Vocabularies in the Virtual Observatory, v0.01

IVOA Working Draft, 2007 December 6 [DRAFT $Revision: 31 $]

Working Group

Semantics

This version

http://www.ivoa.net/Documents/ivoa-thesaurus-0.01

Latest version

http://www.ivoa.net/Documents/ivoa-thesaurus-0.01

Editors

TBD

Authors

Alasdair J G Gray, Norman Gray, Frederic V Hessman and Andrea Preite Martinez

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Abstract

Status of this document

This is an IVOA Working Draft. The first release of this document was 2007 December 6.

This document is an IVOA Working Draft for review by IVOA members and other interested parties. It is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use IVOA Working Drafts as reference materials or to cite them as other than work in progress.

A list of current IVOA Recommendations and other technical documents can be found at http://www.ivoa.net/Documents/.

Acknowledgments

We would like to thank the members of the IVOA semantic working group for many interesting ideas and fruitful discussions.

Table of Contents

1 Introduction

1.1 Vocabularies in astronomy

Astronomical information of relevance to the Virtual Observatory (VO) is not confined to quantities easily expressed in a catalogue or a table. Fairly simple things such as position on the sky, brightness in some units, times measured in some frame, redshits, classifications or other similar quantities are easily manipulated and stored in VOTables and can currently be identified using IVOA Unified Content Descriptors (UCDs) [std:ucd]. However, astrophysical concepts and quantities use a wide variety of names, identifications, classifications and associations, most of which cannot be described or labelled via UCDs.

There are a number of basic forms of organised semantic knowledge of potential use to the VO, ranging from informal folksonomies (where users are free to choose their own labels) at one extreme, to formally structured vocabularies (where the label is drawn from a predefined set of defintions which can include relationships between labels) and ontologies (where the domain is captured in a data model) at the other. More formal definitions are presented later in this document.

An astronomical ontology is necessary if we are to have a computer (appear to) `understand' something of the domain. There has been some progress towards creating an ontology of astronomical object types [std:ivoa-astro-onto] to meet this need. However there are distinct use cases for letting human users find resources of interest through search and navigation of the information space. The most appropriate technology to meet these use cases derives from the Information Science community, that of controlled vocabularies, taxonomies and thesauri. In the present document, we do not distinguish between controlled vocabularies, taxonomies and thesauri, and use the term vocabulary to represent all three.

One of the best examples of the need for a simple vocabulary within the VO is VOEvent [std:voevent], the VO standard for handling astronomical events: if someone broadcasts, or `publishes', the occurrence of an event, the implication is that someone else is going to want to respond to it, but no institution is interested in all possible events, so some standardised information about what the event `is about' is necessary, in a form which ensures that the parties can communicate effectively. If a `burst' is announced, is it a Gamma Ray Burst due to the collapse of a star in a distant galaxy, a solar flare, or the brightening of a stellar or AGN accretion disk? If a publisher doesn't use the label one might have expected, how is one to guess what other equivalent labels might have been used?

There have been a number of attempts to create astronomical vocabularies.

• The Second Reference Dictionary of the Nomenclature of Celestial Objects [lortet94], [lortet94a] contains 500 paper pages of astronomical nomenclature
• For decades professional journals have used a set of reasonably compatible keywords to help classify the content of whole articles. These keywords have been analysed by Preite Martinez & Lesteven [preitemartinez07], from which they derived a set of common keywords constituting one of the potential bases for a fuller VO vocabulary. The same authors also attempted to derive a set of common concepts by analyzing the contents of abstracts in journal articles, which should comprise a list of tokens/concepts more up-to-date than the old list of journal keywords. A similar but less formal attempt was made by Hessman [hessman05] for the VOEvent working group, resulting in a similar list Check differences from the A&A list.
• Astronomical databases generally use simple sets of keywords – sometimes hierarchically organized – to aid the users in the querying of the databases. Two examples from totally different contexts are the list of object types used in the Simbad database and the search keywords used in the educational Hands-On Universe image database portal.
• The Astronomical Outreach Imagery (AOI) working group has created a simple taxonomy for helping to classify images used for educational or public relations [std:aoim].
• In 1993, Shobbrook and Shobbrook published an Astronomy Thesaurus endorsed by the IAU [shobbrook92]. This collection of nearly 3000 terms, in five languages, is a valuable resource, but has seen little use in recent years. Its very size, which gives it expressive power, is a disadvantage to the extent that it is therefore hard to use.
• The Unified Content Descriptors [std:ucd] (UCD) constitute the main controlled vocabulary of the IVOA and contains some taxonometric information. However, UCD suffers from two major problems which makes it difficult to use beyond the present applications of labeling VOTables: firstly, there is no standard means of identifying and processing the contents of the text-based reference document; and secondly, the content cannot be openly extended beyond that set by a formal IVOA committee without going through a laborious and time-consuming negotiation process of extending the primary vocabulary itself.

