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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>
47 <h1>Vocabularies in the Virtual Observatory<br/>Version @VERSION@</h1>
48 <h2>IVOA Working Draft, @RELEASEDATE@ [Editors' draft]</h2>
49 <p><strong>$Revision$ $Date$</strong></p>
51 <dl>
53 <dt>This version</dt>
54 <dd><span class='url'>@DOCURI@.html</span></dd>
56 <dt>Latest version</dt>
57 <dd><span class='url'>http://www.ivoa.net/Documents/latest/vocabularies.html</span><br/>
58 and <a href='@ISSUESLIST@' >issues list</a></dd>
60 <dt>Previous version</dt>
61 <dd><span class='url'>http://www.ivoa.net/Documents/WD/Semantics/vocabularies-20080320.html</span></dd>
63 <dt>Working Group</dt>
64 <dd><em><a href="http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics">Semantics</a></em></dd>
66 <dt>Editors</dt>
67 <dd>Alasdair J G Gray, University of Glasgow, UK<br/>
68 <a href='http://nxg.me.uk/norman/' >Norman Gray</a>, University of
69 Leicester / University of Glasgow, UK<br/>
70 Frederic V Hessman, University of Göttingen, Germany<br/>
71 Andrea Preite Martinez, INAF, Italy</dd>
73 <dt>Authors</dt>
74 <dd>
75 <span property="dc:creator">Sébastien Derriere</span>,
76 <span property="dc:creator">Alasdair J G Gray</span>,
77 <span property="dc:creator">Norman Gray</span>,
78 <span property="dc:creator">Frederic V Hessman</span>,
79 <span property="dc:creator">Tony Linde</span>,
80 <span property="dc:creator">Andrea Preite Martinez</span>,
81 <span property="dc:creator">Rob Seaman</span> and
82 <span property="dc:creator">Brian Thomas</span>
83 </dd>
84 </dl>
85 <hr/>
86 </div>
88 <div class="section-nonum" id="abstract">
89 <p class="title">Abstract</p>
91 <div class="abstract">
92 <p>As the astronomical information processed within the <em>Virtual Observatory
93 </em> becomes more complex, there is an increasing need for a more
94 formal means of identifying quantities, concepts, and processes not
95 confined to things easily placed in a FITS image, or expressed in a
96 catalogue or a table. We propose that the IVOA adopt a standard
97 format for vocabularies based on the W3C's <em>Resource Description
98 Framework</em> (RDF) and <em>Simple Knowledge Organisation System</em>
99 (SKOS). By adopting a standard and simple format, the IVOA will
100 permit different groups to create and maintain their own specialised
101 vocabularies while letting the rest of the astronomical community
102 access, use, and combine them. The use of current, open standards
103 ensures that VO applications will be able to tap into resources of the
104 growing semantic web. Several examples of useful astronomical
105 vocabularies are provided, including work on a common IVOA thesaurus
106 intended to provide a semantic common base for VO applications.</p>
107 </div>
109 </div>
111 <div class="section-nonum" id="status">
112 <p class="title">Status of this document</p>
114 <p>This is an IVOA Working
115 Draft. The first release of this document was
116 <span property="dc:date">2008 March 20</span>.</p>
118 <p>This document is an IVOA Working Draft for review by IVOA members
119 and other interested parties. It is a draft document and may be
120 updated, replaced, or obsoleted by other documents at any time. It is
121 inappropriate to use IVOA Working Drafts as reference materials or to
122 cite them as other than <q>work in progress</q>.</p>
124 <p>A list of current IVOA Recommendations and other technical
125 documents can be found at
126 <span class='url' >http://www.ivoa.net/Documents/</span>.</p>
128 <p>This document includes a normative reference to the W3C SKOS
129 standard <span class='cite'>std:skosref</span>, despite the fact that,
130 at the time this present document was standardised, the SKOS document
131 was still a W3C Working Draft and thus a <q>work in progress</q>. The
132 core part of the SKOS standard which this standard refers to (that is,
133 the concept schemes, documentation and intravocabulary relationship
134 vocabularies) are stable, and are very unlikely to change before
135 Recommendation. When the SKOS document becomes a W3C Recommendation,
136 we will issue a minor update to this present document referring to the
137 finalised SKOS standard, and incorporating any errata which have
138 appeared by then.</p>
140 <h3>Acknowledgments</h3>
142 <p>We would like to thank the members of the IVOA semantic working
143 group for many interesting ideas and fruitful discussions.</p>
144 </div>
146 <h2><a id="contents" name="contents">Table of Contents</a></h2>
147 <?toc?>
149 <hr/>
151 <div class="section" id="introduction">
152 <p class="title">Introduction (informative)</p>
154 <div class="section" id='astrovocab'>
155 <p class="title">Vocabularies in astronomy</p>
157 <p>Astronomical information of relevance to the Virtual Observatory
158 (VO) is not confined to quantities easily expressed in a catalogue or
159 a table.
160 Fairly simple things such as position on the sky, brightness in some
161 units, times measured in some frame, redshifts, classifications or
162 other similar quantities are easily manipulated and stored in VOTables
163 and can currently be identified using IVOA Unified Content Descriptors
164 (UCDs) <span class="cite">std:ucd</span>.
165 However, astrophysical concepts and quantities use a wide variety of
166 names, identifications, classifications and associations, most of
167 which cannot be described or labelled via UCDs.</p>
169 <p>There are a number of basic forms of organised semantic knowledge
170 of potential use to the VO. Informal <q>folksonomies</q> are at one
171 extreme, and are a very lightly coordinated collection of labels
172 chosen by users. In a slightly more formally structured
173 <q>vocabulary</q>, the label is drawn from a predefined set of
174 definitions, and which can include relationships to other labels;
175 vocabularies are primarily associated with searching and browsing
176 tasks. At the other extreme are <q>ontologies</q>, where the domain
177 is formally captured in a set of logical classes, typically related in
178 a subclass hierarchy. More formal definitions are presented later in
179 this document.
180 </p>
182 <p>An astronomical ontology is necessary if we are to have a computer
183 (appear to) <q>understand</q> something of the domain.
184 There has been some progress towards creating an ontology of
185 astronomical object types <span
186 class="cite">std:ivoa-astro-onto</span> to meet this need.
187 However there are distinct use cases for letting human users find
188 resources of interest through search and navigation of the information space.
189 The most appropriate technology to meet these use cases derives from
190 the Information Science community, that of <em>controlled
191 vocabularies, taxonomies and thesauri</em>.
192 In the present document, we do not distinguish between controlled
193 vocabularies, taxonomies and thesauri, and use the term
194 <em>vocabulary</em> to represent all three.
195 </p>
197 <p>One of the best examples of the need for a simple vocabulary within
198 the VO is VOEvent <span class="cite">std:voevent</span>, the VO
199 standard for supporting rapid notification of astronomical events.
200 This standard requires some formalised indication of what a published
201 event is <q>about</q>, in a formalism which can be used straightforwardly
202 by the developer of relevant services. See <span class='xref'
203 >usecases</span> for further discussion.</p>
205 <p>A number of astronomical vocabularies have been created, with a
206 variety of goals and intended uses. Some examples are detailed below. </p>
208 <ul>
210 <li>The <em>Second Reference Dictionary of the Nomenclature of
211 Celestial Objects</em> <span class="cite">lortet94</span>, <span
212 class="cite">lortet94a</span> contains 500 paper pages of astronomical
213 nomenclature</li>
215 <li>For decades professional journals have used a set of reasonably
216 compatible keywords to help classify the content of whole articles.
217 These keywords have been analysed by Preite Martinez &amp; Lesteven
218 <span class="cite">preitemartinez07</span>, who derived a
219 set of common keywords constituting one of the potential bases for a
220 fuller VO vocabulary. The same authors also attempted to derive a set
221 of common concepts by analysing the contents of abstracts in journal
222 articles, which should comprise a list of tokens/concepts more
223 up-to-date than the old list of journal keywords. A similar but less
224 formal attempt was made by Hessman <span class='cite'>hessman05</span>
225 for the VOEvent working group, resulting in a similar list.</li>
227 <li>Astronomical databases generally use simple sets of keywords
228 – sometimes hierarchically organised – to help users make queries.
