' draft]</h2>
54 <p><strong>$Revision$ $Date$</strong></p>
55 -->
56
57 <dl>
58
59 <dt>This version</dt>
60 <dd><span class='url'>@DOCURI@.html</span></dd>
61
62 <dt>Latest version</dt>
63 <dd><span class='url'>http://www.ivoa.net/Documents/latest/Vocabularies.html</span><br/>
64 and <a href='@ISSUESLIST@' >issues list</a></dd>
65
66 <dt>Previous version</dt>
67 <dd><span class='url'>http://www.ivoa.net/Documents/WD/Semantics/vocabularies-20080320.html</span></dd>
68
69 <dt>Working Group</dt>
70 <dd><em><a href="http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics">Semantics</a></em></dd>
71
72 <dt>Editors</dt>
73 <dd>Alasdair J G Gray, University of Glasgow, UK<br/>
74 <a href='http://nxg.me.uk/norman/' >Norman Gray</a>, University of
75 Leicester / University of Glasgow, UK<br/>
76 Frederic V Hessman, University of Göttingen, Germany<br/>
77 Andrea Preite Martinez, INAF, Italy</dd>
78
79 <dt>Authors</dt>
80 <dd>
81 <span property="dc:creator">Sébastien Derriere</span>,
82 <span property="dc:creator">Alasdair J G Gray</span>,
83 <span property="dc:creator">Norman Gray</span>,
84 <span property="dc:creator">Frederic V Hessman</span>,
85 <span property="dc:creator">Tony Linde</span>,
86 <span property="dc:creator">Andrea Preite Martinez</span>,
87 <span property="dc:creator">Rob Seaman</span> and
88 <span property="dc:creator">Brian Thomas</span>
89 </dd>
90 </dl>
91 <hr/>
92 </div>
93
94 <div class="section-nonum" id="abstract">
95 <p class="title">Abstract</p>
96
97 <div class="abstract">
98 <p>As the astronomical information processed within the <em>Virtual Observatory
99 </em> becomes more complex, there is an increasing need for a more
100 formal means of identifying quantities, concepts, and processes not
101 confined to things easily placed in a FITS image, or expressed in a
102 catalogue or a table. We propose that the IVOA adopt a standard
103 format for vocabularies based on the W3C's <em>Resource Description
104 Framework</em> (RDF) and <em>Simple Knowledge Organisation System</em>
105 (SKOS). By adopting a standard and simple format, the IVOA will
106 permit different groups to create and maintain their own specialised
107 vocabularies while letting the rest of the astronomical community
108 access, use, and combine them. The use of current, open standards
109 ensures that VO applications will be able to tap into resources of the
110 growing semantic web. Several examples of useful astronomical
111 vocabularies are provided, including work on a common IVOA thesaurus
112 intended to provide a semantic common base for VO applications.</p>
113 </div>
114
115 </div>
116
117 <div class="section-nonum" id="status">
118 <p class="title">Status of this document</p>
119
120 <p>This is an IVOA Working
121 Draft. The first release of this document was
122 <span property="dc:date">2008 March 20</span>.</p>
123
124 <p>This document is an IVOA Working Draft for review by IVOA members
125 and other interested parties. It is a draft document and may be
126 updated, replaced, or obsoleted by other documents at any time. It is
127 inappropriate to use IVOA Working Drafts as reference materials or to
128 cite them as other than <q>work in progress</q>.</p>
129
130 <p>A list of current IVOA Recommendations and other technical
131 documents can be found at
132 <span class='url' >http://www.ivoa.net/Documents/</span>.</p>
133
134 <p>This document includes a normative reference to the W3C SKOS
135 standard <span class='cite'>std:skosref</span>, despite the fact that,
136 at the time this present document was standardised, the SKOS document
137 was still a W3C Working Draft and thus a <q>work in progress</q>. The
138 core part of the SKOS standard which this standard refers to (that is,
139 the concept schemes, documentation and intravocabulary relationship
140 vocabularies) are stable, and are very unlikely to change before
141 Recommendation. When the SKOS document becomes a W3C Recommendation,
142 we will issue a minor update to this present document referring to the
143 finalised SKOS standard, and incorporating any errata which have
144 appeared by then.</p>
145
146 <h3>Acknowledgments</h3>
147
148 <p>We would like to thank the members of the IVOA semantic working
149 group for many interesting ideas and fruitful discussions.</p>
150 </div>
151
152 <h2><a id="contents" name="contents">Table of Contents</a></h2>
153 <?toc?>
154
155 <hr/>
156
157 <div class="section" id="introduction">
158 <p class="title">Introduction (informative)</p>
159
160 <div class="section" id='astrovocab'>
161 <p class="title">Vocabularies in astronomy</p>
162
163 <p>Astronomical information of relevance to the Virtual Observatory
164 (VO) is not confined to quantities easily expressed in a catalogue or
165 a table.
166 Fairly simple things such as position on the sky, brightness in some
167 units, times measured in some frame, redshifts, classifications or
168 other similar quantities are easily manipulated and stored in VOTables
169 and can currently be identified using IVOA Unified Content Descriptors
170 (UCDs) <span class="cite">std:ucd</span>.
171 However, astrophysical concepts and quantities use a wide variety of
172 names, identifications, classifications and associations, most of
173 which cannot be described or labelled via UCDs.</p>
174
175 <p>There are a number of basic forms of organised semantic knowledge
176 of potential use to the VO. Informal <q>folksonomies</q> are at one
177 extreme, and are a very lightly coordinated collection of labels
178 chosen by users.
179 A slightly more formal structure is a <q>vocabulary</q>, where the label is drawn from a predefined set of definitions which can include relationships to other labels;
180 vocabularies are primarily associated with searching and browsing
181 tasks.
182 At the other extreme are <q>ontologies</q>, where the domain
183 is formally captured in a set of logical classes, typically related in
184 a subclass hierarchy. More formal definitions are presented later in
185 this document.
186 </p>
187
188 <p>An astronomical ontology is necessary if we are to have a computer
189 (appear to) <q>understand</q> something of the domain.
190 There has been some progress towards creating an ontology of
191 astronomical object types <span
192 class="cite">std:ivoa-astro-onto</span> to meet this need.
193 However there are distinct use cases for letting human users find
194 resources of interest through search and navigation of the information space.
195 The most appropriate technology to meet these use cases derives from
196 the Information Science community, that of <em>controlled
197 vocabularies, taxonomies and thesauri</em>.
198 In the present document, we do not distinguish between controlled
199 vocabularies, taxonomies and thesauri, and use the term
200 <em>vocabulary</em> to represent all three.
201 </p>
202
203 <p>One of the best examples of the need for a simple vocabulary within
204 the VO is VOEvent <span class="cite">std:voevent</span>, the VO
205 standard for supporting rapid notification of astronomical events.
206 This standard requires some formalised indication of what a published
207 event is <q>about</q>, in a formalism which can be used straightforwardly
208 by the developer of relevant services. See <span class='xref'
209 >usecases</span> for further discussion.</p>
210
211 <p>A number of astronomical vocabularies have been created, with a
212 variety of goals and intended uses. Some examples are detailed below. </p>
213
214 <ul>
215
216 <li>The <em>Second Reference Dictionary of the Nomenclature of
217 Celestial Objects</em> <span class="cite">lortet94</span>, <span
218 class="cite">lortet94a</span> contains 500 paper pages of astronomical
219 nomenclature</li>
220
221 <li>For decades professional journals have used a set of reasonably
222 compatible keywords to help classify the content of whole articles.
223 These keywords have been analysed by Preite Martinez &amp; Lesteven
224 <span class="cite">preitemartinez07</span>, who derived a
225 set of common keywords constituting one of the potential bases for a
226 fuller VO vocabulary. The same authors also attempted to derive a set
227 of common concepts by analysing the contents of abstracts in journal
228 articles, which should comprise a list of tokens/concepts more
229 up-to-date than the old list of journal keywords. A similar but less
230 formal attempt was made by Hessman <span class='cite'>hessman05</span>
231 for the VOEvent working group, resulting in a similar list.</li>
232
233 <li>Astronomical databases generally use simple sets of keywords
234 – sometimes hierarchically organised – to help users make queries.