1.2 Formalising and managing multiple vocabularies

We find ourselves in the situation where there are multiple vocabularies in use, describing a broad range of resources of interest to professional and amateur astronomers, and members of the public. These different vocabularies use different terms and different relationships to support the different constituencies they cater for. For example, delta Sct and RR Lyr are terms one would find in a vocabulary aimed at professional astronomers, associated with the notion of variable star; however one would not find such technical terms in a vocabulary intended to support outreach activities.

One approach to this problem is to create a single consensus vocabulary, which draws terms from the various existing vocabularies to create a new vocabulary which is able to express anything its users might desire. The problem with this is that such an effort would be very expensive: both in terms of time and effort on the part of those creating it, and to the potential users, who have to learn to navigate around it, recognise the new terms, and who have to be supported in using the new terms correctly (or, more often, incorrectly).

The alternative approach to the problem is to evade it, and this is the approach taken in this document. Rather than deprecating the existence of multiple overlapping vocabularies, we embrace it, formalise all of them, and formally declare the relationships between them. This means that:

• The various vocabularies are allowed to evolve separately, on their own timescales, managed either by the IVOA, individual working groups within the IVOA, or by third parties;
• Specialized vocabularies can be developed and maintained by the community with the most knowledge about a specific topic, ensuring that the vocabulary will have the right breadth, depth, and accuracy;
• Users can choose the vocabulary or combination of vocabularies most appropriate to their situation, either when annotating resources, or when querying them; and
• We can retain the previous investments made in vocabularies by users and resource owners.

The purpose of this proposal is to establish a common format for the grass-roots creation, publishing, use, and manipulation of astronomical vocabularies within the Virtual Observatory, based upon the W3C's SKOS standard. We include as appendices to this proposal formalised versions of a number of existing vocabularies, encoded as SKOS vocabularies [std:skoscore].

2 SKOS-based vocabularies

2.1 Selection of the vocabulary format

After extensive online and face-to-face discussions, the authors have brokered a consensus within the IVOA community that formalised vocabularies should be published at least in SKOS (Simple Knowledge Organising Systems) format, a W3C draft standard application of RDF to the field of knowledge organisation [std:skoscore]. SKOS draws on long experience within the Library and Information Science community, to address a well-defined set of problems to do with the indexing and retrieval of information and resources; as such, it is a close match to the problem this working group is addressing.

ISO 5964 [std:iso5964] defines a number of the relevant terms (ISO 5964:1985=BS 6723:1985; see also [std:bs8723-1] and [std:z39.19]), and some of the (lightweight) theoretical background. The only technical distinction relevant to this document is that between `vocabulary' and `thesaurus': BS-8723-1 defines a thesaurus as a

Controlled vocabulary in which concepts are represented by preferred terms, formally organized so that paradigmatic relationships between the concepts are made explicit, and the preferred terms are accompanied by lead-in entries for synonyms or quasi-synonyms. NOTE: The purpose of a thesaurus is to guide both the indexer and the searcher to select the same preferred term or combination of preferred terms to represent a given subject. (BS-8723-1, sect. 2.39)

with a similar definition in ISO-5964 sect. 3.16. The paradigmatic relationships in question are those relating a term to a broader, narrower or more generically related term, with an operational definition of broader term which is such that a resource retrieved by a given term will also be retrieved by that term's broader term. This is not a subsumption relationship, as there is no implication that the concept referred to by a narrower term is of the same type as a broader term.

Thus a vocabulary (SKOS or otherwise) is not an ontology. It has lighter and looser semantics than an ontology, and is specialised for the restricted case of resource retrieval. Those interested in ontological analyses can easily transfer the vocabulary relationship information from SKOS to a formal ontological format such as OWL [std:owl].

