- DCRDFGuidelines

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Guidelines for encoding DC metadata using the RDF model

This document is currently under development. It is being worked on by the DC RDF Taskforce. Comments should be sent to the dc-rdf-taskforce@jiscmail.ac.uk mailing list.

Introduction

This document provides some recommendations for encoding DC metadata in RDF. It does this by describing how the features of the DCMI Abstract Model are mapped to the RDF model rather than by referring to any specific RDF syntax encoding such as RDF/XML. This will allow DC metadata to be encoded using any of the recognised encoding syntaxes for RDF.

DCMI Abstract Model summary

The abstract model of DCMI metadata descriptions is as follows:

A description is made up of one or more statements (about one, and only one, resource) and zero or one resource URI (a URI that identifies the resource being described).
Each statement instantiates a property/value pair and is made up of a property URI (a URI that identifies a property), zero or one value URI (a URI that identifies a value of the property), zero or one vocabulary encoding scheme URI (a URI that identifies the class of the value) and zero or more value representations of the value.
The value representation may take the form of a value string or a rich representation.
Each value string is a simple, human-readable string that is a representation of the resource that is the value of the property.
Each value string may have an associated syntax encoding scheme URI that identifies a syntax encoding scheme.
Each value string may have an associated value string language that is an ISO language tag (e.g. en-GB).
Each rich representation is some marked-up text, an image, a video, some audio, etc. or some combination thereof that is a representation of the resource that is the value of the property.
Each value may be the subject of a separate related description.
A description set is a set of one or more descriptions about one or more resources.
A DCMI metadata record is a description set that is instantiated according to one of the DCMI encoding guidelines (XHTML meta tags, XML, RDF/XML, etc.)

Mapping the DCMI Abstract Model to the RDF model

The remainder of this document describes how to represent the DCMI Abstract Model using the RDF model.

Descriptions

A DCAM description is represented using an RDF graph originating from a single RDF subject (a single RDF node), where the RDF graph represents one or more DCAM statements as described below.

Statements

A DCAM statement is represented using the RDF predicate (also known as the RDF property) and RDF object that make a up an RDF triple originating from the RDF subject of a DCAM description. A DCAM statement may also include an RDF triple that indicates the =rdf:type= of the RDF object (see the "Vocabulary encoding scheme URIs" section below). A DCAM statement may also include one or more RDF triples that indicate a DCAM value representation (see the "Value strings" and "Rich representations" sections below).

Resource URIs

A DCAM resource URI is represented using the RDF URI Reference corresponding the the RDF subject of a DCAM description.

Property URIs

A DCAM property URI is represented using the RDF URI Reference corresponding to the RDF predicate of an RDF triple originating from the RDF subject of a DCAM description.

Value URIs

A DCAM value URI is represented using the RDF URI Reference corresponding to the RDF object of an RDF triple originating from the RDF subject of a DCAM description.

Vocabulary encoding scheme URIs

A DCAM vocabulary encoding scheme is represented using an RDF triple comprising:

an RDF subject that is the RDF object of an RDF triple originating from the RDF subject of a DCAM description
an RDF predicate that is =rdf:type=
an RDF URI Reference corresponding to the RDF object that is the DCAM vocabulary encoding scheme

Value strings

A DCAM value string is represented using an RDF triple comprising:

an RDF subject that is the RDF object of an RDF triple originating from the RDF subject of a DCAM description
an RDF predicate that is =rdfs:label=
an RDF literal RDF object (an RDF plain literal or RDF typed literal).

Note that in those cases where the range of the DCAM property is =rdfs:Literal= a shorter representation is possible by using an RDF literal that is the RDF object of an RDF triple whose RDF subject is the RDF subject of the DCAM description.

Rich representations

A DCAM rich representation is represented using a DCAM value string that is an RDF typed literal.

Syntax encoding scheme URIs

A DCAM syntax encoding scheme is represented using the RDF URI Reference that is the RDF datatype URI associated with an RDF typed literal DCAM value string.

Value string languages

A DCAM value string language is represented using a language tag as defined by RFC-3066, normalized to lowercase, associated with the RDF plain literal that is the DCAM value string.

Note that in the RDF model, a language tag can only be associated with an RDF plain literal, not with an RDF typed literal. This is a limitation on the DCAM as it is currently worded. We may want to consider tightening up the wording of the DCAM at some point?

Related descriptions

A DCAM related description is represented using any RDF Graph that originates from an RDF node that is not the RDF subject of the DCAM description.

Description sets

A DCAM description set is represented as an RDF Graph that includes one or more DCAM descriptions as described above.

Records

A DCAM record is an RDF Graph that is encoded using any of the recognised RDF encoding syntaxes.

Appendix A: A note about dumb-down

The dumb-down algorithm is applied to a DC description set (in terms of the DCAM), not directly to an RDF graph. Therefore, applying the dumb-down algorithm to a DC/RDF graph should be done by first mapping the graph to a DC description set, then applying the dumb-down algorithm, then mapping the resulting DC description back to a DC/RDF graph.

Appendix B: A few examples using the RDF/XML syntax

Note: The figures used in this document are available in a MS-Powerpoint file.

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