229 Two examples from very
230 different contexts are the list of object types used in the <a
231 href="http://simbad.u-strasbg.fr">Simbad</a> database and the search
232 keywords used in the educational Hands-On Universe image database
233 portal.</li>
235 <li>The Astronomical Outreach Imagery (AOI) working group has created
236 a simple taxonomy for helping to classify images used for educational
237 or public relations <span class="cite">std:aoim</span>. See
238 <span class='xref'>vocab-aoim</span>.</li>
240 <!--
241 <li>The Hands-On Universe project (see <span class='url'
242 >http://sunra.lbl.gov/telescope2/index.html</span>) has maintained a
243 public database of images for use by the general public since the
244 1990s. The images are very heterogeneous, since they are gathered from
245 a variety of professional, semi-professional, amateur, and school
246 observatories, so a simple taxonomy is used to facilitate browsing
247 by the users of the database.</li>
248 -->
250 <li>In 1993, Shobbrook and Shobbrook published an Astronomy Thesaurus
251 endorsed by the IAU <span class='cite' >shobbrook93</span>. This
252 collection of nearly 3000 terms, in five languages, is a valuable
253 resource, but has seen little use in recent years. Its very size,
254 which gives it expressive power, is a disadvantage to the extent that
255 it is consequently hard to use. See <span class='xref'>vocab-iau93</span>.</li>
257 <li>The VO's Unified Content Descriptors <span class='cite'
258 >std:ucd</span> (UCD) constitute the main controlled vocabulary of the
259 IVOA and contain some taxonomic information. However, UCD has some
260 features which supports its goals, but which make it difficult to use
261 beyond the present applications of labelling VOTables: firstly, there
262 is no standard means of identifying and processing the contents of the
263 text-based reference document; secondly, the content cannot be openly
264 extended beyond that set by a formal IVOA committee without going
265 through a laborious and time-consuming negotiation process of
266 extending the primary vocabulary itself; and thirdly, the UCD
267 vocabulary is primarily concerned with data types and their
268 processing, and only peripherally with astronomical objects (for
269 example, it defines formal labels for RA, flux, and bandpass, but does
270 not mention the Sun). See <span class='xref'>vocab-ucd1</span>.</li>
272 </ul>
273 </div>
275 <div class='section' id='usecases'>
276 <p class='title'>Use-cases, and the motivation for formalised vocabularies</p>
278 <p>The most immediate high-level motivation for this work is the
279 requirement of the VOEvent standard <span class='cite'
280 >std:voevent</span> for a controlled vocabulary usable in the
281 VOEvent's <code>&lt;Why/&gt;</code> and <code>&lt;What/&gt;</code>
282 elements, which describe what
283 sort of object the VOEvent packet is describing, in some broadly
284 intelligible way. For example a <q>burst</q> might be a gamma-ray burst
285 due to the collapse of a star in a distant galaxy, a solar flare, or
286 the brightening of a stellar or AGN accretion disk, and having an
287 explicit list of vocabulary terms can help guide the event publisher
288 into using a term which will be usefully precise for the event's
289 consumers. A free-text label can help here (which brings us into the
290 domain sometimes referred to as folksonomies), but the astronomical
291 community, with a culture sympathetic to international agreement, can
292 do better.</p>
294 <p>The purpose of this proposal is to establish a set of conventions for
295 the creation, publication, use, and manipulation of
296 astronomical vocabularies within the Virtual Observatory, based upon
297 the W3C's SKOS standard. We include as appendices to this proposal
298 formalised versions of a number of existing vocabularies, encoded as
299 SKOS vocabularies <span class="cite">std:skosref</span>.</p>
301 <p>Specific use-cases include the following.</p>
302 <ul>
303 <li>A user wishes to process all events concerning supernovae, which
304 means that an event concerning a type 1a supernova must be understood to be
305 relevant. [This supports a system working autonomously, filtering
306 incoming information]</li>
308 <li>A user is searching an archive of VOEvents for microlensing
309 events, and retrieves a large number of them; the search interface may
310 then prompt them to narrow their search using one of a set of terms
311 including, say, binary lens events. [This supports so-called <q>semantic
312 search</q>, providing semantic support to an interface which is in turn
313 supporting a user]</li>
315 <li>A user wishes to search for resources based on the
316 journal-supported keywords in a paper; they might either initiate this by
317 hand, or have this done on their behalf by a tool which can extract
318 the keywords from a PDF. The keywords are in the A&amp;A vocabulary,
319 and mappings have been defined between this vocabulary and others,
320 which means that the query keywords are translated automatically
321 into those appropriate for a search of an outreach image database
322 (everyone likes pretty pictures), the VO Registry, a set of Simbad
323 object types, and one or more concepts in more formal ontologies. The
324 search interface is then able to support the user browsing up and down
325 the AOIM vocabulary, and a specialised Simbad tool is able to take
326 over the search, now it has an appropriate starting place. [This
327 supports interoperability, building on the investments which
328 institutions and users have made in existing vocabularies]</li>
330 <li>A user receives a VOTable of results from a VO application – for
331 example a catalogue of objects or observations – and wants to search a
332 database of old FITS files for potential matches. Because the UCDs
333 labeling the columns of the tables are expressed in well-documented
334 SKOS, both the official descriptions of the UCDs and their semantic
335 matches to a variety of other plain-text vocabularies (such as the IAU
336 or AOIM thesauri) are available to the VO application, providing a basis
337 for massive searches for all kinds of FITS keyword values.</li>
339 </ul>
341 <p>The goal of this standard is to show how vocabularies can be easily
342 expressed in an interoperable and computer-manipulable format, and the
343 sole normative section of this Recommendation (namely section <span
344 class='xref'>publishing</span>) contains requirements and suggestions
345 intended to promote this. Four example vocabularies that have
346 previously been expressed using non-standardized formats – namely the
347 A&amp;A keyword list, the IAU thesaurus and AOIM taxonomy, and UCD1 – are
348 included below as illustrations of how simple it is to publish them in
349 SKOS, without losing any of the information of the original source
350 vocabularies.</p>
352 <p>It is not a goal of this standard, as it is not a goal of SKOS, to
353 produce knowledge-engineering artefacts which can support elaborate
354 machine reasoning – such artefacts would be very valuable, but require
355 much more expensive work on ontologies. As the supernova use-case
356 above illustrates, even simple vocabularies can support useful machine
357 reasoning.</p>
359 <p>It is also not a goal of this standard to produce new vocabularies,
360 or substantially alter existing ones; instead, the vocabularies
361 included below in section <span class='xref'>distvocab</span> are directly
362 derived from existing vocabularies (the exceptions are the IVOAT
363 vocabulary, which is ultimately intended to be a significant update to
364 the IAU-93 original, and the constellations vocabulary, which is
365 intended to be purely didactic). It therefore follows that the ambiguities,
366 redundancies and incompleteness of the source vocabularies are
367 faithfully represented in the distributed SKOS vocabularies. We hope
368 that this formalisation process will create greater visibility and
369 broader use for the various vocabularies, and that this will guide the
370 maintenance efforts of the curating groups.</p>
372 <p>The reason for both of these limitations is that vocabularies are
373 extremely expensive to produce, maintain and deploy, and we must
374 therefore rely on such vocabularies as have been developed, and
375 attached as metadata to resources, by others. Such vocabularies are
376 less rich or less coherent than we might prefer, but they are widely enough
377 deployed to be useful. We hope that the set of example vocabularies
378 we have provided will build on this deployment, by providing material
379 which is useful out of the box.</p>
381 </div>
383 <div class="section" id='formalising'>
384 <p class="title">Formalising and managing multiple vocabularies</p>
386 <p>We find ourselves in the situation where there are multiple
387 vocabularies in use, describing a broad range of resources of interest
388 to professional and amateur astronomers, and members of the public.
389 These different vocabularies use different terms and different
390 relationships to support the different constituencies they cater for.
391 For example, <q>delta Sct</q> and <q>RR Lyr</q> are terms one would
392 find in a vocabulary aimed at professional astronomers, associated
393 with the notion of <q>variable star</q>; however one would
394 <em>not</em> find such technical terms in a vocabulary intended to
395 support outreach activities.</p>
397 <p>One approach to this problem is to create a single consensus
398 vocabulary, which draws terms from the various existing vocabularies
399 to create a new vocabulary which is able to express anything its users
400 might desire. The problem with this is that such an effort would be
401 very expensive, both in terms of time and effort on the part of those
402 creating it, and to the potential users, who have to learn
403 to navigate around it, recognise the new terms, and who have to be
404 supported in using the new terms correctly (or, more often,
405 incorrectly).</p>
407 <p>The alternative approach to the problem is to evade it, and this is
408 the approach taken in this document. Rather than deprecating the
409 existence of multiple overlapping vocabularies, we embrace it,
410 help interest groups formalise as many of them as are appropriate, and
411 standardise the process of formally declaring the relationships between
412 them. This means that:</p>
413 <ul>
414 <li>The various vocabularies are allowed to evolve separately, on
415 their own timescales, managed either by the IVOA, individual working
416 groups within the IVOA, or by third parties;</li>
418 <li>Specialised vocabularies can be developed and maintained by the
419 community with the most knowledge about a specific topic, ensuring
420 that the vocabulary will have the most appropriate breadth, depth, and
421 precision;</li>
423 <li>Users can choose the vocabulary or combination of vocabularies most
424 appropriate to their situation, either when annotating resources, or
425 when querying them; and</li>
427 <li>We can retain the previous investments made in vocabularies by
428 users and resource owners.</li>
430 </ul>
433 </div>
435 </div>
437 <div class='section' id='skos'>
438 <p class='title'>SKOS-based vocabularies (informative)</p>
440 <p>In this section, we introduce the concepts of SKOS-based
441 vocabularies, and the technology of mapping between them. We describe
442 some additional requirements for IVOA vocabularies in the next
443 section, <span class='xref' >publishing</span>.</p>
445 <div class="section" id='vocab'>
446 <p class="title">Selection of the vocabulary format</p>
448 <p>After extensive online and face-to-face discussions, the authors have
449 brokered a consensus within the IVOA community that
450 formalised vocabularies should be published at least in SKOS (Simple Knowledge
451 Organisation System) format, a W3C draft standard application of RDF to the
452 field of knowledge organisation <span
453 class="cite">std:skosref</span>. SKOS draws on long experience
454 within the Library and Information Science community, to address a
455 well-defined set of problems to do with the indexing and retrieval of
456 information and resources; as such, it is a close match to the problem
457 this document is addressing.</p>
459 <p>ISO 5964 <span class='cite' >std:iso5964</span> defines a number of
460 the relevant terms (ISO 5964:1985=BS 6723:1985; see also <span
461 class='cite' >std:bs8723-1</span> and <span class='cite'
462 >std:z39.19</span>), and some of the (lightweight) theoretical
463 background. The only technical distinction relevant to this document
464 is that between vocabulary and thesaurus: BS-8723-1 defines a
465 controlled vocabulary as a</p>
466 <blockquote>
467 prescribed list of terms or headings each one having an assigned meaning
468 [noting that <q>Controlled vocabularies are designed for use in
469 classifying or indexing documents and for searching them.</q>]
470 </blockquote>
471 <p>and a thesaurus as a</p>
472 <blockquote>
473 Controlled vocabulary in which concepts are represented by preferred
474 terms, formally organized so that paradigmatic relationships between
475 the concepts are made explicit, and the preferred terms are
476 accompanied by lead-in entries for synonyms or quasi-synonyms.