2.2 Content and format of a SKOS vocabulary

A published vocabulary in SKOS format consists of a set of concepts – the examples below are shown in both the RDF/XML and the Turtle notation for RDF [std:turtle] (this is similar to the more informal N3 notation). Each concept should contain the following elements:

• A single URI representing the concept, mainly for use by computers but preferably human-readable, e.g. an entry for spiral galaxy might look like:
  
  

  XML Syntax		Turtle Syntax
  <skos:Concept rdf:about="#spiralGalaxy"> ... </skos:Concept>		<#spiralGalaxy> a skos:Concept

• A prefered label in each supported language for the vocabulary for use by humans, e.g.
  
  

  <skos:prefLabel lang="en">spiral galaxy</prefLabel> <skos:prefLabel lang="de">Spiralgalaxie</prefLabel>

  skos:prefLabel "spiral galaxy"@en, "Spiralgalaxie"@de

• Optional alternative labels which applications may encounter or in common use, whether simple synonyms or commonly-used aliases, e.g. GRB for "gamma-ray burst":
  
  

  <skos:altLabel lang="en">GRB</prefLabel>

  skos:altLabel "GRB"@en

• Optional hidden labels which capture common misspellings for either the preferred or alternate labels, e.g. spiral glaxy for spiral galaxy:
  
  

  <skos:hiddenLabel lang="en">spiral glaxy</prefLabel>

  skos:altLabel "spiral glaxy"@en

  .
• A definition for the concept, where one exists in the original vocabulary, to clarify the meaning of the term, e.g.
  
  

  <skos:definition lang="en"> A galaxy having a spiral structure. </skos:definition>

  skos:definition "A galaxy having a spiral structure."@en

  .
• A scope note to further clarify a defintion, or the usage of the concept, e.g.
  
  

  <skos:scopeNote lang="en"> Spiral galaxies fall into one of three catagories: Sa, Sc, and Sd. </skos:scopeNote>

  skos:scopeNote "Spiral galaxies fall into one of three categories: Sa, Sc, and Sd"@en.

  .
• Optional, a concept may be involved in any number of relationships to other concepts. The types of relationships are
  • Narrower or more specific concepts, e.g. a link to the concept representing a barred spiral galaxy:
    
    

    <skos:narrower rdf:resource="#barredSpiralGalaxy"/>

    skos:narrower <#barredSpiralGalaxy>

  • Broader or more general concepts, e.g. a link to the token representing galaxies in general:
    
    

    <skos:broader rdf:resource="#galaxy"/>

    skos:broader <#galaxy>

  • Related concepts, e.g. a link to the token representing spiral arms of galaxies:
    
    

    <skos:related rdf:resource="#spiralArm"/>

    skos:related <#spiralArm>

    
    (note this relationship does not say that spiral galaxies have spiral arms – that would be ontological information of a higher order which is beyond the requirements for information stored in a vocabulary).

In addition to the information about a single concept, a vocabulary can contain information to help users navigate its structure and contents:

• The top concepts of the vocabulary, i.e. those that occur at the top of the vocabulary hierarchy defined by the broader/narrower relationships, can be explicitly stated to make it easier to navigate the vocabulary.
• Concepts that form a natural group can be defined as being members of a collection.
• Versioning information can be added using change notes.
• Additional metadata about the vocabulary, e.g. the publisher, may be documented using the Dublin Core metadata set [std:dublincore].

2.3 Relationships Between Vocabularies

There already exist several vocabularies in the domain of astronomy. Instead of attempting to replace all these existing vocabularies, which have been developed to achieve different aims and user groups, we embrace them. This requires a mechanism to relate the concepts in the different vocabularies. The W3C are in the process of developing a standard for relating the concepts in different SKOS vocabularies [std:skosMapping] and when completed this should be reviewed for use by the IVOA.

Four types of relationship are sufficient to capture the relationships between concepts in vocabularies and are similar to those defined for relationships between concepts within a single vocabulary. The relationships are as follows. [TODO] Add specifics to the examples.

• Equivalence between concepts, i.e. the concpets in the different vocabularies refer to the same real world entity. This is captured with the following RDF statement iau93:#SPIRALGALAXY map:exactMatch ivoat:#spiralGalaxy which states the the spiral galaxy concept in the IAU thesaurus is the same as the spiral galaxy concept in the IVOAT. (Note the use of an external namespaces iau93 and ivoat which must be defined within the document.)
• Broader concept, i.e. there is not an equivalent concept but there is a more general one. This is captured with the RDF statement iau93:#XXX map:broadMatch ivoat:#YYY which states that the IVOAT concept YYY is more general than the IAU93 concept XXX.
• Narrower concept, i.e. there is not an equivalent concept but there is a more specific one. This is captured with the RDF statement iau93:#XXX map:narrowMatch ivoat:#YYY which states that the IVOAT concept YYY is more specific than the IAU93 concept XXX.
• Related concept, i.e. there is some form of relationship. This is captured with the RDF statement iau93:#XXX map:relatedMatch ivoat:#YYY which states that the IAU93 concept XXX has an association with the IVOAT concept YYY.