235 Two examples from very
236 different contexts are the list of object types used in the <a
237 href="http://simbad.u-strasbg.fr">Simbad</a> database and the search
238 keywords used in the educational Hands-On Universe image database
239 portal.</li>
240
241 <li>The Astronomical Outreach Imagery (AOI) working group has created
242 a simple taxonomy for helping to classify images used for educational
243 or public relations <span class="cite">std:avm</span>. See
244 <span class='xref'>vocab-avm</span>.</li>
245
246 <!--
247 <li>The Hands-On Universe project (see <span class='url'
248 >http://sunra.lbl.gov/telescope2/index.html</span>) has maintained a
249 public database of images for use by the general public since the
250 1990s. The images are very heterogeneous, since they are gathered from
251 a variety of professional, semi-professional, amateur, and school
252 observatories, so a simple taxonomy is used to facilitate browsing
253 by the users of the database.</li>
254 -->
255
256 <li>In 1993, Shobbrook and Shobbrook published an Astronomy Thesaurus
257 endorsed by the IAU <span class='cite' >shobbrook93</span>. This
258 collection of nearly 3000 terms, in five languages, is a valuable
259 resource, but has seen little use in recent years. Its very size,
260 which gives it expressive power, is a disadvantage to the extent that
261 it is consequently hard to use. See <span class='xref'>vocab-iau93</span>.</li>
262
263 <li>The VO's Unified Content Descriptors <span class='cite'
264 >std:ucd</span> (UCD) constitute the main controlled vocabulary of the
265 IVOA and contain some taxonomic information. However, UCD has some
266 features which supports its goals, but which make it difficult to use
267 beyond the present applications of labelling VOTables: firstly, there
268 is no standard means of identifying and processing the contents of the
269 text-based reference document; secondly, the content cannot be openly
270 extended beyond that set by a formal IVOA committee without going
271 through a laborious and time-consuming negotiation process of
272 extending the primary vocabulary itself; and thirdly, the UCD
273 vocabulary is primarily concerned with data types and their
274 processing, and only peripherally with astronomical objects (for
275 example, it defines formal labels for RA, flux, and bandpass, but does
276 not mention the Sun). See <span class='xref'>vocab-ucd1</span>.</li>
277
278 </ul>
279 </div>
280
281 <div class='section' id='usecases'>
282 <p class='title'>Use-cases, and the motivation for formalised vocabularies</p>
283
284 <p>The most immediate high-level motivation for this work is the
285 requirement of the VOEvent standard <span class='cite'
286 >std:voevent</span> for a controlled vocabulary usable in the
287 VOEvent's <code>&lt;Why/&gt;</code> and <code>&lt;What/&gt;</code>
288 elements, which describe what
289 sort of object the VOEvent packet is describing, in some broadly
290 intelligible way. For example a <q>burst</q> might be a gamma-ray burst
291 due to the collapse of a star in a distant galaxy, a solar flare, or
292 the brightening of a stellar or AGN accretion disk, and having an
293 explicit list of vocabulary terms can help guide the event publisher
294 into using a term which will be usefully precise for the event's
295 consumers. A free-text label can help here (which brings us into the
296 domain sometimes referred to as folksonomies), but the astronomical
297 community, with a culture sympathetic to international agreement, can
298 do better.</p>
299
300 <p>The purpose of this proposal is to establish a set of conventions for
301 the creation, publication, use, and manipulation of
302 astronomical vocabularies within the Virtual Observatory, based upon
303 the W3C's SKOS standard. We include as appendices to this proposal
304 formalised versions of a number of existing vocabularies, encoded as
305 SKOS vocabularies <span class="cite">std:skosref</span>.</p>
306
307 <p>Specific use-cases include the following.</p>
308 <ul>
309 <li>A user wishes to process all events concerning supernovae, which
310 means that an event concerning a type 1a supernova must be understood to be
311 relevant. [This supports a system working autonomously, filtering
312 incoming information]</li>
313
314 <li>A user is searching an archive of VOEvents for microlensing
315 events, and retrieves a large number of them; the search interface may
316 then prompt them to narrow their search using one of a set of terms
317 including, say, binary lens events. [This supports so-called <q>semantic
318 search</q>, providing semantic support to an interface which is in turn
319 supporting a user]</li>
320
321 <li>A user wishes to search for resources based on the
322 journal-supported keywords in a paper; they might either initiate this by
323 hand, or have this done on their behalf by a tool which can extract
324 the keywords from a PDF. The keywords are in the A&amp;A vocabulary,
325 and mappings have been defined between this vocabulary and others,
326 which means that the query keywords are translated automatically
327 into those appropriate for a search of an outreach image database
328 (everyone likes pretty pictures), the VO Registry, a set of Simbad
329 object types, and one or more concepts in more formal ontologies. The
330 search interface is then able to support the user browsing up and down
331 the AVM vocabulary, and a specialised Simbad tool is able to take
332 over the search, now it has an appropriate starting place. [This
333 supports interoperability, building on the investments which
334 institutions and users have made in existing vocabularies]</li>
335
336 <li>A user receives a VOTable of results from a VO application – for
337 example a catalogue of objects or observations – and wants to search a
338 database of old FITS files for potential matches. Because the UCDs
339 labeling the columns of the tables are expressed in well-documented
340 SKOS, both the official descriptions of the UCDs and their semantic
341 matches to a variety of other plain-text vocabularies (such as the IAU
342 or AVM thesauri) are available to the VO application, providing a basis
343 for massive searches for all kinds of FITS keyword values.</li>
344
345 </ul>
346
347 <p>The goal of this standard is to show how vocabularies can be easily
348 expressed in an interoperable and computer-manipulable format, and the
349 sole normative section of this Recommendation (namely section <span
350 class='xref'>publishing</span>) contains requirements and suggestions
351 intended to promote this. Four example vocabularies that have
352 previously been expressed using non-standardized formats – namely the
353 A&amp;A keyword list, the IAU thesaurus and AVM taxonomy, and UCD1 – are
354 included below as illustrations of how simple it is to publish them in
355 SKOS, without losing any of the information of the original source
356 vocabularies.</p>
357
358 <p>It is not a goal of this standard, as it is not a goal of SKOS, to
359 produce knowledge-engineering artefacts which can support elaborate
360 machine reasoning – such artefacts would be very valuable, but require
361 much more expensive work on ontologies. As the supernova use-case
362 above illustrates, even simple vocabularies can support useful machine
363 reasoning.</p>
364
365 <p>It is also not a goal of this standard to produce new vocabularies,
366 or substantially alter existing ones; instead, the vocabularies
367 included below in section <span class='xref'>distvocab</span> are directly
368 derived from existing vocabularies (the exceptions are the IVOAT
369 vocabulary, which is ultimately intended to be a significant update to
370 the IAU-93 original, and the constellations vocabulary, which is
371 intended to be purely didactic). It therefore follows that the ambiguities,
372 redundancies and incompleteness of the source vocabularies are
373 faithfully represented in the distributed SKOS vocabularies. We hope
374 that this formalisation process will create greater visibility and
375 broader use for the various vocabularies, and that this will guide the
376 maintenance efforts of the curating groups.</p>
377
378 <p>The reason for both of these limitations is that vocabularies are
379 extremely expensive to produce, maintain and deploy, and we must
380 therefore rely on such vocabularies as have been developed, and
381 attached as metadata to resources, by others. Such vocabularies are
382 less rich or less coherent than we might prefer, but they are widely enough
383 deployed to be useful. We hope that the set of example vocabularies
384 we have provided will build on this deployment, by providing material
385 which is useful out of the box.</p>
386
387 </div>
388
389 <div class="section" id='formalising'>
390 <p class="title">Formalising and managing multiple vocabularies</p>
391
392 <p>We find ourselves in the situation where there are multiple
393 vocabularies in use, describing a broad range of resources of interest
394 to professional and amateur astronomers, and members of the public.
395 These different vocabularies use different terms and different
396 relationships to support the different constituencies they cater for.
397 For example, <q>delta Sct</q> and <q>RR Lyr</q> are terms one would
398 find in a vocabulary aimed at professional astronomers, associated
399 with the notion of <q>variable star</q>; however one would
400 <em>not</em> find such technical terms in a vocabulary intended to
401 support outreach activities.</p>
402
403 <p>One approach to this problem is to create a single consensus
404 vocabulary, which draws terms from the various existing vocabularies
405 to create a new vocabulary which is able to express anything its users
406 might desire. The problem with this is that such an effort would be
407 very expensive, both in terms of time and effort on the part of those
408 creating it, and to the potential users, who have to learn
409 to navigate around it, recognise the new terms, and who have to be
410 supported in using the new terms correctly (or, more often,
411 incorrectly).</p>
412
413 <p>The alternative approach to the problem is to evade it, and this is
414 the approach taken in this document. Rather than deprecating the
415 existence of multiple overlapping vocabularies, we embrace it,
416 help interest groups formalise as many of them as are appropriate, and
417 standardise the process of formally declaring the relationships between
418 them. This means that:</p>
419 <ul>
420 <li>The various vocabularies are allowed to evolve separately, on
421 their own timescales, managed either by the IVOA, individual working
422 groups within the IVOA, or by third parties;</li>
423
424 <li>Specialised vocabularies can be developed and maintained by the
425 community with the most knowledge about a specific topic, ensuring
426 that the vocabulary will have the most appropriate breadth, depth, and
427 precision;</li>
428
429 <li>Users can choose the vocabulary or combination of vocabularies most
430 appropriate to their situation, either when annotating resources, or
431 when querying them; and</li>
432
433 <li>We can retain the previous investments made in vocabularies by
434 users and resource owners.</li>
435
436 </ul>
437
438
439 </div>
440
441 </div>
442
443 <div class='section' id='skos'>
444 <p class='title'>SKOS-based vocabularies (informative)</p>
445
446 <p>In this section, we introduce the concepts of SKOS-based
447 vocabularies, and the technology of mapping between them. We describe
448 some additional requirements for IVOA vocabularies in the next
449 section, <span class='xref' >publishing</span>.</p>
450
451 <div class="section" id='vocab'>
452 <p class="title">Selection of the vocabulary format</p>
453
454 <p>After extensive online and face-to-face discussions, the authors have
455 brokered a consensus within the IVOA community that
456 formalised vocabularies should be published at least in SKOS (Simple Knowledge
457 Organisation System) format, a W3C draft standard application of RDF to the
458 field of knowledge organisation <span
459 class="cite">std:skosref</span>. SKOS draws on long experience
460 within the Library and Information Science community, to address a
461 well-defined set of problems to do with the indexing and retrieval of
462 information and resources; as such, it is a close match to the problem
463 this document is addressing.</p>
464
465 <p>ISO 5964 <span class='cite' >std:iso5964</span> defines a number of
466 the relevant terms (ISO 5964:1985=BS 6723:1985; see also <span
467 class='cite' >std:bs8723-1</span> and <span class='cite'
468 >std:z39.19</span>), and some of the (lightweight) theoretical
469 background. The only technical distinction relevant to this document
470 is that between vocabulary and thesaurus: BS-8723-1 defines a
471 controlled vocabulary as a</p>
472 <blockquote>
473 prescribed list of terms or headings each one having an assigned meaning
474 [noting that <q>Controlled vocabularies are designed for use in
475 classifying or indexing documents and for searching them.</q>]
476 </blockquote>
477 <p>and a thesaurus as a</p>
478 <blockquote>
479 Controlled vocabulary in which concepts are represented by preferred
480 terms, formally organized so that paradigmatic relationships between
481 the concepts are made explicit, and the preferred terms are
482 accompanied by lead-in entries for synonyms or quasi-synonyms.
