Vocabularies in the Virtual Observatory
Version 1.0

IVOA Working Draft, 2008 March 20

This version
Latest version
and issues list
Working Group
Alasdair J G Gray, Norman Gray, Frederic V Hessman and Andrea Preite Martinez
Sébastien Derriere, Alasdair J G Gray, Norman Gray, Frederic V Hessman, Tony Linde, Andrea Preite Martinez, Rob Seaman and Brian Thomas

As the astronomical information processed within the Virtual Observatory becomes more complex, there is an increasing need for a more formal means of identifying quantities, concepts, and processes not confined to things easily placed in a FITS image, or expressed in a catalogue or a table. We proposed that the IVOA adopt a standard format for vocabularies based on the W3C's Resource Description Framework (RDF) and Simple Knowledge Organisation System (SKOS). By adopting a standard and simple format, the IVOA will permit different groups to create and maintain their own specialised vocabularies while letting the rest of the astronomical community access, use, and combine them. The use of current, open standards ensures that VO applications will be able to tap into resources of the growing semantic web. Several examples of useful astronomical vocabularies are provided, including work on a common IVOA thesaurus intended to provide a semantic common base for VO applications.

This is an IVOA Working Draft. The first release of this document was 2008 March 20.

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/.


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

Table of Contents

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, redshifts, 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 definitions, and which can include relationships between labels) and “ontologies” (where the domain is captured in a formal 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 supporting rapid notification of astronomical events. This standard requires some formalised indication of what a published event is “about”, in a formalism which can be used straightforwardly by the developer of relevant services. See 1.2. Use-cases, and the motivation for formalised vocabularies for further discussion.

A number of astronomical vocabularies have been created, with a variety of goals and intended uses. Some examples are detailed below.

The most immediate high-level motivation for this work is the requirement of the VOEvent standard [std:voevent] for a controlled vocabulary usable in the VOEvent's <Why/> and <What/> elements, which describe what sort of object the VOEvent packet is describing, in some broadly intelligible way. For example a “burst” might be 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, and having an explicit list of vocabulary terms can help guide the event publisher into using a term which will be usefully precise for the event's consumers. A free-text label can help here (which brings us into the domain sometimes referred to as folksonomies), but the astronomical community, with a culture sympathetic to international agreement, can do better.

The purpose of this proposal is to establish a set of conventions for the creation, publication, 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:skosref].

Specific use-cases include the following.

It is not a goal of this standard, as it is not a goal of SKOS, to produce knowledge-engineering artefacts which can support elaborate machine reasoning – such artefacts would be very valuable, but require much more expensive work on ontologies. As the supernova use-case above illustrates, even simple vocabularies can support useful machine reasoning.

It is also not a goal of this standard to produce new vocabularies, or substantially alter existing ones; instead, the vocabularies included below in section 4. Example vocabularies are directly derived from existing vocabularies (the exceptions are the IVOAT vocabulary, which is ultimately intended to be a significant update to the IAU-93 original, and the constellations vocabulary, which is intended to be purely didactic). It therefore follows that the ambiguities, redundancies and incompleteness of the source vocabularies are faithfully represented in the distributed SKOS vocabularies. We hope that this formalisation process will create greater visibility and broader use for the various vocabularies, and that this will guide the maintenance efforts of the curating groups.

The reason for both of these limitations is that vocabularies are extremely expensive to produce, maintain and deploy, and we must therefore rely on such vocabularies as have been developed, and attached as metadata to resources, by others. Such vocabularies are less rich or less coherent than we might prefer, but widely enough deployed to be useful.

The purpose of this document is not to produce new vocabularies but to show how vocabularies can be easily expressed in a internationally standardized, computer-manipulable format. Four example vocabularies that have previously been expressed using non-standardized formats – namely the A&A keyword list, the IAU and AOIM thesauri, and UCD1 – are included below as an illustration of how simple it is to publish them in SKOS. In all cases, one can easily express all of the information of the original source vocabularies. It is true, that complex vocabularies can be expensive to produce, maintain, and deploy, and it will take a while until VO applications will be able to use them to full advantage. However, the example vocabularies should make it fairly easy to boot-strap the process by providing useful vocabularies that can be used out of the box.

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, help interest groups formalise as many of them as are appropriate, and standardise the process of formally declaring the relationships between them. This means that:

In this section, we introduce the concepts of SKOS-based vocabularies, and the technology of mapping between them. We describe some additional requirements for IVOA vocabularies in the next section, 3. Publishing vocabularies (normative).

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 Organisation System) format, a W3C draft standard application of RDF to the field of knowledge organisation [std:skosref]. 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 document 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. (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. These notions have an operational definition: any resource retrieved as a result of a search on a given term will also be retrievable through a search on that term's “broader term” (“narrower” is a simple inverse, so that for any pair of terms, if A skos:broader B, then B skos:narrower A; a term may have multiple narrower and broader terms). 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].

The purpose of a thesaurus is to help users find resources they might be interested in, be they library books, image archives, or VOEvent packets.

