DL:Lesson 4 Metadata: Dublin Core Luca Dini

RDF Cases where metadata are used – Electronic Yellow Pages – Library – Video store You can search by fields and each field receive a "local" interpretation. On the web search is usually "brute force" Can we do something better? We need descriptions of resources but how?)

RDF Organising data presupposes complete knowledge of the world metadata ( Tim Bray) Being currently unavailable…

RDF The Resource Description Framework is a framework to describe and exchange metadata. A Resource is anything can be referred with a URI (a web page, a node in some xml document…) A property is a specific feature used to describe a resource by relating it to a value A statement is specific resource together with a named property and the value it assumes (subject, predicate, verb) Anna Goy

RDF (desiderata) Independence: every organization can create its own properties Interchange: RDF properties are XML. Scalability: Properties are triple which can be handled very easily (propositional logic) Recursivity: everything in the definition of a resource is a resource, and as such it can receive a definition – values can be resources – predicate can be resource (possibility to query them)

Why not XML? Scalability problem (embedding) Ordering properties are irrelevant. Sometimes one needs more complex relationships than the ones permitted by XML

Packaging Properties cannot (usually) be defined in isolation. Providers of metadata will generally group properties defined in their domain. Properties in one domain are usually visible simultaneously. Packages of properties are usually called Vocabularies or schemas (e.g. DC).

Resources A resource is a qualified URI, i.e an URI possibly followed by a fragment identifier.(#) The base element is the triple. A resource (subject) is linked to another resource (object) by an arc labelled by a third resource (predicate) Ora Lassila is the creator of the resource

Resources Let's try to express something more complex: The individual whose name is Ora Lassila, , is the creator of

Resources We can label the set of object data as a single resource:

Syntax The syntax fro RDF is defined as XML in order to allow interoperability and interchange(massive use of namespaces) The element rdf:RDF signals all the contents which must be mapped into an RDF model. The element rdf:Description provide the context to input statements about a given resource. The resource is identified by the attribute about (URI). The attribute ID always refers to the creation of a new resource.. Within Description it is possible to introduce elements denoting arcs: Ora Lassila

Namespaces <rdf:RDF xmlns:rdf=" xmlns:s=" Ora Lassila <RDF xmlns=" xmlns:s=" Ora Lassila Ora Lassila

Abbreviations <rdf:Description about=" s:Creator="Ora Lassila" /> Ora Lassila

Abbreviations Ora Lassila <s:Creator rdf:resource=" v:Name="Ora Lassila" />

Representation <s:Creator rdf:resource=" v:Name="Ora Lassila" />

Meaning It is crucial that two communicating applications agree about the meaning to associate to the various predicates. The meaning of a predicate is expressed by referring to a unique Vocabulary or RDF Schema. An RDF Schema is the place where all constraint associated to a certain predicate are encoded

RDF Schema rdfs:Resource : All things being described by RDF expressions are called resources, and are considered to be instances of the class rdfs:Resource rdf:Property represents the subset of RDF resources that are properties(attibutes) rdfs:Class corresponds to the generic concept of a Type or Category (OO)

RDF Schema

RDF Shema rdf:type : it indicates that a resource is a member of a class, and thus has all the characteristics that are to be expected of a member of that class. – It is said to be an instance of that class. – Its value is a resource which is an instance of rdfs:Class rdfs:subClassOf : it specifies a subset/superset relation between classes. It is transitive: – If class A is a subclass of some broader class B, and B is a subclass of C, then A is also implicitly a subclass of C. – Thus resources that are instances of class A will also be instances of C, since A is a sub-set of both B and C.

RDF Schema

<rdf:RDF xml:lang="en" xmlns:rdf=" xmlns:rdfs=" <rdfs:subClassOf rdf:resource="

RDF Schema rdfs:subPropertyOf it is an instance of rdf:Property that is used to specify that one property is a specialization of another – a property might be a specialization of one or more properties – If some property P2 is a subPropertyOf another more general property P1, and if a resource A has a P2 property with a value B, this implies that the resource A also has a P1 property with value B.

RDF Schema <rdf:RDF xml:lang="en" xmlns:rdf=" xmlns:rdfs=" <rdf:type resource=" <rdf:type resource="

RDF Schema Constraints are mostly used to relate vocabulary items from multiple independently developed schemas rdfs:range – That the value of a property should be a resource of a designated class rdfs:domain – That a property may be used on resources of a certain class

RDF Schema <rdf:RDF xml:lang="en" xmlns:rdf=" xmlns:rdfs="

DC and resources but… this seems to be too wide for the things we can describe with DC! – can we really describe people using DC? – do people have titles and subjects? no… in general we only use DC to describe a sub-set of all resources anything covered by the DCMIType list… – Collection, Dataset, Event, Image (Still or Moving), Interactive Resource, Service, Software, Sound, Text, Physical Object

DCMI abstract model a description is made up of – one or more statements (about one, and only one, resource) and – optionally, the URI of the resource being described (resource URI ) each statement is made up of – a property URI (that identifies a property) – a value URI (that identifies a value) and/or one or more representations of the value (value representations)

Elements within DCMI, we often use the phrases ‘element’ and ‘element refinement’ an ‘element’ is just another word for a property an ‘element refinement’ is a special kind of property (a sub-property) that shares some meaning with one other property but has narrower semantics – e.g. if “Ben is the illustrator of a Book” then it is also true to say that “Ben is a contributor to the Book”