[TODO:] Enter text regarding the resolution of issue 7.

2.4 Suggested good practices

As long as the vocabularies conform to the SKOS standard and published in a machine processable RDF format, there is nothing keeping a VO application from using the vocabulary to support the human user and to enable new connections between different sources of information. However, we have identified a set of best practice rules which, if followed, will make the creation, management, and use of the vocabularies within the VO simpler and more effective:

1. The SKOS documents defining the vocabulary should be published at a long-term accessible URI and should be mirrored at a central IVOA vocabulary repository. Each version of the vocabulary should be indicated within the name (e.g. "MyFavoriteVocabulary-v3.14") and previous versions should continue to be available even after having been subsumed by newer versions; Published vocabulary updates should be infrequent and individual changes should be documented, e.g. by <skos:changeNote>. The vocabulary namespace should be the same as the location of the vocabulary.
2. Concept identifiers should consist only of the letters a-z, A-Z, and numbers 0-9, i.e. no spaces, no exotic letters (e.g. umlauts), and no characters which would make a token inexpressible as part of a URI; since tokens are for use by computers only, this is not a big restriction - the exotic letters can be used within the labels and documentation if appropriate.
3. Token names should be kept in human-readable form, directly reflect the implied meaning, and not be semi-random identifiers only (e.g. spiralGalaxy, not "t1234567"); tokens should preferably be created via a direct conversion from the preferred label via removable/translation of non-token characters (see above) and sub-token separation via capitalization of the first sub-token character (e.g. the label "My favorite idea-label #42" is converted into "MyFavoriteIdeaLabel42"). Open issue
4. Labels should be in the form of the source vocabulary. When developing a new vocabulary the singular form is preferred, e.g. spiral galaxy, not "spiral galaxies". Open issue
5. Each concept should have a definition (skos:definition) that constitutes a short description of the concept which could be adopted by an application using the vocabulary; The use of additional documentation in standard SKOS or Dublin format (see above) is encouraged. Note distinction between description and SKOS scope-note
6. The language localization should be declared where appropriate, e.g. preferred labels, alternate labels, defintions, etc.
7. Relationships (broader, narrower, related) between concepts are encouraged, but not required; if used, they should be complete (e.g. all broader links have corresponding narrower links in the referenced entries and related entries link each other).
8. TopConcept entries (see above) should be declared and normally consist of those concepts that do not have any broader relationships (i.e. not at a sub-ordinate position in the hierarchy).
9. Publishers are encouraged to publish mappings between their vocabularies and other commonly used vocabularies. These should be external to the defining vocabulary document so that the vocabulary can be used independently of the publisher's mappings.

These suggestions are by no means trivial – there was considerable discussion within the semantic working group on many of these topics, particularly about token formats (some wanted lower-case only), and singular versus plural forms of the labels (different traditions exist within the international library science community). Obviously, no publisher of an astronomical vocabulary has to adopt these rules, but the adoption of these rules will make it easier to use the vocabularly in external generic VO applications. However, VO applications should be developed to accept any vocabulary that complies with the latest SKOS standard [std:skoscore].

3 Example vocabularies

The intent of having the IVOA adopt SKOS as the prefered format for astronomical vocabularies is to encourage the creation and management of diverse vocabularies by competent astronomical groups, so that users of the VO and related resources can benefit directly and dynamically without the intervention of the IAU or IVOA. However, we felt it important to provide several examples of vocabularies in the SKOS format as part of the proposal, to illustrate their simplicity and power, and to provide an immediate vocabular basis for VO applications.

We provide a set of SKOS files representing the vocabularies which have been developed, and mappings between them. These can be downloaded at the URL

http://www.ivoa.net/Documents/ivoa-thesaurus-0.01/dist-XXX.tar.gz

[To be expanded:] there are no mappings at the moment. Also, the vocabularies are all in a single language, though translations of the IAU93 thesaurus are available.