A published vocabulary in SKOS format consists of a set of “concepts” – an example concept capturing the vocabulary information about spiral galaxies is provided in the Figure below, with the RDF shown in both RDF/XML [std:rdfxml] and Turtle notation [std:turtle] (Turtle is similar to the more informal N3 notation). The elements of a concept are detailed below.

Figure: examples of SKOS vocabularies

XML Syntax Turtle Syntax
<skos:Concept rdf:about="#spiralGalaxy">
  <skos:prefLabel lang="en">
    spiral galaxy
  <skos:prefLabel lang="de">
  <skos:altLabel lang="en">
    spiral nebula
  <skos:hiddenLabel lang="en">
    spiral glaxy
  <skos:definition lang="en">
    A galaxy having a spiral structure.
  <skos:scopeNote lang="en">
    Spiral galaxies fall into one of 
    three catagories: Sa, Sc, and Sd.
<#spiralGalaxy> a skos:Concept;
    "spiral galaxy"@en, 
  skos:altLabel "spiral nebula"@en;
  skos:hiddenLabel "spiral glaxy"@en;
  skos:definition """A galaxy having a 
    spiral structure."""@en;
  skos:scopeNote """Spiral galaxies fall
    into one of three categories:
    Sa, Sc, and Sd"""@en;
  skos:narrower <#barredSpiralGalaxy>;
  skos:broader <#galaxy>;
  skos:related <#spiralArm> .

A SKOS vocabulary includes the following features.

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

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.

Part of the SKOS standard [std:skosref] allows a concept in one vocabulary to be related to a concept in another vocabulary. There are four types of relationship provided to capture the relationships between concepts in vocabularies, which are similar to those defined for relationships between concepts within a single vocabulary. The types of mapping relationships are:

The semantic mapping relationships have certain properties. The broadMatch relationship has the narrowMatch relationship as its inverse and the exactMatch and relatedMatch relationships are symmetrical. The consequence of these properties is that if you have a mapping from concept A in one vocabulary to concept B in another vocabulary then you can infer a mapping from concept B to concept A.

A vocabulary which conforms to this IVOA standard has the following features. In this section, the keywords must, should and so on, are to be interpreted as described in [std:rfc2119].

3.1.1. Dereferenceable namespace

The namespace of the vocabulary must be dereferenceable on the web. That is, typing the namespace URL into a web browser will produce human-readable documentation about the vocabulary. In addition, the namespace URL should return the RDF version of the vocabulary if it is retrieved with an HTTP Accept header of application/rdf+xml.

Rationale: These prescriptions are intended to be compatible with the patterns described in [berrueta08] and [sauermann07], and vocabulary distributors should follow these patterns where possible.

3.1.2. Long-term availability

The files defining a vocabulary, including those of superseded versions, should remain permanently available. There is no requirement that the namespace URL be at any particular location, although the IVOA web pages, or the online sections of the A&A journal would likely be suitable archival locations.

3.1.3. Distribution format

Vocabularies must be made available for distribution as SKOS RDF files, in either RDF/XML [std:rdfxml] or Turtle [std:turtle] format; vocabularies should be made available in both formats. See issue [distformat-2].

A publisher may make available documentation and supporting files in other formats.

Rationale: this does imply that the vocabulary source files can only realistically be parsed using an RDF parser. An alternative is to require that vocabularies be distributed using a subset of RDF/XML which can also be naively handled as traditional XML; however as well as creating an extra standardisation requirement, this would make it effectively infeasible to write out the distribution version of the vocabulary using an RDF or general SKOS tool.

3.1.4. Clearly versioned vocabulary

Open issue. There are interactions with the requirements for long-term availability and a dereferenceable namespace, since this implies that the vocabulary version should be manifestly encoded in the namespace URI. See issue [versioning-3].

3.1.5. No restrictions on source files

This standard does not place any restrictions on the format of the files managed by the maintenance process, as long as the distributed files are as specified above. See issue [masterformat-1].

This standard imposes a number of requirements on conformant vocabularies (see 3. Publishing vocabularies (normative)). In this section we list a number of good practices that IVOA vocabularies should abide by. Some of the prescriptions below are more specific than good-practice guidelines for vocabularies in general.

The adoption of the following guidelines will make it easier to use vocabularies in generic VO applications. However, VO applications should be able to accept any vocabulary that complies with the latest SKOS standard [std:skosref] (this does not imply, of course, that an application will necessarily understand the terms in an alien vocabulary, although the presence of mappings to a known vocabulary should allow it to derive some benefit).