483 <!-- NOTE:
484 The purpose of a thesaurus is to guide both the indexer and the
485 searcher to select the same preferred term or combination of preferred
486 terms to represent a given subject. -->
487 (BS-8723-1, sect. 2.39)
488 </blockquote>
489 <p>with a similar definition in ISO-5964 sect. 3.16.</p>
490
491 <p>The paradigmatic relationships in question are those relating a
492 term to a <q>broader</q>, <q>narrower</q> or more generically
493 <q>related</q> term. These notions have an operational definition:
494 any resource retrieved as a result of a search on a given term will
495 also be retrievable through a search on that term's <q>broader
496 term</q> (<q>narrower</q> is a simple inverse, so that for any pair of
497 terms, if <code>A skos:broader B</code>, then <code>B skos:narrower
498 A</code>; a term may have multiple narrower and broader terms). This
499 is not a subsumption relationship, as there is no implication that the
500 concept referred to by a narrower term is of the same <em>type</em> as
501 a broader term. Further, the <code>skos:broader</code> and
502 <code>skos:narrower</code> relationships are not transitive (that is,
503 declaring that if <code>A skos:broader B</code> and <code>B
504 skos:broader C</code> does not imply that <code>A skos:broader
505 C</code>). However the SKOS standard includes the notions of
506 <code>skos:broaderTransitive</code> and
507 <code>skos:narrowerTransitive</code> relations for the subset of
508 vocabularies and systems which would find these useful.</p>
509
510 <p>Thus <strong>a vocabulary (SKOS or otherwise) is not an
511 ontology</strong>. It has lighter and looser semantics than an
512 ontology, and is specialised for the restricted case of resource
513 retrieval. Those interested in ontological analyses can easily
514 transfer the vocabulary relationship information from SKOS to a formal
515 ontological format such as OWL <span class='cite' >std:owl</span>.</p>
516
517 <p>The purpose of a thesaurus is to help users find resources they
518 might be interested in, be they library books, image archives, or VOEvent
519 packets.</p>
520
521 </div>
522
523 <div class='section' id='skos-format'>
524 <p class='title'>Content and format of a SKOS vocabulary</p>
525
526 <p>A published vocabulary in SKOS format consists of a set of
527 <q>concepts</q> – an example concept capturing the
528 vocabulary information about spiral galaxies is provided in the <a
529 href='#figexample' >Figure below</a>, with the RDF shown in both
530 RDF/XML <span class='cite' >std:rdfxml</span> and Turtle notation <span
531 class='cite' >std:turtle</span> (Turtle is similar to the more
532 informal <em>Notation3</em>). The elements of a concept are detailed
533 below.</p>
534
535 <center>
536 <p><a name='figexample' >Figure: examples of SKOS vocabularies</a></p>
537 <table>
538 <tr>
539 <th class='rdfxml'>XML Syntax</th>
540 <th width="10"/>
541 <th class='turtle'>Turtle Syntax</th>
542 </tr>
543 <tr><td/></tr>
544 <tr>
545 <td class='rdfxml'>
546 <pre class='rdfxml'>
547 &lt;skos:Concept rdf:about="#spiralGalaxy"&gt;
548 &lt;skos:prefLabel lang="en"&gt;
549 spiral galaxy
550 &lt;/prefLabel&gt;
551 &lt;skos:prefLabel lang="de"&gt;
552 Spiralgalaxie
553 &lt;/prefLabel&gt;
554 &lt;skos:altLabel lang="en"&gt;
555 spiral nebula
556 &lt;/skos:altLabel&gt;
557 &lt;skos:hiddenLabel lang="en"&gt;
558 spiral glaxy
559 &lt;/hiddenLabel&gt;
560 &lt;skos:definition lang="en"&gt;
561 A galaxy having a spiral structure.
562 &lt;/skos:definition&gt;
563 &lt;skos:scopeNote lang="en"&gt;
564 Spiral galaxies fall into one of
565 three catagories: Sa, Sc, and Sd.
566 &lt;/skos:scopeNote&gt;
567 &lt;skos:narrower
568 rdf:resource="#barredSpiralGalaxy"/&gt;
569 &lt;skos:broader
570 rdf:resource="#galaxy"/&gt;
571 &lt;skos:related
572 rdf:resource="#spiralArm"/&gt;
573 &lt;/skos:Concept&gt;
574 </pre>
575 </td>
576 <td/>
577 <td class='turtle'>
578 <pre class='turtle'>
579 &lt;#spiralGalaxy&gt; a skos:Concept;
580 skos:prefLabel
581 "spiral galaxy"@en,
582 "Spiralgalaxie"@de;
583 skos:altLabel "spiral nebula"@en;
584 skos:hiddenLabel "spiral glaxy"@en;
585 skos:definition """A galaxy having a
586 spiral structure."""@en;
587 skos:scopeNote """Spiral galaxies fall
588 into one of three categories:
589 Sa, Sc, and Sd"""@en;
590 skos:narrower &lt;#barredSpiralGalaxy&gt;;
591 skos:broader &lt;#galaxy&gt;;
592 skos:related &lt;#spiralArm&gt; .
593 </pre>
594 </td>
595 </tr>
596 </table>
597 </center>
598
599 <p>A SKOS vocabulary includes the following features.</p>
600
601 <ul>
602
603 <li>A single URI representing the concept, mainly for use by computers.
604 <!--
605 <code>&lt;#spiralGalaxy&gt; a skos:Concept</code>.
606 <code>&lt;skos:Concept rdf:about="#spiralGalaxy"&gt;</code>
607 -->
608 </li>
609
610 <li>A single prefered label in each supported language of the
611 vocabulary, for use by humans.
612 <!--
613 <code>skos:prefLabel "spiral galaxy"@en, "Spiralgalaxie"@de</code>.
614 <code>&lt;skos:prefLabel&gt;spiral galaxy&lt;/skos:prefLabel&gt;</code>
615 -->
616 </li>
617
618 <li>Optional alternative labels which applications may encounter or in
619 common use, whether simple synonyms or commonly-used aliases such as
620 <q>GRB</q> for "gamma-ray burst", or <q>Spiral nebula</q> for
621 spiral galaxies.
622 <!--
623 <code>skos:altLabel "GRB"@en</code>
624 <code>&lt;skos:altLabel lang="de"&gt;Spiralgalaxie&lt;/skos:altLabel&gt;</code>
625 -->
626 </li>
627
628 <li>Optional hidden labels which capture terms which are sometimes
629 used for the corresponding concept, but which are deprecated in some
630 sense. This might include common misspellings for
631 either the preferred or alternate labels, for example <q>glaxy</q> for
632 <q>galaxy</q>.
633 </li>
634
635 <li>A definition for the concept, where one exists in the original
636 vocabulary, to clarify the meaning of the term.
637 <!--
638 <code>skos:definition "A galaxy having a spiral structure."@en</code>
639 <code>&lt;skos:definition lang="en"&gt;<br/>A galaxy having a spiral structure.<br/>&lt;/skos:definition&gt;</code>
640 -->
641 </li>
642
643 <li>A scope note to further clarify a definition, or the usage of the
644 concept.
645 <!--
646 <code>skos:scopeNote "Spiral galaxies fall into one of three categories: Sa, Sc, and Sd"@en</code>
647 <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>
648 -->
649 </li>
650
651 <li>Optionally, a concept may be involved in any number of relationships
652 to other concepts. The types of relationships are
653 <ul>
654 <li>Narrower or more specific concepts, for example a link to the concept
655 representing a <q>barred spiral galaxy</q>.
656 <!--
657 <code>skos:narrower &lt;#barredSpiralGalaxy&gt;</code>.
658 <code>&lt;skos:narrower rdf:resource="#barredSpiralGalaxy"&gt;</code>
659 -->
660 </li>
661 <li>Broader or more general concepts, for example a link to the token
662 representing galaxies in general.
663 <!--
664 <code>skos:broader &lt;#galaxy&gt;</code>.