477 <!-- NOTE:
478 The purpose of a thesaurus is to guide both the indexer and the
479 searcher to select the same preferred term or combination of preferred
480 terms to represent a given subject. -->
481 (BS-8723-1, sect. 2.39)
482 </blockquote>
483 <p>with a similar definition in ISO-5964 sect. 3.16.</p>
485 <p>The paradigmatic relationships in question are those relating a
486 term to a <q>broader</q>, <q>narrower</q> or more generically
487 <q>related</q> term. These notions have an operational definition:
488 any resource retrieved as a result of a search on a given term will
489 also be retrievable through a search on that term's <q>broader
490 term</q> (<q>narrower</q> is a simple inverse, so that for any pair of
491 terms, if <code>A skos:broader B</code>, then <code>B skos:narrower
492 A</code>; a term may have multiple narrower and broader terms). This
493 is not a subsumption relationship, as there is no implication that the
494 concept referred to by a narrower term is of the same <em>type</em> as
495 a broader term. Further, the <code>skos:broader</code> and
496 <code>skos:narrower</code> relationships are not transitive (that is,
497 declaring that if <code>A skos:broader B</code> and <code>B
498 skos:broader C</code> does not imply that <code>A skos:broader
499 C</code>). However the SKOS standard includes the notions of
500 <code>skos:broaderTransitive</code> and
501 <code>skos:narrowerTransitive</code> relations for the subset of
502 vocabularies and systems which would find these useful.</p>
504 <p>Thus <strong>a vocabulary (SKOS or otherwise) is not an
505 ontology</strong>. It has lighter and looser semantics than an
506 ontology, and is specialised for the restricted case of resource
507 retrieval. Those interested in ontological analyses can easily
508 transfer the vocabulary relationship information from SKOS to a formal
509 ontological format such as OWL <span class='cite' >std:owl</span>.</p>
511 <p>The purpose of a thesaurus is to help users find resources they
512 might be interested in, be they library books, image archives, or VOEvent
513 packets.</p>
515 </div>
517 <div class='section' id='skos-format'>
518 <p class='title'>Content and format of a SKOS vocabulary</p>
520 <p>A published vocabulary in SKOS format consists of a set of
521 <q>concepts</q> – an example concept capturing the
522 vocabulary information about spiral galaxies is provided in the <a
523 href='#figexample' >Figure below</a>, with the RDF shown in both
524 RDF/XML <span class='cite' >std:rdfxml</span> and Turtle notation <span
525 class='cite' >std:turtle</span> (Turtle is similar to the more
526 informal <em>Notation3</em>). The elements of a concept are detailed
527 below.</p>
529 <center>
530 <p><a name='figexample' >Figure: examples of SKOS vocabularies</a></p>
531 <table>
532 <tr>
533 <th class='rdfxml'>XML Syntax</th>
534 <th width="10"/>
535 <th class='turtle'>Turtle Syntax</th>
536 </tr>
537 <tr><td/></tr>
538 <tr>
539 <td class='rdfxml'>
540 <pre class='rdfxml'>
541 &lt;skos:Concept rdf:about="#spiralGalaxy"&gt;
542 &lt;skos:prefLabel lang="en"&gt;
543 spiral galaxy
544 &lt;/prefLabel&gt;
545 &lt;skos:prefLabel lang="de"&gt;
546 Spiralgalaxie
547 &lt;/prefLabel&gt;
548 &lt;skos:altLabel lang="en"&gt;
549 spiral nebula
550 &lt;/skos:altLabel&gt;
551 &lt;skos:hiddenLabel lang="en"&gt;
552 spiral glaxy
553 &lt;/hiddenLabel&gt;
554 &lt;skos:definition lang="en"&gt;
555 A galaxy having a spiral structure.
556 &lt;/skos:definition&gt;
557 &lt;skos:scopeNote lang="en"&gt;
558 Spiral galaxies fall into one of
559 three catagories: Sa, Sc, and Sd.
560 &lt;/skos:scopeNote&gt;
561 &lt;skos:narrower
562 rdf:resource="#barredSpiralGalaxy"/&gt;
563 &lt;skos:broader
564 rdf:resource="#galaxy"/&gt;
565 &lt;skos:related
566 rdf:resource="#spiralArm"/&gt;
567 &lt;/skos:Concept&gt;
568 </pre>
569 </td>
570 <td/>
571 <td class='turtle'>
572 <pre class='turtle'>
573 &lt;#spiralGalaxy&gt; a skos:Concept;
574 skos:prefLabel
575 "spiral galaxy"@en,
576 "Spiralgalaxie"@de;
577 skos:altLabel "spiral nebula"@en;
578 skos:hiddenLabel "spiral glaxy"@en;
579 skos:definition """A galaxy having a
580 spiral structure."""@en;
581 skos:scopeNote """Spiral galaxies fall
582 into one of three categories:
583 Sa, Sc, and Sd"""@en;
584 skos:narrower &lt;#barredSpiralGalaxy&gt;;
585 skos:broader &lt;#galaxy&gt;;
586 skos:related &lt;#spiralArm&gt; .
587 </pre>
588 </td>
589 </tr>
590 </table>
591 </center>
593 <p>A SKOS vocabulary includes the following features.</p>
595 <ul>
597 <li>A single URI representing the concept, mainly for use by computers.
598 <!--
599 <code>&lt;#spiralGalaxy&gt; a skos:Concept</code>.
600 <code>&lt;skos:Concept rdf:about="#spiralGalaxy"&gt;</code>
601 -->
602 </li>
604 <li>A single prefered label in each supported language of the
605 vocabulary, for use by humans.
606 <!--
607 <code>skos:prefLabel "spiral galaxy"@en, "Spiralgalaxie"@de</code>.
608 <code>&lt;skos:prefLabel&gt;spiral galaxy&lt;/skos:prefLabel&gt;</code>
609 -->
610 </li>
612 <li>Optional alternative labels which applications may encounter or in
613 common use, whether simple synonyms or commonly-used aliases such as
614 <q>GRB</q> for "gamma-ray burst", or <q>Spiral nebula</q> for
615 spiral galaxies.
616 <!--
617 <code>skos:altLabel "GRB"@en</code>
618 <code>&lt;skos:altLabel lang="de"&gt;Spiralgalaxie&lt;/skos:altLabel&gt;</code>
619 -->
620 </li>
622 <li>Optional hidden labels which capture terms which are sometimes
623 used for the corresponding concept, but which are deprecated in some
624 sense. This might include common misspellings for
625 either the preferred or alternate labels, for example <q>glaxy</q> for
626 <q>galaxy</q>.
627 </li>
629 <li>A definition for the concept, where one exists in the original
630 vocabulary, to clarify the meaning of the term.
631 <!--
632 <code>skos:definition "A galaxy having a spiral structure."@en</code>
633 <code>&lt;skos:definition lang="en"&gt;<br/>A galaxy having a spiral structure.<br/>&lt;/skos:definition&gt;</code>
634 -->
635 </li>
637 <li>A scope note to further clarify a definition, or the usage of the
638 concept.
639 <!--
640 <code>skos:scopeNote "Spiral galaxies fall into one of three categories: Sa, Sc, and Sd"@en</code>
641 <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>
642 -->
643 </li>
645 <li>Optionally, a concept may be involved in any number of relationships
646 to other concepts. The types of relationships are
647 <ul>
648 <li>Narrower or more specific concepts, for example a link to the concept
649 representing a <q>barred spiral galaxy</q>.
650 <!--
651 <code>skos:narrower &lt;#barredSpiralGalaxy&gt;</code>.
652 <code>&lt;skos:narrower rdf:resource="#barredSpiralGalaxy"&gt;</code>
653 -->
654 </li>
655 <li>Broader or more general concepts, for example a link to the token
656 representing galaxies in general.
657 <!--
658 <code>skos:broader &lt;#galaxy&gt;</code>.