3.1 A Constellation Name Vocabulary (normative)

This vocabulary is presented as a simple example of an astronomical vocabulary for a very particular purpose, e.g. handling constellation information like that commonly encountered in variable star research. For example, SS Cygni is a cataclysmic variable located in the constellation Cygnus. The name of the star uses the genitive form Cygni, but the alternate label SS Cyg uses the standard abbreviation Cyg. Given the constellation vocabulary, all of these forms are recorded together in a computer-manipulatable format. `Incorrect' forms should probably be represented in SKOS `hidden labels'

The <skos:ConceptScheme> contains a single <skos:TopConcept>, constellation

<skos:Concept rdf:about="#constellation">
  <skos:inScheme rdf:resource=""/>
  <skos:prefLabel>
    constellation
  </skos:prefLabel>
  <skos:definition>
    IAU-sanctioned constellation names
  </skos:definition>
  <skos:narrower rdf:resource="#Andromeda"/>
  ...
  <skos:narrower rdf:resource="#Vulpecula"/>
</skos:Concept>

<#constellation> a :Concept;
  :inScheme <>;
  :prefLabel "constellation";
  :definition "IAU-sanctioned constellation names";
  :narrower <#Andromeda>;
  ...
  :narrower <#Vulpecula>.

and the entry for Cygnus is

<skos:Concept rdf:about="#Cygnus">
  <skos:inScheme rdf:resource=""/>
  <skos:prefLabel>Cygnus</skos:prefLabel>
  <skos:definition>Cygnus</skos:definition>
  <skos:altLabel>Cygni</skos:altLabel>
  <skos:altLabel>Cyg</skos:altLabel>
  <skos:broader rdf:resource="#constellation"/>
  <skos:scopeNote>
    Cygnus is nominative form; the alternative
    labels are the genitive and short forms
  </skos:scopeNote>
</skos:Concept>

<#Cygnus> a :Concept;
  :inScheme <>;
  :prefLabel "Cygnus";
  :definition "Cygnus";
  :altLabel "Cygni";
  :altLabel "Cyg";
  :broader <#constellation>;
  :scopeNote "Cygnus is nominative form; the alternative
    labels are the genitive and short forms".

Note that SKOS alone does not permit the distinct differentiation of genitive forms and abbreviations, but the use of alternate labels is more than adequate enough for processing by VO applications where the difference between SS Cygni, SS Cyg, and the incorrect form SS Cygnus is probably irrelevant.

3.2 The 1993 IAU Thesaurus (normative)

The IAU Thesaurus consists of concepts with mostly capitalized labels and a rich set of thesaurus relationships (BF for "broader form", NF for narrower form, and RF for related form). The thesaurus also contains U (for use) and UF (use for) relationships. In a SKOS model of a vocabulary these are captured as alternative labels. A separate document contains translations of the vocabulary terms in five languages: English, French, German, Italian, and Spanish. Enumeratable concepts are plural (e.g. SPIRAL GALAXIES) and non-enumerable concepts are singular (e.g. STABILITY). Finally, there are some useage hints like combine with other

In converting the IAU Thesaurus to SKOS, we have been as faithful as possible to the original format of the thesaurus. Thus, preferred labels have been kept in their uppercase format.

3.3 The Astronomy & Astrophysics Keyword List (normative)

[TODO] AG to write a short description here

3.4 The AOIM Taxonomy (normative)

[TODO] AG to write a short description here

3.5 The UCD1+ Vocabulary (non-normative)

The UCD standard is an officially sanctioned and managed vocabulary of the IVOA. The normative document is a simple text file containing entries consisting of tokens (e.g. em.IR), a short description, and usage information (syntax codes which permit UCD tokens to be concatenated). The form of the tokens implies a natural hierarchy: em.IR.8-15um is obviously a narrower term than em.IR, which in turn is narrower than em.

Given the structure of the UCD1+ vocabulary, the natural translation to SKOS consists of preferred labels equal to the original tokens (the UCD1 words include dashes and periods), vocabulary tokens created using the "5th Commandment" (e.g. "emIR815Um" for em.IR.8-15um), direct use of the definitions, and the syntax codes placed in usage documentation: <skos:scopeNote>UCD syntax code: P</skos:scopeNote> NOTE: THIS IS THE FORMAT I USED IN MY VERSION - MAY NOT BE THE SAME AS NORMAN'S [FVH]

Note that the SKOS document containing the UCD1+ vocabulary does NOT consistute the official version: the normative document is still the text list. However, on the long term, the IVOA may decide to make the SKOS version normative, since the SKOS version contains all of the information contained in the original text document but has the advantage of being in a standard format easily read and used by any application on the semantic web.