665 <code>&lt;skos:broader rdf:resource="#galaxy"&gt;</code>
666 -->
667 </li>
668 <li>Related concepts, for example a link to the token representing spiral
669 arms of galaxies
670 <!--
671 <code>skos:related &lt;#spiralArm&gt;</code>
672 <code>&lt;skos:related rdf:resource="#spiralArm"&gt;</code>
673 -->
674 <br/>
675 (note this relationship does not say that spiral galaxies have spiral
676 arms – that would be ontological information of a higher order which
677 is beyond the requirements for information stored in a vocabulary).</li>
678 </ul>
679 </li>
680 </ul>
681
682 <p>In addition to the information about a single concept, a vocabulary
683 can contain information to help users navigate its structure and
684 contents:</p>
685 <ul>
686 <li>The <q>top concepts</q> of the vocabulary, i.e. those that occur
687 at the top of the vocabulary hierarchy defined by the broader/narrower
688 relationships, can be explicitly stated to make it easier to navigate
689 the vocabulary.</li>
690
691 <li>Concepts that form a natural group can be defined as being members
692 of a <q>collection</q>.</li>
693
694 <li>Versioning information can be added using change notes.</li>
695
696 <li>Additional metadata about the vocabulary, for example indicating
697 the publisher, may be documented using the Dublin Core metadata set
698 <span class='cite' >std:dublincore</span>, <span
699 class='cite'>std:pubguide</span>. At a minimum, the vocabulary's
700 <code>skos:ConceptScheme</code> should be annotated with DC title,
701 creator, description and date terms.</li>
702
703 <li>The SKOS standard describes a number of <q>documentation
704 properties</q>; these should be used to document provenance of and
705 changes to vocabulary terms.</li>
706
707 <li>A set of mappings between vocabularies has the potential to be
708 circular or create inconsistencies, though this is probably reasonably
709 unlikely in fact. This is in principle out of the
710 control of the vocabulary authors, since vocabularies do not contain
711 mappings, and so this can only be detected dynamically by applications
712 which use the vocabularies.</li>
713 </ul>
714 </div>
715
716
717 <div class='section' id='skos-relationships'>
718 <p class='title'>Mapping relationships between vocabularies</p>
719
720 <p>There already exist several vocabularies in the domain of astronomy.
721 Instead of attempting to replace all these existing vocabularies,
722 which have been developed to achieve different aims and user groups,
723 we embrace them.
724 This requires a mechanism to relate the concepts in the different
725 vocabularies.</p>
726
727 <p>Part of the SKOS working draft standard <span class='cite'>std:skosref</span>
728 allows a concept in one vocabulary to be related to a concept in
729 another vocabulary.
730 There are four types of relationship provided to capture the
731 relationships between concepts in vocabularies, which are similar to
732 those defined for relationships between concepts within a single
733 vocabulary.
734 The types of mapping relationships are as follows.</p>
735
736 <ul>
737
738 <li>
739 Equivalence between concepts, i.e. the concepts in the different
740 vocabularies refer to the same real world entity.
741 This is captured with the RDF statement
742 <blockquote>
743 <code>AAkeys:#Cosmology skos:exactMatch avm:#Cosmology</code>
744 </blockquote>
745 which states that the cosmology concept in the A&amp;A Keywords is the
746 same as the cosmology concept in the AVM.
747 (Note the use of an external namespaces <code>AAkeys</code> and
748 <code>avm</code> which must be defined within the document.)
749 </li>
750
751 <li>
752 Broader concept, i.e. there is not an equivalent concept but there is
753 a more general one.
754 This is captured with the RDF statement
755 <blockquote>
756 <code>AAkeys:#Moon skos:broadMatch avm:PlanetSatellite</code>
757 </blockquote>
758 which states that the AVM concept <q>Planet Satellite</q> is a more general
759 term than the A&amp;A Keywords concept <q>Moon</q>.
760 </li>
761
762 <li>
763 Narrower concept, i.e. there is not an equivalent concept but there is
764 a more specific one.
765 This is captured with the RDF statement
766 <blockquote>
767 <code>AAkeys:#IsmClouds skos:narrowMatch
768 avm:#NebulaAppearanceDarkMolecularCloud</code>
769 </blockquote>
770 which states that the AVM concept <q>Nebula Appearance Dark Molecular
771 Cloud</q> is more specific than the A&amp;A Keywords concept <q>ISM Clouds</q>.
772 </li>
773
774 <li>
775 Related concept, i.e. there is some form of relationship.
776 This is captured with the RDF statement
777 <blockquote>
778 <code>AAkeys:#BlackHolePhysics skos:relatedMatch
779 avm:#StarEvolutionaryStageBlackHole</code>
780 </blockquote>
781 which states that the A&amp;A Keywords concept <q>Black Hole Physics</q> has
782 an association with the AVM concept <q>Star Evolutionary Stage Black Hole</q>.
783 </li>
784
785 </ul>
786
787 <p>The semantic mapping relationships have certain properties.
788 The broadMatch relationship has the narrowMatch relationship as its
789 inverse and the exactMatch and relatedMatch relationships are
790 symmetrical.
791 The consequence of these properties is that if you have a mapping from
792 concept <code>A</code> in one vocabulary to concept <code>B</code> in
793 another vocabulary then you can infer a mapping from concept
794 <code>B</code> to concept <code>A</code>.
795 </p>
796
797 <p class='todo'>At the time of writing, the SKOS document is still a
798 working draft, and may or may not end up with support for mappings in
799 the core document rather than in a companion document. This section
800 of this Working Draft, and other references to mappings below, should
801 therefore be considered as current best practice and could be updated
802 in a subsequent version of this document once the SKOS document has
803 become a standard.</p>
804
805 </div>
806
807 <div class='section' id='vocabversions'>
808 <p class='title'>Vocabulary versions</p>
809
810 <p>The document <span class='cite'>kendall08</span> discusses good
811 practice for managing RDF vocabularies. At the time of writing (2008
812 May) this is still an editor's draft, and it itself notes that good
813 practice in this area is not yet fully stable, so our recommendations
814 here are necessarily tentative, and in some places restricted to the
815 relatively small vocabularies (100s to 1000s of terms) we expect to
816 encounter in the VO. We expect to adjust or enhance this advice in
817 future editions of this Recommendation, as best practice evolves, or
818 as we gain more experience with the relevant vocabularies.</p>
819
820 <p>We must distinguish between versions of a vocabulary, and versions
821 of the description of a vocabulary. In the former case, we are
822 concerned with the presence or absence of certain concepts, such as
823 <q>star</q> or <q>GRB</q>, and expect that there will be some
824 reasonably stable relationship between the concept URI and the
825 real-world concept it refers to. In the latter case, we are concerned
826 with the technicalities of associating a concept URI with its
827 labels, its description, and with other related concept URIs. While
828 it is true that there are epistemological commitments involved in the
829 simple act of naming (and the terms <q>GRB</q> and <q>planet</q>
830 remind us that there is knowledge implicit within a name), it is the
831 latter case that generally represents the <em>knowledge</em> we have
832 of an object, and it is this knowledge which we must version.</p>
833
834 <p>In consequence, <em>the concept URIs should not carry
835 version information</em>. The partial exception to this is when a
836 vocabulary undergoes a major restructuring, as a result of the terms
837 in it becoming significantly incoherent – for example, we might
838 imagine the IAU93 thesaurus being updated to form an IAU 200x
839 thesaurus – but in this case we should regard the result as a new
840 vocabulary, rather than simply an adjusted version of an old one.</p>
841
842 <p>All the terms in the SKOS vocabulary appear in an unversioned
843 namespace, and once in the vocabulary they are not removed <span
844 class='cite'>kendall08</span>. Successive versions of the vocabulary
845 description describe the vocabulary terms as <q>unstable</q>, <q>testing</q>,
846 <q>stable</q> or <q>deprecated</q>.</p>
847 <!-- there seems to be no
848 discussion of this in a SKOS document, as opposed to commentary on
849 SKOS -->
850
851 <p>The Dublin Core namespaces are managed in a similar way <span
852 class='cite'>dc:namespaces</span>. The namespace URIs, which act as
853 common prefixes to the DC terms, and which are defined using a <q>hash
854 URI</q> strategy, in RDF terms, have no version numbers, so that
855 the namespace for the DC terms vocabulary is <span
856 class='url'>http://purl.org/dc/terms/</span>. Terms such as <span
857 class='url'>http://purl.org/dc/terms/extent</span> then 302-redirect
858 to a URL which, for administrative convenience, happens to contain a
859 release date, but which resolves to RDF which defines the unversioned
860 term <span class='url'>http://purl.org/dc/terms/extent</span>. This
861 file includes the following content (translated into Turtle from the
862 original RDF/XML for legibility).</p>
863 <pre>@prefix rdf: &lt;http://www.w3.org/1999/02/22-rdf-syntax-ns#&gt; .
864 @prefix skos: &lt;http://www.w3.org/2004/02/skos/core#&gt; .
865 @prefix dcam: &lt;http://purl.org/dc/dcam/&gt; .
866 @prefix dcterms: &lt;http://purl.org/dc/terms/&gt; .
867 @prefix rdfs: &lt;http://www.w3.org/2000/01/rdf-schema#&gt; .
868
869 &lt;http://purl.org/dc/terms/&gt;
870 dcterms:title """DCMI Namespace for metadata terms
871 in the http://purl.org/dc/terms/ namespace"""@en-us;
872 rdfs:comment """To comment on this schema,
873 please contact dcmifb@dublincore.org.""";
874 dcterms:publisher "The Dublin Core Metadata Initiative"@en-us;
875 dcterms:modified "2008-01-14" .