659 <code>&lt;skos:broader rdf:resource="#galaxy"&gt;</code>
660 -->
661 </li>
662 <li>Related concepts, for example a link to the token representing spiral
663 arms of galaxies
664 <!--
665 <code>skos:related &lt;#spiralArm&gt;</code>
666 <code>&lt;skos:related rdf:resource="#spiralArm"&gt;</code>
667 -->
668 <br/>
669 (note this relationship does not say that spiral galaxies have spiral
670 arms – that would be ontological information of a higher order which
671 is beyond the requirements for information stored in a vocabulary).</li>
672 </ul>
673 </li>
674 </ul>
676 <p>In addition to the information about a single concept, a vocabulary
677 can contain information to help users navigate its structure and
678 contents:</p>
679 <ul>
680 <li>The <q>top concepts</q> of the vocabulary, i.e. those that occur
681 at the top of the vocabulary hierarchy defined by the broader/narrower
682 relationships, can be explicitly stated to make it easier to navigate
683 the vocabulary.</li>
685 <li>Concepts that form a natural group can be defined as being members
686 of a <q>collection</q>.</li>
688 <li>Versioning information can be added using change notes.</li>
690 <li>Additional metadata about the vocabulary, for example indicating
691 the publisher, may be documented using the Dublin Core metadata set
692 <span class='cite' >std:dublincore</span>, <span
693 class='cite'>std:pubguide</span>. At a minimum, the vocabulary's
694 <code>skos:ConceptScheme</code> should be annotated with DC title,
695 creator, description and date terms.</li>
697 <li>The SKOS standard describes a number of <q>documentation
698 properties</q>; these should be used to document provenance of and
699 changes to vocabulary terms.</li>
701 <li>A set of mappings between vocabularies has the potential to be
702 circular or create inconsistencies, though this is probably reasonably
703 unlikely in fact. This is in principle out of the
704 control of the vocabulary authors, since vocabularies do not contain
705 mappings, and so this can only be detected dynamically by applications
706 which use the vocabularies.</li>
707 </ul>
708 </div>
711 <div class='section' id='skos-relationships'>
712 <p class='title'>Mapping relationships between vocabularies</p>
714 <p>There already exist several vocabularies in the domain of astronomy.
715 Instead of attempting to replace all these existing vocabularies,
716 which have been developed to achieve different aims and user groups,
717 we embrace them.
718 This requires a mechanism to relate the concepts in the different
719 vocabularies.</p>
721 <p>Part of the SKOS standard <span class='cite'>std:skosref</span>
722 allows a concept in one vocabulary to be related to a concept in
723 another vocabulary.
724 There are four types of relationship provided to capture the
725 relationships between concepts in vocabularies, which are similar to
726 those defined for relationships between concepts within a single
727 vocabulary.
728 The types of mapping relationships are as follows.</p>
730 <ul>
732 <li>
733 Equivalence between concepts, i.e. the concepts in the different
734 vocabularies refer to the same real world entity.
735 This is captured with the RDF statement
736 <blockquote>
737 <code>AAkeys:#Cosmology skos:exactMatch aoim:#Cosmology</code>
738 </blockquote>
739 which states that the cosmology concept in the A&amp;A Keywords is the
740 same as the cosmology concept in the AOIM.
741 (Note the use of an external namespaces <code>AAkeys</code> and
742 <code>aoim</code> which must be defined within the document.)
743 </li>
745 <li>
746 Broader concept, i.e. there is not an equivalent concept but there is
747 a more general one.
748 This is captured with the RDF statement
749 <blockquote>
750 <code>AAkeys:#Moon skos:broadMatch aoim:PlanetSatellite</code>
751 </blockquote>
752 which states that the AOIM concept <q>Planet Satellite</q> is a more general
753 term than the A&amp;A Keywords concept <q>Moon</q>.
754 </li>
756 <li>
757 Narrower concept, i.e. there is not an equivalent concept but there is
758 a more specific one.
759 This is captured with the RDF statement
760 <blockquote>
761 <code>AAkeys:#IsmClouds skos:narrowMatch
762 aoim:#NebulaAppearanceDarkMolecularCloud</code>
763 </blockquote>
764 which states that the AOIM concept <q>Nebula Appearance Dark Molecular
765 Cloud</q> is more specific than the A&amp;A Keywords concept <q>ISM Clouds</q>.
766 </li>
768 <li>
769 Related concept, i.e. there is some form of relationship.
770 This is captured with the RDF statement
771 <blockquote>
772 <code>AAkeys:#BlackHolePhysics skos:relatedMatch
773 aoim:#StarEvolutionaryStageBlackHole</code>
774 </blockquote>
775 which states that the A&amp;A Keywords concept <q>Black Hole Physics</q> has
776 an association with the AOIM concept <q>Star Evolutionary Stage Black Hole</q>.
777 </li>
779 </ul>
781 <p>The semantic mapping relationships have certain properties.
782 The broadMatch relationship has the narrowMatch relationship as its
783 inverse and the exactMatch and relatedMatch relationships are
784 symmetrical.
785 The consequence of these properties is that if you have a mapping from
786 concept <code>A</code> in one vocabulary to concept <code>B</code> in
787 another vocabulary then you can infer a mapping from concept
788 <code>B</code> to concept <code>A</code>.
789 </p>
791 <p class='todo'>At the time of writing, the SKOS document is still a
792 working draft, and may or may not end up with support for mappings in
793 the core document rather than in a companion document. This section
794 of this Working Draft, and other references to mappings below, should
795 therefore be considered as current best practice and could be updated
796 in a subsequent version of this document once the SKOS document has
797 become a standard.</p>
799 </div>
801 <div class='section' id='vocabversions'>
802 <p class='title'>Vocabulary versions</p>
804 <p>The document <span class='cite'>kendall08</span> discusses good
805 practice for managing RDF vocabularies. At the time of writing (2008
806 May) this is still an editor's draft, and it itself notes that good
807 practice in this area is not yet fully stable, so our recommendations
808 here are necessarily tentative, and in some places restricted to the
809 relatively small vocabularies (100s to 1000s of terms) we expect to
810 encounter in the VO. We expect to adjust or enhance this advice in
811 future editions of this Recommendation, as best practice evolves, or
812 as we gain more experience with the relevant vocabularies.</p>
814 <p>We must distinguish between versions of a vocabulary, and versions
815 of the description of a vocabulary. In the former case, we are
816 concerned with the presence or absence of certain concepts, such as
817 <q>star</q> or <q>GRB</q>, and expect that there will be some
818 reasonably stable relationship between the concept URI and the
819 real-world concept it refers to. In the latter case, we are concerned
820 with the technicalities of associating a concept URI with its
821 labels, its description, and with other related concept URIs. While
822 it is true that there are epistemological commitments involved in the
823 simple act of naming (and the terms <q>GRB</q> and <q>planet</q>
824 remind us that there is knowledge implicit within a name), it is the
825 latter case that generally represents the <em>knowledge</em> we have
826 of an object, and it is this knowledge which we must version.</p>
828 <p>In consequence, <em>the concept URIs should not carry
829 version information</em>. The partial exception to this is when a
830 vocabulary undergoes a major restructuring, as a result of the terms
831 in it becoming significantly incoherent – for example, we might
832 imagine the IAU92 thesaurus being updated to form an IAU 200x
833 thesaurus – but in this case we should regard the result as a new
834 vocabulary, rather than simply an adjusted version of an old one.</p>
836 <p>All the terms in the SKOS vocabulary appear in an unversioned
837 namespace, and once in the vocabulary they are not removed <span
838 class='cite'>kendall08</span>. Successive versions of the vocabulary
839 description describe the vocabulary terms as <q>unstable</q>, <q>testing</q>,
840 <q>stable</q> or <q>deprecated</q>.</p>
841 <!-- there seems to be no
842 discussion of this in a SKOS document, as opposed to commentary on
843 SKOS -->
845 <p>The Dublin Core namespaces are managed in a similar way <span
846 class='cite'>dc:namespaces</span>. The namespace URIs, which act as
847 common prefixes to the DC terms, and which are defined using a <q>hash
848 URI</q> strategy, in RDF terms, have no version numbers, so that
849 the namespace for the DC terms vocabulary is <span
850 class='url'>http://purl.org/dc/terms/</span>. Terms such as <span
851 class='url'>http://purl.org/dc/terms/extent</span> then 302-redirect
852 to a URL which, for administrative convenience, happens to contain a
853 release date, but which resolves to RDF which defines the unversioned
854 term <span class='url'>http://purl.org/dc/terms/extent</span>. This
855 file includes the following content (translated into Turtle from the
856 original RDF/XML for legibility).</p>
857 <pre>@prefix rdf: &lt;http://www.w3.org/1999/02/22-rdf-syntax-ns#&gt; .
858 @prefix skos: &lt;http://www.w3.org/2004/02/skos/core#&gt; .
859 @prefix dcam: &lt;http://purl.org/dc/dcam/&gt; .
860 @prefix dcterms: &lt;http://purl.org/dc/terms/&gt; .
861 @prefix rdfs: &lt;http://www.w3.org/2000/01/rdf-schema#&gt; .
863 &lt;http://purl.org/dc/terms/&gt;
864 dcterms:title """DCMI Namespace for metadata terms
865 in the http://purl.org/dc/terms/ namespace"""@en-us;
866 rdfs:comment """To comment on this schema,
867 please contact dcmifb@dublincore.org.""";
868 dcterms:publisher "The Dublin Core Metadata Initiative"@en-us;
869 dcterms:modified "2008-01-14" .