Values values and value strings can be ‘qualified’ by using encoding schemes a vocabulary encoding scheme is used to indicate the class of the value – e.g. the value is taken from LCSH a syntax encoding scheme is used to indicate how the value string is structured – e.g. the value string is a date structured according to the W3CDTF rules (“ ”)

Simple vs. qualified DC? within DCMI, we often use the phrases “simple DC” and “qualified DC” “simple DC” only supports a single description using the 15 DCMES elements with value strings “qualified DC” supports all the features of the abstract model, and allows the use of all DCMI terms as well as other, non-DCMI, terms

Dumb-down the process of translating qualified DC into simple DC is normally referred to as ‘dumbing- down’ ignore any property that isn't in the Dublin Core Metadata Element Set use value URI (if present) or value string as new value string recursively resolve sub- property relationships until one of the 15 properties in the DCMES is reached, otherwise ignore use knowledge of rich values, related descriptions or the value string and the syntax encoding scheme to create a new value string elementvalue uninformed informed

RDF and DC: why? RDF provides shared metadata ‘model’… …shared ‘meaning’ metadata can be shared between applications that have little or no knowledge about each other e.g. an RDF-based bibliographic application can consume RDF-based geospatial metadata and have 'some' knowledge of what it means …with (X)HTML and XML encodings, software applications must have ‘understanding’ hard-coded into them…

DC abstract model maps easily onto the RDF model (because RDF was the basis for it!) DC in RDF/XML syntax is an encoding of the RDF model in XML simple DC is similar to the non-RDF XML we've seen already… …but with the addition of and container elements

dc.xml

Qualified DC. Element Refinement. These qualifiers make the meaning of an element narrower or more specific. A refined element shares the meaning of the unqualified element, but with a more restricted scope. A client that does not understand a specific element refinement term should be able to ignore the qualifier and treat the metadata value as if it were an unqualified (broader) element. The definitions of element refinement terms for qualifiers must be publicly available. Encoding Scheme. These qualifiers identify schemes that aid in the interpretation of an element value. These schemes include controlled vocabularies and formal notations or parsing rules. A value expressed using an encoding scheme will thus be a token selected from a controlled vocabulary (e.g., a term from a classification system or set of subject headings) or a string formatted in accordance with a formal notation (e.g., " " as the standard expression of a date). If an encoding scheme is not understood by a client or agent, the value may still be useful to a human reader. The definitive description of an encoding scheme for qualifiers must be clearly identified and available for public use. The normative reference for DC Qualifiers is at

QDC examples (abstract.xml)

QDC Examples (subject.xml) LCSH MeSH DDC LCC UDC

QDC examples (schema.xml)

Date schema W3C-DTF Instances of this class are dates and times encoded with the W3C Encoding rules - a profile based on ISO8601

DCMES Element Element Refinement(s) Element Encoding Scheme(s) TitleAlternative- Creator-- Subject-LCSH MeSH DDC LCC UDC DescriptionTable Of Contents Abstract - Publisher-- Contributor-- DateCreated Valid Available Issued Modified DCMI Period W3C-DTF Type-DCMI Type Vocabulary Format IMT Extent- Medium Identifier-URI Source-URI Language-ISO RFC 1766 RelationIs Version Of Has Version Is Replaced By Replaces Is Required By Requires Is Part Of Has Part Is Referenced By References Is Format Of Has Format URI CoverageSpatialDCMI Point ISO 3166 DCMI Box TGN TemporalDCMI Period W3C-DTF Rights--

DCMES Element Element Refinement(s) Element Encoding Scheme(s) TitleAlternative- Creator-- Subject-LCSH MeSH DDC LCC UDC DescriptionTable Of Contents Abstract - Publisher-- Contributor-- DateCreated Valid Available Issued Modified DCMI Period W3C-DTF Type-DCMI Type Vocabulary Format IMT Extent- Medium Identifier-URI Source-URI Language-ISO RFC 1766 RelationIs Version Of Has Version Is Replaced By Replaces Is Required By Requires Is Part Of Has Part Is Referenced By References Is Format Of Has Format URI CoverageSpatialDCMI Point ISO 3166 DCMI Box TGN TemporalDCMI Period W3C-DTF Rights--

DCMES Element Element Refinement(s) Element Encoding Scheme(s) TitleAlternative- Creator-- Subject-LCSH MeSH DDC LCC UDC DescriptionTable Of Contents Abstract - Publisher-- Contributor-- DateCreated Valid Available Issued Modified DCMI Period W3C-DTF Type-DCMI Type Vocabulary Format IMT Extent- Medium Identifier-URI Source-URI Language-ISO RFC 1766 RelationIs Version Of Has Version Is Replaced By Replaces Is Required By Requires Is Part Of Has Part Is Referenced By References Is Format Of Has Format URI CoverageSpatialDCMI Point ISO 3166 DCMI Box TGN TemporalDCMI Period W3C-DTF Rights--

Important links:

Project/exercise Select 20 web docs in your favouite field: – 10 HTML – 5 PDF – 3 images – 2 video For each one provide a well formed rdf QDC decription using your favourite editor (one file per doc) For the time being avoid SH classification.