876
877 dcterms:extent
878 rdfs:label "Extent"@en-us;
879 rdfs:comment "The size or duration of the resource."@en-us;
880 rdfs:isDefinedBy &lt;http://purl.org/dc/terms/&gt;;
881 dcterms:issued "2000-07-11";
882 dcterms:modified "2008-01-14";
883 a rdf:Property;
884 dcterms:hasVersion &lt;http://dublincore.org/usage/terms/history/#extent-003&gt;;
885 rdfs:range dcterms:SizeOrDuration;
886 rdfs:subPropertyOf &lt;http://purl.org/dc/elements/1.1/format&gt;,
887 dcterms:format .
888 ...
889 </pre>
890 <p>This includes the definition of the (unversioned) <span class='url'
891 >http://purl.org/dc/terms/extent</span> concept, along with semantic
892 knowledge about the concept (<code>rdfs:subPropertyOf</code>) as of
893 2008-01-14, plus other editorial (<code>dcterms:modified</code>) and
894 definitional (<code>rdfs:isDefinedBy</code>) metadata.</p>
895
896 </div>
897
898 </div>
899
900 <div class='section' id='publishing'>
901 <p class='title'>Publishing vocabularies (normative)</p>
902
903 <div class='section' id='pubreq'>
904 <p class='title'>Requirements</p>
905
906 <p>A vocabulary which conforms to this IVOA standard has the following
907 features. In this section, the keywords
908 <span class='rfc2119' >must</span>,
909 <span class='rfc2119' >should</span>
910 and so on, are to be interpreted as described in <span
911 class='cite'>std:rfc2119</span>.</p>
912
913 <div class='section'>
914 <p class='title'>Dereferenceable namespace</p>
915
916 <p>The namespace of the vocabulary <span class='rfc2119'>must</span>
917 be dereferenceable on the web. That is, typing the namespace URL into
918 a web browser will produce human-readable documentation about the
919 vocabulary. In addition, the namespace URL <span class='rfc2119'
920 >should</span> return an RDF version of the vocabulary if it is
921 retrieved with one of the RDF MIME types in the HTTP Accept header.
922 At the time of writing, the only fully standardised RDF MIME type is
923 <code>application/rdf+xml</code> for RDF/XML, but
924 <code>text/rdf+n3</code> and <code>text/turtle</code> are the proposed
925 types for Notation3 <span class='cite'>notation3</span> and Turtle
926 <span class='cite'>std:turtle</span>, respectively.</p>
927
928 <p><em>Rationale: These prescriptions are intended to be compatible
929 with the patterns described in <span class='cite'>berrueta08</span>
930 and <span class='cite'>sauermann08</span>, and vocabulary distributors
931 <span class='rfc2119' >should</span> follow these patterns where
932 possible.</em></p>
933 </div>
934
935 <div class='section'>
936 <p class='title'>Long-term availability</p>
937
938 <p>The files defining a vocabulary, including those of superseded
939 versions, <span class='rfc2119' >should</span> remain permanently
940 available. There is no requirement that the namespace URL be at any
941 particular location, although the IVOA web pages, or a journal
942 publisher's web pages, would likely be suitable archival
943 locations.</p> </div>
944
945 <div class='section'>
946 <p class='title'>Distribution format</p>
947
948 <p>Vocabularies <span class='rfc2119'>must</span> be made available
949 for distribution as SKOS RDF files in RDF/XML <span
950 class='cite'>std:rdfxml</span> format. A human readable version in
951 Turtle <span class='cite'>std:turtle</span> format <span
952 class='rfc2119'>should</span> also be made available. As an
953 alternative to Turtle, vocabularies may be made available in that
954 subset of Notation3 <span class='cite'>notation3</span> which is
955 compatible with Turtle; if Turtle or Notation3 is being served, it is
956 prudent to support both <code>text/rdf+n3</code> and
957 <code>text/turtle</code> as MIME types in the <code>Accept</code>
958 header of the HTTP request. <!-- See issue <a
959 href='@ISSUESLIST@#distformat-2'>[distformat-2]</a> -->.</p>
960
961 <p>A publisher <span class='rfc2119'>may</span> make available RDF in
962 other formats, or other supporting files. A publisher <span
963 class='rfc2119'>must</span> make available at least some
964 human-readable documentation – see section <span
965 class='xref'>servevocab</span> for a discussion of the mechanics here.</p>
966
967 <p><em>Rationale: this does imply that the vocabulary source files can only
968 realistically be parsed using an RDF parser. An alternative is to
969 require that vocabularies be distributed using a subset of RDF/XML
970 which can also be naively handled as traditional XML; however as well
971 as creating an extra standardisation requirement, this would make it
972 effectively infeasible to write out the distribution version of the
973 vocabulary using an RDF or general SKOS tool.</em></p>
974 </div>
975
976 <div class='section' id='versioning'>
977 <p class='title'>Clearly versioned vocabulary</p>
978
979 <p>The vocabulary <em>namespace</em> <span class='rfc2119' >should
980 not</span> be versioned, but it <span class='rfc2119' >should</span>
981 be easy to retrieve earlier versions of the RDF describing the
982 vocabulary. See the discussion in section <span
983 class='xref'>vocabversions</span> for the rationale for this, and see
984 section <span class='xref'>servevocab</span> for a discussion of its
985 implications for the way that vocabularies are served on the web.</p>
986
987 <!-- Issue <a href='@ISSUESLIST@#versioning-3' >[versioning-3]</a>-->
988
989 </div>
990
991 <div class='section'>
992 <p class='title'>No restrictions on source files</p>
993
994 <p>This Recommendation does not place any restrictions on the format of the
995 files managed by the maintenance process, as long as the distributed
996 files are as specified above.
997 <!-- See issue
998 <a href='@ISSUESLIST@#masterformat-1' >[masterformat-1]</a> -->
999 </p>
1000 </div>
1001
1002 </div>
1003
1004 <div class='section' id='practices'>
1005 <p class='title'>Good practices of vocabulary design</p>
1006
1007 <p>This standard imposes a number of requirements on conformant
1008 vocabularies (see <span class='xref' >pubreq</span>). In
1009 this section we list a number of good practices that IVOA vocabularies
1010 <span class='rfc2119'>should</span> abide by. Some of the
1011 prescriptions below are more specific than good-practice guidelines
1012 for vocabularies in general.</p>
1013
1014 <p>The adoption of the following guidelines will make it easier to use
1015 vocabularies in generic VO applications. However, VO applications
1016 <span class='rfc2119'>should</span> be able to accept any vocabulary
1017 that complies with the latest SKOS standard
1018 <span class="cite">std:skosref</span> (this is a syntactical
1019 requirement, and does not imply that an application will necessarily
1020 understand the terms in
1021 an alien vocabulary, although the presence of mappings to a known
1022 vocabulary should allow it to derive some benefit).</p>
1023
1024 <ol>
1025
1026 <li>Concept identifiers <span class='rfc2119'>should</span> consist
1027 only of the letters a-z, A-Z, and numbers 0-9, i.e. no spaces, no
1028 exotic letters (for example umlauts), and no characters which would make a
1029 token inexpressible as part of a URI; since tokens are for use by
1030 computers only, this is not a big restriction, since the exotic
1031 letters can be used within the labels and documentation if
1032 appropriate.</li>
1033
1034 <li>The concept identifiers <span class='rfc2119'>should</span> be
1035 kept in human-readable form, directly reflecting the implied meaning, and
1036 not be semi-random identifiers only (for example, use
1037 <code>spiralGalaxy</code>, not <code>t1234567</code>); tokens <span
1038 class='rfc2119'>should</span> preferably be created via a direct
1039 conversion from the preferred label via removable/translation of
1040 non-token characters (see above) and sub-token separation via
1041 capitalisation of the first sub-token character (for example the label <code>My
1042 favourite idea-label #42</code> is converted into
1043 <code>MyFavouriteIdeaLabel42</code>).</li>
1044
1045 <li>Labels <span class='rfc2119'>should</span> be unchanged from the
1046 labels appearing in the source vocabulary. When
1047 developing a new vocabulary standard thesaurus practice indicates that
1048 english language labels for concrete concepts should be pluralised –
1049 thus <code>"galaxies"@en</code>, but <code>"astronomy"@en</code>;
1050 thesaurus practice in other european languages uses the singular for
1051 all cases.</li>
1052 <!--
1053 the singular form <span
1054 class='rfc2119'>should</span> be preferred,
1055 for example <code>spiral galaxy</code>, not <code>spiral galaxies</code>.
1056 <span class='todo'><a href="http://code.google.com/p/volute/issues/detail?id=1">Open issue</a></span></li>
1057 -->
1058
1059 <li>Each concept <span class='rfc2119'>should</span> have a definition
1060 (<code>skos:definition</code>) that constitutes a short description of
1061 the concept which could be adopted by an application using the
1062 vocabulary. Each concept <span class='rfc2119'>should</span> have
1063 additional documentation using SKOS Notes or
1064 Dublin Core terms as appropriate
1065 (see <span class='cite'>std:skosref</span>). In practice, this
1066 requirement is rather difficult to satisfy, since pre-existing
1067 structured vocabularies, being convered to SKOS, frequently provide
1068 only labels, and not fuller descriptions or scope notes.</li>
1069
1070 <li>The language localisation <span class='rfc2119'>should</span> be
1071 declared where appropriate, in preferred labels, alternate labels,
1072 definitions, and the like.</li>
1073
1074 <li>Relationships (<q>broader</q>, <q>narrower</q>, <q>related</q>)
1075 between concepts <span class='rfc2119'>should</span> be present, but
1076 are not required; if used, they <span class='rfc2119'>should</span> be
1077 complete (thus all <q>broader</q> links have corresponding
1078 <q>narrower</q> links in the referenced entries and <q>related</q>
1079 entries link each other).</li>
1080
1081 <li><q>TopConcept</q> entries (see above) <span
1082 class='rfc2119'>should</span> be declared and normally consist of
1083 those concepts that do not have any <q>broader</q> relationships
1084 (i.e. not at a sub-ordinate position in the hierarchy).</li>
1085
1086 <li>The SKOS standard describes some good practices for vocabulary
1087 maintenance, such as using <code>&lt;skos:changeNote&gt;</code> and
1088 the like, and these are elaborated in the (currently draft) note <span
1089 class='cite'>kendall08</span>. At a minimum, the vocabulary's
1090 <code>skos:ConceptScheme</code> <span class='rfc2119'>must</span> be
1091 annotated with DC title, creator, description and date terms
1092 <span class='cite'>std:dublincore</span>,
1093 <span class='cite'>std:pubguide</span>.