871 dcterms:extent
872 rdfs:label "Extent"@en-us;
873 rdfs:comment "The size or duration of the resource."@en-us;
874 rdfs:isDefinedBy &lt;http://purl.org/dc/terms/&gt;;
875 dcterms:issued "2000-07-11";
876 dcterms:modified "2008-01-14";
877 a rdf:Property;
878 dcterms:hasVersion &lt;http://dublincore.org/usage/terms/history/#extent-003&gt;;
879 rdfs:range dcterms:SizeOrDuration;
880 rdfs:subPropertyOf &lt;http://purl.org/dc/elements/1.1/format&gt;,
881 dcterms:format .
882 ...
883 </pre>
884 <p>This includes the definition of the (unversioned) <span class='url'
885 >http://purl.org/dc/terms/extent</span> concept, along with semantic
886 knowledge about the concept (<code>rdfs:subPropertyOf</code>) as of
887 2008-01-14, plus other editorial (<code>dcterms:modified</code>) and
888 definitional (<code>rdfs:isDefinedBy</code>) metadata.</p>
890 </div>
892 </div>
894 <div class='section' id='publishing'>
895 <p class='title'>Publishing vocabularies (normative)</p>
897 <div class='section' id='pubreq'>
898 <p class='title'>Requirements</p>
900 <p>A vocabulary which conforms to this IVOA standard has the following
901 features. In this section, the keywords
902 <span class='rfc2119' >must</span>,
903 <span class='rfc2119' >should</span>
904 and so on, are to be interpreted as described in <span
905 class='cite'>std:rfc2119</span>.</p>
907 <div class='section'>
908 <p class='title'>Dereferenceable namespace</p>
910 <p>The namespace of the vocabulary <span class='rfc2119'>must</span>
911 be dereferenceable on the web. That is, typing the namespace URL into
912 a web browser will produce human-readable documentation about the
913 vocabulary. In addition, the namespace URL <span class='rfc2119'
914 >should</span> return an RDF version of the vocabulary if it is
915 retrieved with one of the RDF MIME types in the HTTP Accept header.
916 At the time of writing, the only fully standardised RDF MIME type is
917 <code>application/rdf+xml</code> for RDF/XML, but
918 <code>text/rdf+n3</code> and <code>text/turtle</code> are the proposed
919 types for Notation3 <span class='cite'>notation3</span> and Turtle
920 <span class='cite'>std:turtle</span>, respectively.</p>
922 <p><em>Rationale: These prescriptions are intended to be compatible
923 with the patterns described in <span class='cite'>berrueta08</span>
924 and <span class='cite'>sauermann08</span>, and vocabulary distributors
925 <span class='rfc2119' >should</span> follow these patterns where
926 possible.</em></p>
927 </div>
929 <div class='section'>
930 <p class='title'>Long-term availability</p>
932 <p>The files defining a vocabulary, including those of superseded
933 versions, <span class='rfc2119' >should</span> remain permanently
934 available. There is no requirement that the namespace URL be at any
935 particular location, although the IVOA web pages, or a journal
936 publisher's web pages, would likely be suitable archival
937 locations.</p> </div>
939 <div class='section'>
940 <p class='title'>Distribution format</p>
942 <p>Vocabularies <span class='rfc2119'>must</span> be made available
943 for distribution as SKOS RDF files in RDF/XML <span
944 class='cite'>std:rdfxml</span> format. A human readable version in
945 Turtle <span class='cite'>std:turtle</span> format <span
946 class='rfc2119'>should</span> also be made available. As an
947 alternative to Turtle, vocabularies may be made available in that
948 subset of Notation3 <span class='cite'>notation3</span> which is
949 compatible with Turtle; if Turtle or Notation3 is being served, it is
950 prudent to support both <code>text/rdf+n3</code> and
951 <code>text/turtle</code> as MIME types in the <code>Accept</code>
952 header of the HTTP request. <!-- See issue <a
953 href='@ISSUESLIST@#distformat-2'>[distformat-2]</a> -->.</p>
955 <p>A publisher <span class='rfc2119'>may</span> make available RDF in
956 other formats, or other supporting files. A publisher <span
957 class='rfc2119'>must</span> make available at least some
958 human-readable documentation – see section <span
959 class='xref'>servevocab</span> for a discussion of the mechanics here.</p>
961 <p><em>Rationale: this does imply that the vocabulary source files can only
962 realistically be parsed using an RDF parser. An alternative is to
963 require that vocabularies be distributed using a subset of RDF/XML
964 which can also be naively handled as traditional XML; however as well
965 as creating an extra standardisation requirement, this would make it
966 effectively infeasible to write out the distribution version of the
967 vocabulary using an RDF or general SKOS tool.</em></p>
968 </div>
970 <div class='section' id='versioning'>
971 <p class='title'>Clearly versioned vocabulary</p>
973 <p>The vocabulary <em>namespace</em> <span class='rfc2119' >should
974 not</span> be versioned, but it <span class='rfc2119' >should</span>
975 be easy to retrieve earlier versions of the RDF describing the
976 vocabulary. See the discussion in section <span
977 class='xref'>vocabversions</span> for the rationale for this, and see
978 section <span class='xref'>servevocab</span> for a discussion of its
979 implications for the way that vocabularies are served on the web.</p>
981 <!-- Issue <a href='@ISSUESLIST@#versioning-3' >[versioning-3]</a>-->
983 </div>
985 <div class='section'>
986 <p class='title'>No restrictions on source files</p>
988 <p>This Recommendation does not place any restrictions on the format of the
989 files managed by the maintenance process, as long as the distributed
990 files are as specified above.
991 <!-- See issue
992 <a href='@ISSUESLIST@#masterformat-1' >[masterformat-1]</a> -->
993 </p>
994 </div>
996 </div>
998 <div class='section' id='practices'>
999 <p class='title'>Good practices of vocabulary design</p>
1001 <p>This standard imposes a number of requirements on conformant
1002 vocabularies (see <span class='xref' >pubreq</span>). In
1003 this section we list a number of good practices that IVOA vocabularies
1004 <span class='rfc2119'>should</span> abide by. Some of the
1005 prescriptions below are more specific than good-practice guidelines
1006 for vocabularies in general.</p>
1008 <p>The adoption of the following guidelines will make it easier to use
1009 vocabularies in generic VO applications. However, VO applications
1010 <span class='rfc2119'>should</span> be able to accept any vocabulary
1011 that complies with the latest SKOS standard
1012 <span class="cite">std:skosref</span> (this is a syntactical
1013 requirement, and does not imply that an application will necessarily
1014 understand the terms in
1015 an alien vocabulary, although the presence of mappings to a known
1016 vocabulary should allow it to derive some benefit).</p>
1018 <ol>
1020 <li>Concept identifiers <span class='rfc2119'>should</span> consist
1021 only of the letters a-z, A-Z, and numbers 0-9, i.e. no spaces, no
1022 exotic letters (for example umlauts), and no characters which would make a
1023 token inexpressible as part of a URI; since tokens are for use by
1024 computers only, this is not a big restriction, since the exotic
1025 letters can be used within the labels and documentation if
1026 appropriate.</li>
1028 <li>The concept identifiers <span class='rfc2119'>should</span> be
1029 kept in human-readable form, directly reflect the implied meaning, and
1030 not be semi-random identifiers only (for example, use
1031 <code>spiralGalaxy</code>, not <code>t1234567</code>); tokens <span
1032 class='rfc2119'>should</span> preferably be created via a direct
1033 conversion from the preferred label via removable/translation of
1034 non-token characters (see above) and sub-token separation via
1035 capitalisation of the first sub-token character (for example the label <code>My
1036 favourite idea-label #42</code> is converted into
1037 <code>MyFavouriteIdeaLabel42</code>).</li>
1039 <li>Labels <span class='rfc2119'>should</span> be unchanged from the
1040 labels appearing in the source vocabulary. When
1041 developing a new vocabulary standard thesaurus practice indicates that
1042 english language labels for concrete concepts should be pluralised –
1043 thus <code>"galaxies"@en</code>, but <code>"astronomy"@en</code>;
1044 thesaurus practice in other european languages uses the singular for
1045 all cases.</li>
1046 <!--
1047 the singular form <span
1048 class='rfc2119'>should</span> be preferred,
1049 for example <code>spiral galaxy</code>, not <code>spiral galaxies</code>.