3.6 The proposed IVOA Thesaurus

While it is true that the adoption of SKOS will make it easy to publish and access different astronomical vocabularies, the fact is that there is no vocabulary which makes it easy to jump-start the use of vocabularies in generic astrophysical VO applications: each of the previously developed vocabularies has their own limits and biases. For example, the IAU Thesaurus provides a large number of entries, copious relationships, and translations to four other languages, but there are no definitions, many concepts are now only useful for historical purposes (e.g. many photographic or historical instrument entries), some of the relationships are false or outdated, and many important or newer concepts and their common abbreviations are missing.

Despite its faults, the IAU Thesaurus constitutes a very extensive vocabulary which could easily serve as the basis vocabulary once we have removed its most egregrious faults and extended it to cover the most obvious semantic holes. To this end, a heavily revised IAU thesaurus is in preparation for use within the IVOA and other astronomical contexts. The goal is to provide a general vocabulary foundation to which other, more specialized, vocabularies can be added as needed, and to provide a good lingua franca for the creation of vocabulary mappings.

Appendices

Bibliography

[hessman05] F V Hessman.

VOConcepts - a proposed UCD for astronomical objects, events, and processes. [Online, cited January 2008].

[lortet94] M-C Lortet, S Borde, and F Ochsenbein.

Second reference dictionary of the nomenclature of celestial objects. Astron.\ Ap.\ Supp, 107 pp. 193-218, 1994. [Online].

[lortet94a] M-C Lortet, S Borde, and F Ochsenbein.

The second reference dictionary of the nomenclature of celestial objects (solar system excluded). volumes i, ii.. Technical Report 24, Centre des Données astronomique des Strasbourg, 1994. [Online].

[preitemartinez07] Andrea Preite Martinez and Soizick Lesteven.

Astronomical keywords in the era of the virtual observatory. IVOA Note, IVOA, 2007. [Online].

[shobbrook92] R.M. Shobbrook and R.R. Shobbrook.

The IAU thesaurus for improved on-line access to information. Proc. Astron. Soc. of Australia, 10 pp. 134, 1992. [Online].

[std:aoim] Robert Hurt, Lars Lindberg Christensen, and Adrienne Gauthier.

Astronomical outreach imagery metadata tags for the virtual observatory. [Online, cited January 2008].

[std:bs8723-1] Structured vocabularies for information retrieval - guide - definitions, symbols and abbreviations (BS 8723-1:2005).

British Standard, 2005.

[std:dublincore] DCMI Usage Board.

DCMI metadata terms. DCMI Recommendation, 2006. [Online].

[std:iso5964] Documentation - guidelines for the establishment and development of multilingual thesauri (ISO 5964:1985=BS 6723:1985).

International Standard, 1985.

[std:ivoa-astro-onto] L. Cambrésy, S. Derriere, P. Padovani, A. Preite Martinez, and A. Richard.

Ontology of astronomical object types. IVOA Working Draft, 2007. [Online].

[std:owl] World Wide Web Consortium.

The web ontology language. [Online].

[std:skosMapping] Alistair Miles and Dan Brickley.

SKOS mapping vocabulary specification. W3C Note, nov 2004. [Online].

[std:skoscore] Alistair Miles and Dan Brickley.

SKOS core guide. W3C Working Draft, nov 2005. [Online].

[std:turtle] Dave Beckett.

Turtle - terse RDF triple language. Draft Standard, nov 2007. [Online].

[std:ucd] Sébastien Derriere, Norman Gray, Robert Mann, Andrea Preite Martinez, Jonathan McDowell, Thomas McGlynn, François Ochsenbein, Pedro Osuna, Guy Rixon, and Roy Williams.

UCD (Unified Content Descriptor) - moving to UCD1+. [Online, cited July 2005].

[std:voevent] Sky event reporting metadata (voevent).

IVOA Recommendation, 2006. [Online].

[std:z39.19] Guidelines for the construction, format and management of monolingual thesauri (ANSI/NISO Z39.19-2005).

American National Standard, 2005. Closely corresponds to BS 8723:2005, parts 1 and 2, which replaces BS 5723; and to forthcoming ISO 25964, which replaces ISO 2788. [Online].

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