1094 Publishers <span class='rfc2119'>should</span> respect such good practices
1095 as are available to direct vocabulary development and
1096 maintenance.</li>
1097
1098 <li>Publishers <span class='rfc2119'>should</span> publish
1099 <q>mappings</q> between their vocabularies and other commonly used
1100 vocabularies. These <span class='rfc2119'>must</span> be external to
1101 the defining vocabulary document so that the vocabulary can be used
1102 independently of the publisher's mappings. The mapping file <span
1103 class='rfc2119' >must</span> be annotated with DC title, creator, description and date terms <span class='cite'>std:dublincore</span>,
1104 <span class='cite'>std:pubguide</span>.</li>
1105
1106 <li>When adapting an existing vocabulary into the SKOS format,
1107 implementors <span class='rfc2119'>should not</span> change the form
1108 of labels (for example changing the grammatical number) beyond any
1109 changes necessarily required by SKOS.</li>
1110
1111 </ol>
1112
1113 <!--
1114 <p>These suggestions are by no means trivial – there was
1115 considerable discussion within the semantic working group on many of
1116 these topics, particularly about token formats (some wanted lower-case
1117 only), and singular versus plural forms of the labels (different
1118 traditions exist within the international library science
1119 community). Obviously, no publisher of an astronomical vocabulary has
1120 to adopt these rules, but the adoption of these rules will make it
1121 easier to use the vocabularly in external generic VO
1122 applications. However, VO applications should be developed to accept
1123 any vocabulary that complies with the latest SKOS standard <span
1124 class="cite">std:skosref</span>.</p>
1125 -->
1126 </div>
1127
1128 <div class='section' id='servevocab'>
1129 <p class='title'>Good practices when serving vocabularies on the web</p>
1130
1131 <p>The W3C Interest Group Note <em>Cool URIs for the Semantic Web</em>
1132 <span class='cite'>sauermann08</span> presents guidelines for the
1133 effective use of URIs when serving web documents and concepts on the
1134 Semantic Web. When providing vocabularies to the VO, we recommend
1135 that publishers conform to these guidelines in general. We make some
1136 further observations below.</p>
1137
1138 <p>The “Cool URIs” guidelines describe a number of desirable
1139 features of URIs in this context, namely simplicity, stability and
1140 manageability. Section 4.5 of the document describes
1141 these features as follows (quoted directly).</p>
1142 <dl>
1143 <dt>Simplicity</dt>
1144 <dd>Short, mnemonic URIs will not break as easily when sent in emails
1145 and are in general easier to remember, e.g. when debugging your
1146 Semantic Web server.</dd>
1147 <dt>Stability</dt>
1148 <dd>Once you set up a URI to identify a certain resource, it should
1149 remain this way as long as possible. Think about the next ten
1150 years. Maybe twenty. Keep implementation-specific bits and pieces such
1151 as .php and .asp out of your URIs, you may want to change technologies
1152 later.</dd>
1153 <dt>Manageability</dt>
1154 <dd>Issue your URIs in a way that you can manage. One good practice is
1155 to include the current year in the URI path, so that you can change
1156 the URI-schema each year without breaking older URIs. Keeping all 303
1157 URIs on a dedicated subdomain,
1158 e.g. <code>http://id.example.com/alice</code>, eases later migration
1159 of the URI-handling subsystem.</dd>
1160 </dl>
1161 <p>We endorse this advice in this Recommendation: VO vocabularies
1162 <span class='rfc2119'>should</span> use URIs which have these
1163 properties. The advice in the third point is a general point about
1164 maintaining the general URI namespace on a particular server, and is
1165 not about versioning vocabulary namespaces.</p>
1166
1167 <p>The “Cool URIs” document also describes two broad strategies for
1168 making these URIs available on the web, which they name <em>303
1169 URIs</em> and <em>hash URIs</em> (see the document, section 4, for
1170 descriptions). They note that the <em>hash URI</em> strategy <q>should
1171 be preferred for rather small and stable sets of resources that evolve
1172 together. The ideal case[s] are RDF Schema vocabularies and OWL
1173 ontologies, where the terms are often used together, and the number of
1174 terms is unlikely to grow out of control in the future.</q> Since
1175 this is the case for the (relatively small) SKOS vocabularies this
1176 Recommendation discusses, and since an application will generally want
1177 to use the complete vocabulary rather than only single concepts, we
1178 suggest that vocabularies conformant to this Recommendation <span
1179 class='rfc2119'>should</span> be distributed as <em>hash URI</em>
1180 ones.</p>
1181
1182 <p>Common to the two strategies above is the insistence that the
1183 vocabulary URIs <em>are HTTP URIs which are retrievable on the
1184 web</em> – they differ only in the practicalities of achieving this.
1185 The strategies also share the expectation that the vocabulary URIs are
1186 retrievable both as RDF (machine-readable) and as HTML (providing
1187 documentation for humans). We elevate this to a requirement of this
1188 Recommendation: vocabulary terms <span class='rfc2119'>must</span> be
1189 HTTP URIs which <span class='rfc2119'>must</span> be dereferenceable
1190 as both RDF and HTML using the mechanism appropriate to the URI naming
1191 strategy.</p>
1192
1193 </div>
1194
1195 <div class='section'>
1196 <p class='title'>Example: serving the A&amp;A vocabulary</p>
1197
1198 <p>While <span class='cite'>sauermann08</span> discusses the design of
1199 the URIs naming concepts, it says little about the mechanics of making
1200 these available on the web. We refer vocabulary publishers to the
1201 recipe advice contained in <span class='cite'>berrueta08</span>, which
1202 we illustrate here in the case of the <em>hash URI</em> strategy.</p>
1203
1204 <p>The A&amp;A vocabulary has the namespace <span
1205 class='url'>@BASEURI@/AAkeys</span>. In accordance with the above
1206 guidelines, this namespace URI is dereferenceable, and if you enter
1207 the URI into a web browser, you will end up at a page describing the
1208 vocabulary. The way this works can be illustrated by using
1209 <code>curl</code> to dereference the URI (URIs are cropped for legibility):</p>
1210 <pre>% curl http://[...]/rdf/AAkeys
1211 HTTP/1.1 303 See Other
1212 Date: Thu, 08 May 2008 14:07:12 GMT
1213 Server: Apache
1214 Location: http://[...]/rdf/vocabularies-2008-05-08/AAkeys/AAkeys.html
1215 Connection: close
1216 Content-Type: text/html; charset=utf-8
1217
1218 &lt;title&gt;Redirected&lt;/title&gt;
1219 &lt;p&gt;See &lt;a href='http://[...]/rdf/vocabularies-2008-05-08/AAkeys/AAkeys.html'
1220 &gt;elsewhere&lt;/a&gt;&lt;/p&gt;
1221 </pre>
1222 <p>The server has responded to the HTTP GET for the URI with a 303
1223 response, and a <code>Location</code> header, pointing to the HTML
1224 representation of this thing. In this example, the server has
1225 included a brief HTML explanation in case a human happens to see this response.</p>
1226
1227 <p>If we instead request an RDF representation, by stating a desired
1228 MIME type in the HTTP <code>Accept</code> header, we get a slightly
1229 different response:</p>
1230 <pre>% curl --head -H accept:text/turtle http://[...]/rdf/AAkeys
1231 HTTP/1.1 303 See Other
1232 Date: Thu, 08 May 2008 14:11:28 GMT
1233 Server: Apache
1234 Location: http://[...]/rdf/vocabularies-2008-05-08/AAkeys/AAkeys.ttl
1235 Connection: close
1236 Content-Type: text/html; charset=iso-8859-1
1237 </pre>
1238 <p>This is also a 303 response, but the <code>Location</code> header
1239 this time points to an RDF file in Turtle syntax, which we can now retrieve normally.</p>
1240 <pre>% curl --include http://[...]/rdf/vocabularies-2008-05-08/AAkeys/AAkeys.ttl
1241 HTTP/1.1 200 OK
1242 Date: Thu, 08 May 2008 14:13:35 GMT
1243 Server: Apache
1244 Content-Type: text/turtle; charset=utf-8
1245
1246 @base &lt;http://[...]/rdf/AAkeys&gt; .
1247 @prefix rdf: &lt;http://www.w3.org/1999/02/22-rdf-syntax-ns#&gt; .
1248 @prefix dc: &lt;http://purl.org/dc/elements/1.1/&gt; .
1249 @prefix rdfs: &lt;http://www.w3.org/2000/01/rdf-schema#&gt; .