1050 <span class='todo'><a href="http://code.google.com/p/volute/issues/detail?id=1">Open issue</a></span></li>
1051 -->
1053 <li>Each concept <span class='rfc2119'>should</span> have a definition
1054 (<code>skos:definition</code>) that constitutes a short description of
1055 the concept which could be adopted by an application using the
1056 vocabulary. Each concept <span class='rfc2119'>should</span> have
1057 additional documentation using SKOS Notes or
1058 Dublin Core terms as appropriate
1059 (see <span class='cite'>std:skosref</span>). In practice, this
1060 requirement is rather difficult to satisfy, since pre-existing
1061 structured vocabularies, being convered to SKOS, frequently provide
1062 only labels, and not fuller descriptions or scope notes.</li>
1064 <li>The language localisation <span class='rfc2119'>should</span> be
1065 declared where appropriate, in preferred labels, alternate labels,
1066 definitions, and the like.</li>
1068 <li>Relationships (<q>broader</q>, <q>narrower</q>, <q>related</q>)
1069 between concepts <span class='rfc2119'>should</span> be present, but
1070 are not required; if used, they <span class='rfc2119'>should</span> be
1071 complete (thus all <q>broader</q> links have corresponding
1072 <q>narrower</q> links in the referenced entries and <q>related</q>
1073 entries link each other).</li>
1075 <li><q>TopConcept</q> entries (see above) <span
1076 class='rfc2119'>should</span> be declared and normally consist of
1077 those concepts that do not have any <q>broader</q> relationships
1078 (i.e. not at a sub-ordinate position in the hierarchy).</li>
1080 <li>The SKOS standard describes some good practices for vocabulary
1081 maintenance, such as using <code>&lt;skos:changeNote&gt;</code> and
1082 the like, and these are elaborated in the (currently draft) note <span
1083 class='cite'>kendall08</span>. At a minimum, the vocabulary's
1084 <code>skos:ConceptScheme</code> <span class='rfc2119'>must</span> be
1085 annotated with DC title, creator, description and date terms
1086 <span class='cite'>std:dublincore</span>,
1087 <span class='cite'>std:pubguide</span>.
1088 Publishers <span class='rfc2119'>should</span> respect such good practices
1089 as are available to direct vocabulary development and
1090 maintenance.</li>
1092 <li>Publishers <span class='rfc2119'>should</span> publish
1093 <q>mappings</q> between their vocabularies and other commonly used
1094 vocabularies. These <span class='rfc2119'>must</span> be external to
1095 the defining vocabulary document so that the vocabulary can be used
1096 independently of the publisher's mappings. The mapping file <span
1097 class='rfc2119' >should</span> contain metadata using suitable Dublin
1098 Core terms.</li>
1100 <li>When adapting an existing vocabulary into the SKOS format,
1101 implementors <span class='rfc2119'>should not</span> change the form
1102 of labels (for example changing the grammatical number) beyond any
1103 changes necessarily required by SKOS.</li>
1105 </ol>
1107 <!--
1108 <p>These suggestions are by no means trivial – there was
1109 considerable discussion within the semantic working group on many of
1110 these topics, particularly about token formats (some wanted lower-case
1111 only), and singular versus plural forms of the labels (different
1112 traditions exist within the international library science
1113 community). Obviously, no publisher of an astronomical vocabulary has
1114 to adopt these rules, but the adoption of these rules will make it
1115 easier to use the vocabularly in external generic VO
1116 applications. However, VO applications should be developed to accept
1117 any vocabulary that complies with the latest SKOS standard <span
1118 class="cite">std:skosref</span>.</p>
1119 -->
1120 </div>
1122 <div class='section' id='servevocab'>
1123 <p class='title'>Good practices when serving vocabularies on the web</p>
1125 <p>The W3C Interest Group Note <em>Cool URIs for the Semantic Web</em>
1126 <span class='cite'>sauermann08</span> presents guidelines for the
1127 effective use of URIs when serving web documents and concepts on the
1128 Semantic Web. When providing vocabularies to the VO, we recommend
1129 that publishers conform to these guidelines in general. We make some
1130 further observations below.</p>
1132 <p>The “Cool URIs” guidelines describe a number of desirable
1133 features of URIs in this context, namely simplicity, stability and
1134 manageability. Section 4.5 of the document describes
1135 these features as follows (quoted directly).</p>
1136 <dl>
1137 <dt>Simplicity</dt>
1138 <dd>Short, mnemonic URIs will not break as easily when sent in emails
1139 and are in general easier to remember, e.g. when debugging your
1140 Semantic Web server.</dd>
1141 <dt>Stability</dt>
1142 <dd>Once you set up a URI to identify a certain resource, it should
1143 remain this way as long as possible. Think about the next ten
1144 years. Maybe twenty. Keep implementation-specific bits and pieces such
1145 as .php and .asp out of your URIs, you may want to change technologies
1146 later.</dd>
1147 <dt>Manageability</dt>
1148 <dd>Issue your URIs in a way that you can manage. One good practice is
1149 to include the current year in the URI path, so that you can change
1150 the URI-schema each year without breaking older URIs. Keeping all 303
1151 URIs on a dedicated subdomain,
1152 e.g. <code>http://id.example.com/alice</code>, eases later migration
1153 of the URI-handling subsystem.</dd>
1154 </dl>
1155 <p>We endorse this advice in this Recommendation: VO vocabularies
1156 <span class='rfc2119'>should</span> use URIs which have these
1157 properties. The advice in the third point is a general point about
1158 maintaining the general URI namespace on a particular server, and is
1159 not about versioning vocabulary namespaces.</p>
1161 <p>The “Cool URIs” document also describes two broad strategies for
1162 making these URIs available on the web, which they name <em>303
1163 URIs</em> and <em>hash URIs</em> (see the document, section 4, for
1164 descriptions). They note that the <em>hash URI</em> strategy <q>should
1165 be preferred for rather small and stable sets of resources that evolve
1166 together. The ideal case[s] are RDF Schema vocabularies and OWL
1167 ontologies, where the terms are often used together, and the number of
1168 terms is unlikely to grow out of control in the future.</q> Since
1169 this is the case for the (relatively small) SKOS vocabularies this
1170 Recommendation discusses, and since an application will generally want
1171 to use the complete vocabulary rather than only single concepts, we
1172 suggest that vocabularies conformant to this Recommendation <span
1173 class='rfc2119'>should</span> be distributed as <em>hash URI</em>
1174 ones.</p>
1176 <p>Common to the two strategies above is the insistence that the
1177 vocabulary URIs <em>are HTTP URIs which are retrievable on the
1178 web</em> – they differ only in the practicalities of achieving this.
1179 The strategies also share the expectation that the vocabulary URIs are
1180 retrievable both as RDF (machine-readable) and as HTML (providing
1181 documentation for humans). We elevate this to a requirement of this
1182 Recommendation: vocabulary terms <span class='rfc2119'>must</span> be
1183 HTTP URIs which <span class='rfc2119'>must</span> be dereferenceable
1184 as both RDF and HTML using the mechanism appropriate to the URI naming
1185 strategy.</p>
1187 </div>
1189 <div class='section'>
1190 <p class='title'>Example: serving the A&amp;A vocabulary</p>
1192 <p>While <span class='cite'>sauermann08</span> discusses the design of
1193 the URIs naming concepts, it says little about the mechanics of making
1194 these available on the web. We refer vocabulary publishers to the
1195 recipe advice contained in <span class='cite'>berrueta08</span>, which
1196 we illustrate here in the case of the <em>hash URI</em> strategy.</p>
1198 <p>The A&amp;A vocabulary has the namespace <span
1199 class='url'>@BASEURI@/AAkeys</span>. In accordance with the above
1200 guidelines, this namespace URI is dereferenceable, and if you enter
1201 the URI into a web browser, you will end up at a page describing the
1202 vocabulary. The way this works can be illustrated by using
1203 <code>curl</code> to dereference the URI (URIs are cropped for legibility):</p>
1204 <pre>% curl http://[...]/rdf/AAkeys
1205 HTTP/1.1 303 See Other
1206 Date: Thu, 08 May 2008 14:07:12 GMT
1207 Server: Apache
1208 Location: http://[...]/rdf/vocabularies-2008-05-08/AAkeys/AAkeys.html
1209 Connection: close
1210 Content-Type: text/html; charset=utf-8
1212 &lt;title&gt;Redirected&lt;/title&gt;
1213 &lt;p&gt;See &lt;a href='http://[...]/rdf/vocabularies-2008-05-08/AAkeys/AAkeys.html'
1214 &gt;elsewhere&lt;/a&gt;&lt;/p&gt;
1215 </pre>
1216 <p>The server has responded to the HTTP GET for the URI with a 303
1217 response, and a <code>Location</code> header, pointing to the HTML
1218 representation of this thing. In this example, the server has
1219 included a brief HTML explanation in case a human happens to see this response.</p>
1221 <p>If we instead request an RDF representation, by stating a desired
1222 MIME type in the HTTP <code>Accept</code> header, we get a slightly
1223 different response:</p>
1224 <pre>% curl --head -H accept:text/turtle http://[...]/rdf/AAkeys
1225 HTTP/1.1 303 See Other
1226 Date: Thu, 08 May 2008 14:11:28 GMT
1227 Server: Apache
1228 Location: http://[...]/rdf/vocabularies-2008-05-08/AAkeys/AAkeys.ttl
1229 Connection: close
1230 Content-Type: text/html; charset=iso-8859-1
1231 </pre>
1232 <p>This is also a 303 response, but the <code>Location</code> header
1233 this time points to an RDF file in Turtle syntax, which we can now retrieve normally.</p>
1234 <pre>% curl --include http://[...]/rdf/vocabularies-2008-05-08/AAkeys/AAkeys.ttl
1235 HTTP/1.1 200 OK
1236 Date: Thu, 08 May 2008 14:13:35 GMT
1237 Server: Apache
1238 Content-Type: text/turtle; charset=utf-8
1240 @base &lt;http://[...]/rdf/AAkeys&gt; .