  1. Concept identifiers should consist only of the letters a-z, A-Z, and numbers 0-9, i.e. no spaces, no exotic letters (for example 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, since the exotic letters can be used within the labels and documentation if appropriate.
  2. The concept identifiers should be kept in human-readable form, directly reflect the implied meaning, and not be semi-random identifiers only (for example, use 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 capitalisation of the first sub-token character (for example the label My favourite idea-label #42 is converted into MyFavouriteIdeaLabel42).
  3. Labels should be in the form of the source vocabulary. When developing a new vocabulary the singular form should be preferred, for example spiral galaxy, not spiral galaxies. Open issue
  4. 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. Each concept should have additional documentation using SKOS Notes or Dublin Core terms as appropriate (see [std:skosref])
  5. The language localisation should be declared where appropriate, in preferred labels, alternate labels, definitions, and the like.
  6. Relationships (“broader”, “narrower”, “related”) between concepts should be present, but are not required; if used, they should be complete (thus all “broader” links have corresponding “narrower” links in the referenced entries and “related” entries link each other).
  7. “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).
  8. The SKOS standard describes some good practices for vocabulary maintenance, such as using <skos:changeNote> and the like. Publishers should respect such good maintenance practices are are available.
  9. Publishers should 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. Open issue.

The intent of having the IVOA adopt SKOS as the preferred 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 vocabulary basis for VO applications.

The vocabularies described below are included, as SKOS files, in the distributed version of this standard. These vocabularies have stable URLs Format TBD, see [versioning-3], and may be cited and used indefinitely. These vocabularies will not, however, be developed as part of the maintenance of this standard. Interested groups, within and outwith the IVOA, are encouraged to take these as a starting point and absorb them within existing processes.

The exceptions to this rule are the constellation vocabulary, provided here mainly for didactic purposes, and the proposed IVOA Thesaurus, which is being developed as a separate project and whose aim is to provide a corrected, more user-friendly, more complete, and updated version of the 1993 IAU thesaurus. Although work on the IVOA Thesaurus is on-going, the fact that it is largely based on the IAU thesaurus means that it is already a very useful resource, so a usable snapshot of this vocabulary will be published with the other examples.

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


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, “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. Various incorrect forms should probably be represented in SKOS hidden labels.

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

XML Syntax Turtle Syntax
<skos:Concept rdf:about="#constellation">
  <skos:inScheme rdf:resource=""/>
    IAU-sanctioned constellation names
  <skos:narrower rdf:resource="#Andromeda"/>
  <skos:narrower rdf:resource="#Vulpecula"/>
<#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:broader rdf:resource="#constellation"/>
    Cygnus is nominative form; the alternative
    labels are the genitive and short forms
<#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.

This vocabulary is a set of keywords made available on a web page by the publisher of the journal. The intended usage of the vocabulary is to tag articles with descriptive keywords to aid searching for articles on a particular topic.

The keywords are organised into categories which have been modelled as hierarchical relationships. Additionally, some of the keywords are grouped into collections which has been mirrored in the SKOS version. The vocabulary contains no definitions or related links as these are not provided in the original keyword list, and only a handful of alternative labels and scope notes that are present in the original keyword list.

This vocabulary is published by the IVOA to allow images to be tagged with keywords that are relevant for the public. It consists of a set of keywords organised into an enumerated hierarchical structure. Each term consists of a taxonomic number and a label. There are no definitions, scope notes, or cross references.

When converting the AOIM into SKOS, it was decided to model the taxonomic number as an alternative label. Since there are duplication of terms, the token for a term consists of the full hierarchical location of the term. Thus, it is possible to distinguish between

Planet -> Feature -> Surface -> Canyon


Planet -> Satellite -> Feature -> Surface -> Canyon

which have the tokens PlanetFeatureSurfaceCanyon and PlanetSatelliteFeatureSurfaceCanyon respectively.

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 (for example 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 guidelines in 3.2. Suggested good practices (for example, "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 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 whilst still being usable in the current ways.

The IAU Thesaurus consists of concepts with mostly capitalised labels and a rich set of thesaurus relationships (“BT” for "broader term", “NT” for “narrower term”, and “RT” for “related term”). 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. Enumerable concepts are plural (for example “SPIRAL GALAXIES”) and non-enumerable concepts are singular (for example “STABILITY”). Finally, there are some usage hints like “combine with other”, which have been modelled as scope notes.

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.

The IAU Thesaurus has been unmaintained since its initial production in 1993; it is therefore significantly out of date in places. This vocabulary is published for the sake of completeness, and to make the link between the evolving vocabulary work and any uses of the 1993 vocabulary which come to light. We do not expect to make any future maintenance changes to this vocabulary, and would expect the IVOAT vocabulary, based on this one, to be used instead (see 4.6. Towards an 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 (for example 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 egregious 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 specialised, vocabularies can be added as needed, and to provide a good “lingua franca” for the creation of vocabulary mappings.

To show how mappings can be expressed between two vocabularies, we have provided one example mapping document which maps the concepts in the A&A Keywords vocabulary to the concepts in the AOIM vocabulary. All four types of mappings were required. Since all the mapping relationships have inverse relationships defined, the mapping document can also be used to infer the set of mappings from the AOIM vocabulary to the A&A keywords.

To provide provenence information about the set of mappings in a document, Dublin Core metadata is included in the mapping document.

Mappings to appear: see issue [mappings-6].


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Revision: 104 Date: 2008-03-19 12:24:39 +0100 (Wed, 19 Mar 2008)