1250 @prefix : &lt;http://www.w3.org/2004/02/skos/core#&gt; .
1251
1252 &lt;&gt;
1253 dc:created "2008-05-08" ;
1254 dc:title "Vocabulary for Astronomy &amp; Astrophysics Journal keywords (Version wd-1.0)"@en ;
1255 a :ConceptScheme ;
1256
1257 # and so on...
1258 </pre>
1259 <p>Note that the base URI in the returned RDF still refers to the
1260 unversioned concept names.</p>
1261
1262 <p>This behaviour is controlled by (in this case) an Apache
1263 <code>.htaccess</code> file which looks like this:</p>
1264 <pre>AddType application/rdf+xml .rdf
1265 # The MIME type for .n3 should be text/rdf+n3, not application/n3:
1266 # see MIME notes at &lt;http://www.w3.org/2000/10/swap/doc/changes.html&gt;
1267 #
1268 # The MIME type for Turtle is text/turtle, though this has not
1269 # completed its registration: see
1270 # &lt;http://www.w3.org/TeamSubmission/turtle/#sec-mediaReg&gt;
1271 AddType text/rdf+n3 .n3
1272 AddType text/turtle .ttl
1273 # For Charset types, see &lt;http://www.iana.org/assignments/character-sets&gt;
1274 AddCharset UTF-8 .n3
1275 AddCharset UTF-8 .ttl
1276
1277 RewriteEngine On
1278 # This will match the directory where this file is located.
1279 RewriteBase /users/norman/ivoa/vocabularies/rdf
1280
1281 RewriteCond %{HTTP_ACCEPT} application/rdf\+xml
1282 RewriteRule ^(AAkeys|AVM|UCD|IVOAT|IAUT93)$ vocabularies-2008-05-08/$1/$1.rdf [R=303]
1283
1284 RewriteCond %{HTTP_ACCEPT} text/rdf\+n3 [OR]
1285 RewriteCond %{HTTP_ACCEPT} application/n3 [OR]
1286 RewriteCond %{HTTP_ACCEPT} text/turtle
1287 RewriteRule ^(AAkeys|AVM|UCD|IVOAT|IAUT93)$ vocabularies-2008-05-08/$1/$1.ttl [R=303]
1288
1289 # Any other accept header, including none: make the .html version the default
1290 RewriteRule ^(AAkeys|AVM|UCD|IVOAT|IAUT93)$ vocabularies-2008-05-08/$1/$1.html [R=303]
1291 </pre>
1292 <p>These various <code>RewriteRule</code> statements examine the
1293 content of the HTTP <code>Accept</code> header, and return
1294 303-redirections to the appropriate actual resource.</p>
1295
1296 <p>Note that the namespace remains unversioned throughout the
1297 maintainance history of this vocabulary, even though the actual RDF
1298 files being returned might change as labels or relationships are
1299 adjusted. Previous versions of the vocabulary RDF will remain
1300 available, though they will no longer be served by dereferencing the
1301 namespace URL.</p>
1302
1303 </div>
1304
1305 </div>
1306
1307
1308 <div class="section" id='distvocab'>
1309 <p class="title">Example vocabularies (informative)</p>
1310
1311 <p>The intent of having the IVOA adopt SKOS as the preferred format for
1312 astronomical vocabularies is to encourage the creation and management
1313 of diverse vocabularies by competent astronomical groups, so that
1314 users of the VO and related resources can benefit directly and
1315 dynamically without the intervention of the IAU or IVOA. However, we
1316 felt it important to provide several examples of vocabularies in the
1317 SKOS format as part of the proposal, to illustrate their simplicity
1318 and power, and to provide an immediate vocabulary basis for VO
1319 applications.</p>
1320
1321 <p>The vocabularies described below are included, as SKOS files, in
1322 the distributed version of this standard. These vocabularies have
1323 stable URLs, and may be cited and used indefinitely. These
1324 vocabularies will not, however, be developed as part of the
1325 maintenance of this standard. Interested groups, within and outwith
1326 the IVOA, are encouraged to take these as a starting point and absorb
1327 them within existing processes.</p>
1328
1329 <p>The exceptions to this rule are the constellation vocabulary,
1330 provided here mainly for didactic purposes, and the proposed IVOA
1331 Thesaurus, which is being developed as a separate project and whose
1332 aim is to provide a corrected, more user-friendly, more complete, and
1333 updated version of the 1993 IAU thesaurus. Although work on the IVOA
1334 Thesaurus is on-going, the fact that it is largely based on the IAU
1335 thesaurus means that it is already a very useful resource, so a usable
1336 snapshot of this vocabulary will be published with the other
1337 examples.</p>
1338
1339 <p>We provide a set of SKOS files representing the vocabularies which
1340 have been developed, and mappings between them. These vocabularies
1341 have base URIs starting <span class='url'>@BASEURI@</span>, and can be
1342 downloaded at the URL</p>
1343 <blockquote>
1344 <span class='url'>@BASEURI@/@DISTNAME@.tar.gz</span>
1345 </blockquote>
1346
1347 <!--
1348 <p class='todo'>Not yet: instead go to
1349 <span class='url'>http://code.google.com/p/volute/downloads/list</span></p>-->
1350
1351 <div class='section' id='vocab-constellation'>
1352 <p class='title'>A Constellation Name Vocabulary</p>
1353
1354 <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>
1355
1356 <p>The <code>&lt;skos:ConceptScheme&gt;</code> contains a single
1357 <code>&lt;skos:TopConcept&gt;</code>, <q>constellation</q></p>
1358
1359 <center>
1360 <table>
1361 <tr><th bgcolor="#eecccc">XML Syntax</th>
1362 <th width="10"/><th bgcolor="#cceecc">Turtle Syntax</th></tr>
1363 <tr><td/></tr>
1364 <tr>
1365 <td class='rdfxml'>
1366 <pre class='rdfxml'>&lt;skos:Concept rdf:about="#constellation"&gt;
1367 &lt;skos:inScheme rdf:resource=""/&gt;
1368 &lt;skos:prefLabel&gt;
1369 constellation
1370 &lt;/skos:prefLabel&gt;
1371 &lt;skos:definition&gt;
1372 IAU-sanctioned constellation names
1373 &lt;/skos:definition&gt;
1374 &lt;skos:narrower rdf:resource="#Andromeda"/&gt;
1375 ...
1376 &lt;skos:narrower rdf:resource="#Vulpecula"/&gt;
1377 &lt;/skos:Concept&gt;
1378 </pre>
1379 </td>
1380 <td/>
1381 <td class='turtle'>
1382 <pre class='turtle'>&lt;#constellation&gt; a :Concept;
1383 :inScheme &lt;&gt;;
1384 :prefLabel "constellation";
1385 :definition "IAU-sanctioned constellation names";
1386 :narrower &lt;#Andromeda&gt;;
1387 ...
1388 :narrower &lt;#Vulpecula&gt;.
1389 </pre>
1390 </td></tr>
1391 </table></center>
1392 <p>and the entry for <q>Cygnus</q> is</p>
1393 <center><table><tr>
1394 <td class='rdfxml'>
1395 <pre class='rdfxml'>&lt;skos:Concept rdf:about="#Cygnus"&gt;
1396 &lt;skos:inScheme rdf:resource=""/&gt;
1397 &lt;skos:prefLabel&gt;Cygnus&lt;/skos:prefLabel&gt;
1398 &lt;skos:definition&gt;Cygnus&lt;/skos:definition&gt;
1399 &lt;skos:altLabel&gt;Cygni&lt;/skos:altLabel&gt;
1400 &lt;skos:altLabel&gt;Cyg&lt;/skos:altLabel&gt;
1401 &lt;skos:broader rdf:resource="#constellation"/&gt;
1402 &lt;skos:scopeNote&gt;
1403 Cygnus is nominative form; the alternative
1404 labels are the genitive and short forms
1405 &lt;/skos:scopeNote&gt;
1406 &lt;/skos:Concept&gt;
1407 </pre>
1408 </td>
1409 <td width="10"/>
1410 <td class='turtle'>
1411 <pre class='turtle'>&lt;#Cygnus&gt; a :Concept;
1412 :inScheme &lt;&gt;;
1413 :prefLabel "Cygnus";
1414 :definition "Cygnus";
1415 :altLabel "Cygni";
1416 :altLabel "Cyg";
1417 :broader &lt;#constellation&gt;;
1418 :scopeNote """Cygnus is nominative form;
1419 the alternative labels are the genitive and
1420 short forms""" .
1421 </pre>
1422 </td>
1423 </tr></table></center>
1424
1425 <p>Note that SKOS alone does not permit the distinct differentiation
1426 of genitive forms and abbreviations, but the use of alternate labels
1427 is more than adequate enough for processing by VO applications where
1428 the difference between <q>SS Cygni</q>, <q>SS Cyg</q>, and the incorrect form
1429 <q>SS Cygnus</q> is probably irrelevant.</p>
1430 </div>
1431
1432 <div class='section' id='vocab-aa'>
1433 <p class='title'>The Astronomy &amp; Astrophysics Keyword List</p>
1434
1435 <p>Namespace: <span class='url'>@BASEURI@/AAkeys</span>.</p>
1436
1437 <p>This vocabulary is a set of keywords maintained jointly by the
1438 publishers of the journals <em>Astronomy and Astrophysics</em>
1439 (A&amp;A), <em>Monthly Notices of the Royal Astronomical Society</em>
1440 (MNRAS) and the <em>Astrophysical Journal</em> (ApJ). As noted in the
1441 introduction, an analysis of these keywords <span
1442 class='cite'>preitemartinez07</span> indicates that the different
1443 journals are slightly inconsistent with each other; we have rather
1444 arbitrarily used the list from the A&amp;A web site. The intended
1445 usage of the vocabulary is to tag articles with descriptive keywords
1446 to aid searching for articles on a particular topic.</p>
1447
1448 <p>The keywords are organised into categories which have been modelled as
1449 hierarchical relationships.