1241 @prefix rdf: &lt;http://www.w3.org/1999/02/22-rdf-syntax-ns#&gt; .
1242 @prefix dc: &lt;http://purl.org/dc/elements/1.1/&gt; .
1243 @prefix rdfs: &lt;http://www.w3.org/2000/01/rdf-schema#&gt; .
1244 @prefix : &lt;http://www.w3.org/2004/02/skos/core#&gt; .
1246 &lt;&gt;
1247 dc:created "2008-05-08" ;
1248 dc:title "Vocabulary for Astronomy &amp; Astrophysics Journal keywords (Version wd-1.0)"@en ;
1249 a :ConceptScheme ;
1251 # and so on...
1252 </pre>
1253 <p>Note that the base URI in the returned RDF still refers to the
1254 unversioned concept names.</p>
1256 <p>This behaviour is controlled by (in this case) an Apache
1257 <code>.htaccess</code> file which looks like this:</p>
1258 <pre>AddType application/rdf+xml .rdf
1259 # The MIME type for .n3 should be text/rdf+n3, not application/n3:
1260 # see MIME notes at &lt;http://www.w3.org/2000/10/swap/doc/changes.html&gt;
1261 #
1262 # The MIME type for Turtle is text/turtle, though this has not
1263 # completed its registration: see
1264 # &lt;http://www.w3.org/TeamSubmission/turtle/#sec-mediaReg&gt;
1265 AddType text/rdf+n3 .n3
1266 AddType text/turtle .ttl
1267 # For Charset types, see &lt;http://www.iana.org/assignments/character-sets&gt;
1268 AddCharset UTF-8 .n3
1269 AddCharset UTF-8 .ttl
1271 RewriteEngine On
1272 # This will match the directory where this file is located.
1273 RewriteBase /users/norman/ivoa/vocabularies/rdf
1275 RewriteCond %{HTTP_ACCEPT} application/rdf\+xml
1276 RewriteRule ^(AAkeys|AOIM|UCD|IVOAT|IAUT93)$ vocabularies-2008-05-08/$1/$1.rdf [R=303]
1278 RewriteCond %{HTTP_ACCEPT} text/rdf\+n3 [OR]
1279 RewriteCond %{HTTP_ACCEPT} application/n3 [OR]
1280 RewriteCond %{HTTP_ACCEPT} text/turtle
1281 RewriteRule ^(AAkeys|AOIM|UCD|IVOAT|IAUT93)$ vocabularies-2008-05-08/$1/$1.ttl [R=303]
1283 # Any other accept header, including none: make the .html version the default
1284 RewriteRule ^(AAkeys|AOIM|UCD|IVOAT|IAUT93)$ vocabularies-2008-05-08/$1/$1.html [R=303]
1285 </pre>
1286 <p>These various <code>RewriteRule</code> statements examine the
1287 content of the HTTP <code>Accept</code> header, and return
1288 303-redirections to the appropriate actual resource.</p>
1290 <p>Note that the namespace remains unversioned throughout the
1291 maintainance history of this vocabulary, even though the actual RDF
1292 files being returned might change as labels or relationships are
1293 adjusted. Previous versions of the vocabulary RDF will remain
1294 available, though they will no longer be served by dereferencing the
1295 namespace URL.</p>
1297 </div>
1299 </div>
1302 <div class="section" id='distvocab'>
1303 <p class="title">Example vocabularies (informative)</p>
1305 <p>The intent of having the IVOA adopt SKOS as the preferred format for
1306 astronomical vocabularies is to encourage the creation and management
1307 of diverse vocabularies by competent astronomical groups, so that
1308 users of the VO and related resources can benefit directly and
1309 dynamically without the intervention of the IAU or IVOA. However, we
1310 felt it important to provide several examples of vocabularies in the
1311 SKOS format as part of the proposal, to illustrate their simplicity
1312 and power, and to provide an immediate vocabulary basis for VO
1313 applications.</p>
1315 <p>The vocabularies described below are included, as SKOS files, in
1316 the distributed version of this standard. These vocabularies have
1317 stable URLs, and may be cited and used indefinitely. These
1318 vocabularies will not, however, be developed as part of the
1319 maintenance of this standard. Interested groups, within and outwith
1320 the IVOA, are encouraged to take these as a starting point and absorb
1321 them within existing processes.</p>
1323 <p>The exceptions to this rule are the constellation vocabulary,
1324 provided here mainly for didactic purposes, and the proposed IVOA
1325 Thesaurus, which is being developed as a separate project and whose
1326 aim is to provide a corrected, more user-friendly, more complete, and
1327 updated version of the 1993 IAU thesaurus. Although work on the IVOA
1328 Thesaurus is on-going, the fact that it is largely based on the IAU
1329 thesaurus means that it is already a very useful resource, so a usable
1330 snapshot of this vocabulary will be published with the other
1331 examples.</p>
1333 <p>We provide a set of SKOS files representing the vocabularies which
1334 have been developed, and mappings between them. These vocabularies
1335 have base URIs starting <span class='url'>@BASEURI@</span>, and can be
1336 downloaded at the URL</p>
1337 <blockquote>
1338 <span class='url'>@BASEURI@/@DISTNAME@.tar.gz</span>
1339 </blockquote>
1341 <!--
1342 <p class='todo'>Not yet: instead go to
1343 <span class='url'>http://code.google.com/p/volute/downloads/list</span></p>-->
1345 <div class='section' id='vocab-constellation'>
1346 <p class='title'>A Constellation Name Vocabulary</p>
1348 <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>
1350 <p>The <code>&lt;skos:ConceptScheme&gt;</code> contains a single
1351 <code>&lt;skos:TopConcept&gt;</code>, <q>constellation</q></p>
1353 <center>
1354 <table>
1355 <tr><th bgcolor="#eecccc">XML Syntax</th>
1356 <th width="10"/><th bgcolor="#cceecc">Turtle Syntax</th></tr>
1357 <tr><td/></tr>
1358 <tr>
1359 <td class='rdfxml'>
1360 <pre class='rdfxml'>&lt;skos:Concept rdf:about="#constellation"&gt;
1361 &lt;skos:inScheme rdf:resource=""/&gt;
1362 &lt;skos:prefLabel&gt;
1363 constellation
1364 &lt;/skos:prefLabel&gt;
1365 &lt;skos:definition&gt;
1366 IAU-sanctioned constellation names
1367 &lt;/skos:definition&gt;
1368 &lt;skos:narrower rdf:resource="#Andromeda"/&gt;
1369 ...
1370 &lt;skos:narrower rdf:resource="#Vulpecula"/&gt;
1371 &lt;/skos:Concept&gt;
1372 </pre>
1373 </td>
1374 <td/>
1375 <td class='turtle'>
1376 <pre class='turtle'>&lt;#constellation&gt; a :Concept;
1377 :inScheme &lt;&gt;;
1378 :prefLabel "constellation";
1379 :definition "IAU-sanctioned constellation names";
1380 :narrower &lt;#Andromeda&gt;;
1381 ...
1382 :narrower &lt;#Vulpecula&gt;.
1383 </pre>
1384 </td></tr>
1385 </table></center>
1386 <p>and the entry for <q>Cygnus</q> is</p>
1387 <center><table><tr>
1388 <td class='rdfxml'>
1389 <pre class='rdfxml'>&lt;skos:Concept rdf:about="#Cygnus"&gt;
1390 &lt;skos:inScheme rdf:resource=""/&gt;
1391 &lt;skos:prefLabel&gt;Cygnus&lt;/skos:prefLabel&gt;
1392 &lt;skos:definition&gt;Cygnus&lt;/skos:definition&gt;
1393 &lt;skos:altLabel&gt;Cygni&lt;/skos:altLabel&gt;
1394 &lt;skos:altLabel&gt;Cyg&lt;/skos:altLabel&gt;
1395 &lt;skos:broader rdf:resource="#constellation"/&gt;
1396 &lt;skos:scopeNote&gt;
1397 Cygnus is nominative form; the alternative
1398 labels are the genitive and short forms
1399 &lt;/skos:scopeNote&gt;
1400 &lt;/skos:Concept&gt;
1401 </pre>
1402 </td>
1403 <td width="10"/>
1404 <td class='turtle'>
1405 <pre class='turtle'>&lt;#Cygnus&gt; a :Concept;
1406 :inScheme &lt;&gt;;
1407 :prefLabel "Cygnus";
1408 :definition "Cygnus";
1409 :altLabel "Cygni";
1410 :altLabel "Cyg";
1411 :broader &lt;#constellation&gt;;
1412 :scopeNote """Cygnus is nominative form;
1413 the alternative labels are the genitive and
1414 short forms""" .
1415 </pre>
1416 </td>
1417 </tr></table></center>
1419 <p>Note that SKOS alone does not permit the distinct differentiation
1420 of genitive forms and abbreviations, but the use of alternate labels
1421 is more than adequate enough for processing by VO applications where
1422 the difference between <q>SS Cygni</q>, <q>SS Cyg</q>, and the incorrect form
1423 <q>SS Cygnus</q> is probably irrelevant.</p>
1424 </div>
1426 <div class='section' id='vocab-aa'>
1427 <p class='title'>The Astronomy &amp; Astrophysics Keyword List</p>
1429 <p>Namespace: <span class='url'>@BASEURI@/AAkeys</span>.</p>
1431 <p>This vocabulary is a set of keywords maintained jointly by the
1432 publishers of the journals <em>Astronomy and Astrophysics</em>
1433 (A&amp;A), <em>Monthly Notices of the Royal Astronomical Society</em>
1434 (MNRAS) and the <em>Astrophysical Journal</em> (ApJ). As noted in the
1435 introduction, an analysis of these keywords <span
1436 class='cite'>preitemartinez07</span> indicates that the different
1437 journals are slightly inconsistent with each other; we have rather
1438 arbitrarily used the list from the A&amp;A web site. The intended
1439 usage of the vocabulary is to tag articles with descriptive keywords
1440 to aid searching for articles on a particular topic.</p>
1442 <p>The keywords are organised into categories which have been modelled as
1443 hierarchical relationships.
1444 Additionally, some of the keywords are grouped into collections which
1445 has been mirrored in the SKOS version.
1446 The vocabulary contains no definitions or related links as these are
1447 not provided in the original keyword list, and only a handful of
1448 alternative labels and scope notes that are present in the original
1449 keyword list.</p>
1451 </div>
1453 <div class='section' id='vocab-aoim'>
1454 <p class='title'>The AOIM Taxonomy</p>
1456 <p>Namespace: <span class='url'>@BASEURI@/AOIM</span>.</p>
1458 <p>This vocabulary is published by the IVOA to allow images to be tagged
1459 with keywords that are relevant for the public.
1460 It consists of a set of keywords organised into an enumerated
1461 hierarchical structure.
1462 Each term consists of a taxonomic number and a label.
1463 There are no definitions, scope notes, or cross references.
1464 </p>
1466 <p>When converting the AOIM into SKOS, it was decided to model the
1467 taxonomic number as an alternative label.
1468 Since there are duplication of terms, the token for a term consists of
1469 the full hierarchical location of the term.
1470 Thus, it is possible to distinguish between</p>
1471 <pre>Planet -> Feature -> Surface -> Canyon</pre>
1472 <p>and</p>
1473 <pre>Planet -> Satellite -> Feature -> Surface -> Canyon</pre>
1474 <p>which have the tokens <code>PlanetFeatureSurfaceCanyon</code> and
1475 <code>PlanetSatelliteFeatureSurfaceCanyon</code> respectively.
1476 </p>
1478 </div>
1480 <div class='section' id='vocab-ucd1'>
1481 <p class='title'>The UCD1+ Vocabulary</p>
1483 <p>Namespace: <span class='url'>@BASEURI@/UCD</span>.</p>
1485 <p>The UCD standard is an officially sanctioned and managed vocabulary
1486 of the IVOA. The normative document is a simple text file containing
1487 entries consisting of tokens (for example <code>em.IR</code>), a short
1488 description, and usage information (<q>syntax codes</q> which permit
1489 UCD tokens to be concatenated). The form of the tokens implies a
1490 natural hierarchy: <code>em.IR.8-15um</code> is obviously a narrower
1491 term than <code>em.IR</code>, which in turn is narrower than
1492 <code>em</code>.</p>
1494 <p>Given the structure of the UCD1+ vocabulary, the natural
1495 translation to SKOS consists of preferred labels equal to the original
1496 tokens (the UCD1 words include dashes and periods), vocabulary tokens
1497 created using guidelines in <span class='xref'
1498 >practices</span> (for example, "emIR815Um" for
1499 <code>em.IR.8-15um</code>), direct use of the definitions, and the syntax codes
1500 placed in usage documentation: <code>&lt;skos:scopeNote&gt;UCD syntax code: P&lt;/skos:scopeNote&gt;</code></p>
1502 <p>Note that the SKOS document containing the UCD1+ vocabulary does
1503 NOT consistute the official version: the normative document is still
1504 the text list. However, on the long term, the IVOA may decide to make
1505 the SKOS version normative, since the SKOS version contains all of the
1506 information contained in the original text document but has the
1507 advantage of being in a standard format easily read and used by any
1508 application on the semantic web whilst still being usable in the
1509 current ways.</p>
1511 </div>
1513 <div class='section' id='vocab-iau93'>
1514 <p class='title'>The 1993 IAU Thesaurus</p>
1516 <p>Namespace: <span class='url'>@BASEURI@/IAUT93</span>.</p>
1518 <p>The IAU Thesaurus consists of concepts with mostly capitalised
1519 labels and a rich set of thesaurus relationships (<q>BT</q> for
1520 "broader term", <q>NT</q> for <q>narrower term</q>, and <q>RT</q> for
1521 <q>related term</q>). The thesaurus also contains <q>U</q> (for
1522 <q>use</q>) and <q>UF</q> (<q>use for</q>) relationships. In a SKOS
1523 model of a vocabulary these are captured as alternative labels. A
1524 separate document contains translations of the vocabulary terms in
1525 five languages: English, French, German, Italian, and
1526 Spanish. Enumerable concepts are plural (for example <q>SPIRAL
1527 GALAXIES</q>) and non-enumerable concepts are singular
1528 (for example <q>STABILITY</q>). Finally, there are some usage hints like
1529 <q>combine with other</q>, which have been modelled as scope notes.</p>
1531 <p>In converting the IAU Thesaurus to SKOS, we have been as faithful
1532 as possible to the original format of the thesaurus. Thus, preferred
1533 labels have been kept in their uppercase format.</p>
1535 <p>The IAU Thesaurus has been unmaintained since its initial production in
1536 1993; it is therefore significantly out of date in places. This
1537 vocabulary is published for the sake of completeness, and to make the
1538 link between the evolving vocabulary work and any uses of the 1993
1539 vocabulary which come to light. We do not expect to make any future
1540 maintenance changes to this vocabulary, and would expect the IVOAT
1541 vocabulary, based on this one, to be used instead (see <span class='xref'>vocab-ivoat</span>).</p>
1543 </div>
1545 <div class='section' id='vocab-ivoat'>
1546 <p class='title'>Towards an IVOA Thesaurus</p>
1548 <p>While it is true that the adoption of SKOS will make it easy to
1549 publish and access different astronomical vocabularies, the fact is
1550 that there is no vocabulary which makes it easy to jump-start the
1551 use of vocabularies in generic astrophysical VO applications: each of
1552 the previously developed vocabularies has their own limits and
1553 biases. For example, the IAU Thesaurus provides a large number of
1554 entries, copious relationships, and translations to four other languages,
1555 but there are no definitions, many concepts are now only useful for
1556 historical purposes (for example many photographic or historical instrument
1557 entries), some of the relationships are false or outdated, and many
1558 important or newer concepts and their common abbreviations are
1559 missing.</p>
1561 <p>Despite its faults, the IAU Thesaurus constitutes a very extensive
1562 vocabulary which could easily serve as the basis vocabulary once
1563 we have removed its most egregious faults and extended it to cover the
1564 most obvious semantic holes. To this end, a heavily revised IAU
1565 thesaurus is in preparation for use within the IVOA and other
1566 astronomical contexts. The goal is to provide a general vocabulary
1567 foundation to which other, more specialised, vocabularies can be added
1568 as needed, and to provide a good <q>lingua franca</q> for the creation of
1569 vocabulary mappings.</p>
1570 </div>
1571 </div> <!-- End: Example vocabularies -->
1573 <div class='section' id='distmappings'>
1574 <p class='title'>Mapping vocabularies (informative)</p>
1576 <p>Part of the motivation for formalising vocabularies within the VO
1577 is to support <em>mapping between vocabularies</em>, so that an
1578 application which understands, or can natively process, one
1579 vocabulary, can use a mapping to provide at least partial support for
1580 data described using another vocabulary. The SKOS standard describes
1581 a number of properties for expressing such matches, and we anticipate
1582 that we will shortly see explicit mappings between vocabularies,
1583 produced either by vocabulary maintainers, describing the
1584 relationships between their own vocabularies and others, or by third
1585 parties, asserting such relationships as an intellectual contribution
1586 of their own.</p>
1588 <p>The vocabularies distributed in association with this document
1589 include some non-exhaustive mappings between vocabularies, as an
1590 example of how such mappings will appear.</p>
1592 <!--
1593 <p>To show how mappings can be expressed between two vocabularies, we
1594 have provided one example mapping document which maps the concepts in
1595 the A&amp;A Keywords vocabulary to the concepts in the AOIM
1596 vocabulary.
1597 All four types of mappings were required.
1598 Since all the mapping relationships have inverse relationships
1599 defined, the mapping document can also be used to infer the set of
1600 mappings from the AOIM vocabulary to the A&amp;A keywords.</p>
1602 <p>To provide provenence information about the set of mappings in a
1603 document, Dublin Core metadata is included in the mapping
1604 document.</p>
1606 -->
1608 </div>
1610 <div class="appendices">
1612 <!-- <p><span class='todo'>To come</span></p>-->
1615 <div class="section-nonum" id="bibliography">
1616 <p class="title">References</p>
1617 <?bibliography rm-refs ?>
1618 </div>
1620 <p style="text-align: right; font-size: x-small; color: #888;">
1621 $Revision$ $Date$
1622 </p>
1624 </div>
1626 </body>
1627 </html>


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