1450 Additionally, some of the keywords are grouped into collections which
1451 has been mirrored in the SKOS version.
1452 The vocabulary contains no definitions or related links as these are
1453 not provided in the original keyword list, and only a handful of
1454 alternative labels and scope notes that are present in the original
1455 keyword list.</p>
1456
1457 </div>
1458
1459 <div class='section' id='vocab-avm'>
1460 <p class='title'>The AVM Taxonomy</p>
1461
1462 <p>Namespace: <span class='url'>@BASEURI@/AVM</span>.</p>
1463
1464 <p>This vocabulary is published by the IVOA to allow images to be tagged
1465 with keywords that are relevant for the public.
1466 It consists of a set of keywords organised into an enumerated
1467 hierarchical structure.
1468 Each term consists of a taxonomic number and a label.
1469 There are no definitions, scope notes, or cross references.
1470 </p>
1471
1472 <p>When converting the AVM into SKOS, it was decided to model the
1473 taxonomic number as an alternative label.
1474 Since there are duplication of terms, the token for a term consists of
1475 the full hierarchical location of the term.
1476 Thus, it is possible to distinguish between</p>
1477 <pre>Planet -> Feature -> Surface -> Canyon</pre>
1478 <p>and</p>
1479 <pre>Planet -> Satellite -> Feature -> Surface -> Canyon</pre>
1480 <p>which have the tokens <code>PlanetFeatureSurfaceCanyon</code> and
1481 <code>PlanetSatelliteFeatureSurfaceCanyon</code> respectively.
1482 </p>
1483
1484 </div>
1485
1486 <div class='section' id='vocab-ucd1'>
1487 <p class='title'>The UCD1+ Vocabulary</p>
1488
1489 <p>Namespace: <span class='url'>@BASEURI@/UCD</span>.</p>
1490
1491 <p>The UCD standard is an officially sanctioned and managed vocabulary
1492 of the IVOA. The normative document is a simple text file containing
1493 entries consisting of tokens (for example <code>em.IR</code>), a short
1494 description, and usage information (<q>syntax codes</q> which permit
1495 UCD tokens to be concatenated). The form of the tokens implies a
1496 natural hierarchy: <code>em.IR.8-15um</code> is obviously a narrower
1497 term than <code>em.IR</code>, which in turn is narrower than
1498 <code>em</code>.</p>
1499
1500 <p>Given the structure of the UCD1+ vocabulary, the natural
1501 translation to SKOS consists of preferred labels equal to the original
1502 tokens (the UCD1 words include dashes and periods), vocabulary tokens
1503 created using guidelines in <span class='xref'
1504 >practices</span> (for example, "emIR815Um" for
1505 <code>em.IR.8-15um</code>), direct use of the definitions, and the syntax codes
1506 placed in usage documentation: <code>&lt;skos:scopeNote&gt;UCD syntax code: P&lt;/skos:scopeNote&gt;</code></p>
1507
1508 <p>Note that the SKOS document containing the UCD1+ vocabulary does
1509 NOT consistute the official version: the normative document is still
1510 the text list. However, on the long term, the IVOA may decide to make
1511 the SKOS version normative, since the SKOS version contains all of the
1512 information contained in the original text document but has the
1513 advantage of being in a standard format easily read and used by any
1514 application on the semantic web whilst still being usable in the
1515 current ways.</p>
1516
1517 </div>
1518
1519 <div class='section' id='vocab-iau93'>
1520 <p class='title'>The 1993 IAU Thesaurus</p>
1521
1522 <p>Namespace: <span class='url'>@BASEURI@/IAUT93</span>.</p>
1523
1524 <p>The IAU Thesaurus consists of concepts with mostly capitalised
1525 labels and a rich set of thesaurus relationships (<q>BT</q> for
1526 "broader term", <q>NT</q> for <q>narrower term</q>, and <q>RT</q> for
1527 <q>related term</q>). The thesaurus also contains <q>U</q> (for
1528 <q>use</q>) and <q>UF</q> (<q>use for</q>) relationships. In a SKOS
1529 model of a vocabulary these are captured as alternative labels. A
1530 separate document contains translations of the vocabulary terms in
1531 five languages: English, French, German, Italian, and
1532 Spanish. Enumerable concepts are plural (for example <q>SPIRAL
1533 GALAXIES</q>) and non-enumerable concepts are singular
1534 (for example <q>STABILITY</q>). Finally, there are some usage hints like
1535 <q>combine with other</q>, which have been modelled as scope notes.</p>
1536
1537 <p>In converting the IAU Thesaurus to SKOS, we have been as faithful
1538 as possible to the original format of the thesaurus. Thus, preferred
1539 labels have been kept in their uppercase format.</p>
1540
1541 <p>The IAU Thesaurus has been unmaintained since its initial production in
1542 1993; it is therefore significantly out of date in places. This
1543 vocabulary is published for the sake of completeness, and to make the
1544 link between the evolving vocabulary work and any uses of the 1993
1545 vocabulary which come to light. We do not expect to make any future
1546 maintenance changes to this vocabulary, and would expect the IVOAT
1547 vocabulary, based on this one, to be used instead (see <span class='xref'>vocab-ivoat</span>).</p>
1548
1549 </div>
1550
1551 <div class='section' id='vocab-ivoat'>
1552 <p class='title'>Towards an IVOA Thesaurus</p>
1553
1554 <p>While it is true that the adoption of SKOS will make it easy to
1555 publish and access different astronomical vocabularies, the fact is
1556 that there is no vocabulary which makes it easy to jump-start the
1557 use of vocabularies in generic astrophysical VO applications: each of
1558 the previously developed vocabularies has their own limits and
1559 biases. For example, the IAU Thesaurus provides a large number of
1560 entries, copious relationships, and translations to four other languages,
1561 but there are no definitions, many concepts are now only useful for
1562 historical purposes (for example many photographic or historical instrument
1563 entries), some of the relationships are false or outdated, and many
1564 important or newer concepts and their common abbreviations are
1565 missing.</p>
1566
1567 <p>Despite its faults, the IAU Thesaurus constitutes a very extensive
1568 vocabulary which could easily serve as the basis vocabulary once
1569 we have removed its most egregious faults and extended it to cover the
1570 most obvious semantic holes. To this end, a heavily revised IAU
1571 thesaurus is in preparation for use within the IVOA and other
1572 astronomical contexts. The goal is to provide a general vocabulary
1573 foundation to which other, more specialised, vocabularies can be added
1574 as needed, and to provide a good <q>lingua franca</q> for the creation of
1575 vocabulary mappings.</p>
1576 </div>
1577 </div> <!-- End: Example vocabularies -->
1578
1579 <div class='section' id='distmappings'>
1580 <p class='title'>Mapping vocabularies (informative)</p>
1581
1582 <p>Part of the motivation for formalising vocabularies within the VO
1583 is to support <em>mapping between vocabularies</em>, so that an
1584 application which understands, or can natively process, one
1585 vocabulary, can use a mapping to provide at least partial support for
1586 data described using another vocabulary. The SKOS standard describes
1587 a number of properties for expressing such matches, and we anticipate
1588 that we will shortly see explicit mappings between vocabularies,
1589 produced either by vocabulary maintainers, describing the
1590 relationships between their own vocabularies and others, or by third
1591 parties, asserting such relationships as an intellectual contribution
1592 of their own.</p>
1593
1594 <p>The vocabularies distributed in association with this document
1595 include some non-exhaustive mappings between vocabularies, as an
1596 example of how such mappings will appear.</p>
1597
1598 <p><span class='todo'>To do:</span> Add a link to the one example mapping file we have for relating the A&amp;A Keywords to the AVM.</p>
1599
1600 <!--
1601 <p>To show how mappings can be expressed between two vocabularies, we
1602 have provided one example mapping document which maps the concepts in
1603 the A&amp;A Keywords vocabulary to the concepts in the AVM
1604 vocabulary.
1605 All four types of mappings were required.
1606 Since all the mapping relationships have inverse relationships
1607 defined, the mapping document can also be used to infer the set of
1608 mappings from the AVM vocabulary to the A&amp;A keywords.</p>
1609
1610 <p>To provide provenence information about the set of mappings in a
1611 document, Dublin Core metadata is included in the mapping
1612 document.</p>
1613
1614 -->
1615
1616 </div>
1617
1618 <div class="appendices">
1619
1620 <!-- <p><span class='todo'>To come</span></p>-->
1621
1622
1623 <div class="section-nonum" id="bibliography">
1624 <p class="title">References</p>
1625 <?bibliography rm-refs ?>
1626 </div>
1627
1628 <p style="text-align: right; font-size: x-small; color: #888;">
1629 $Revision$ $Date$
1630 </p>
1631
1632 </div>
1633
1634 </body>
1635 </html>

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svn:keywords Author Date Revision

msdemlei@ari.uni-heidelberg.de
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