A gazetteer of period definitions for linking and visualizing data.

Linked Data

The PeriodO period gazetteer documents definitions of historical period names. Each entry of the gazetteer identifies the definition of a single period. To be included in the gazetteer, a definition must

Much care has been put into giving period definitions stable identifiers that can be resolved to RDF representations of period definitions. PeriodO models period definitions as SKOS concepts. These are grouped into concept schemes sharing the same bibliographic source. Temporal extent is expressed via a direct textual quotation from the source, as well as via a structured approximation of this expression modeled using the OWL-Time ontology. Similarly spatial extent is represented both by a textual quote (where one was given) and a set of identifiers referring to spatial entities in external resources such as DBpedia.

Examples:

Period definitions

The PeriodO dataset is essentially a collection of period definitions.

Labels and documentation

A period definition is a skos:Concept, “an idea or notion,” as the SKOS Reference puts it. We use a number of SKOS properties to describe period definitions:

  • skos:prefLabel is used for the name of the period exactly as given in the original source. The value of this property is a simple literal xsd:string, with no language tag.

  • skos:altLabel is used for language-specific names of the period, assigned by PeriodO curators. The value of this property is a language-tagged string. The language tag consists of (at least)

    1. a three-character primary language subtag, as defined in ISO 639-2 or ISO 639-3, and

    2. a four-character script subtag, as defined in ISO 15924.

    There will always be at least one skos:altLabel, with the language tag eng-latn. If the source definition was not written in English, there will always be another skos:altLabel with a language tag indicating the language and script of the source definition. The language of the source definition is indicated via a dc:language property, the value of which is a language tag as described above.

  • skos:note is used for notes about the period definition taken from or attributed to the original source. For example, the original Pleiades definition of “Ottoman Rise (AD 1300-1453)” includes the note “ends with the conquest of Constantinople.” The value of this property is a simple literal xsd:string, with no language tag.

  • skos:editorialNote is used for administrative or editorial notes added by the PeriodO curators; these do not appear in the original source. The value of this property is a simple literal xsd:string, with no language tag.

  • skos:inScheme is used to link a period definition to the period collection of which it is a part.

Source

Usually the bibliographic information about the source of a period definition is provided through properties of the period collection to which it belongs. However, in some cases there may be additional bibliographic information that is specific to an individual definition. In these cases, we use a dcterms:source to provide this additional information, and dcterms:isPartOf to link the source of the definition to the source of the collection to which it belongs. For example, we might use the following to indicate that the specific book page from which a definition was sourced:

<p0tns5v4kdf>
    dcterms:source [
        dcterms:isPartOf <http://www.worldcat.org/oclc/63807908> ;
        bibo:locator "page 25"
    ] .

(bibo is the Bibliographic Ontology.)

Temporal extent

We use properties from the Time Ontology to describe the temporal extent of period definitions. A period definition is a time:ProperInterval, an interval of time with different beginning and end points. We assume that these (instantaneous) beginning and end points can never be precisely identified, hence our descriptions focus on describing the intervals that start and finish the period:

  • time:intervalStartedBy links the period definition to an (anonymous) time interval that has the same (unknown) beginning point as the period, and an (unknown) end point that comes before the end point of the period. We call this the start interval for the period.

  • time:intervalFinishedBy links the period definition to an (anonymous) time interval that has the same (unknown) end point as the period, and an (unknown) beginning point that comes after the beginning point of the period. We call this the stop interval for the period.

Diagram showing the relation between a period’s temporal extent and its start and stop intervals
The relation between a period’s temporal extent and its start and stop intervals

We describe the start and stop intervals in two ways. Both ways of describing the interval are required; these are complementary descriptions, not alternatives:

  1. skos:prefLabel is used to textually describe the interval exactly as given in the original source, for example “end of the first century BC”. The value of this property is a simple literal xsd:string, with no language tag.

  2. time:hasDateTimeDescription is used to describe the interval in a more structured fashion. This property links the interval to an (anonymous) time:DateTimeDescription. These structured descriptions are created by PeriodO curators.

Currently we use the following properties in our datetime descriptions:

  • time:year for descriptions of intervals that can be represented with a single year. For example, an interval with the textual description “600 BC” can be described with a datetime description having a time:year value of -0599.

  • periodo:earliestYear and periodo:latestYear for descriptions of intervals that need to be represented as ranges. For example, an interval with the textual description “eight century BC” can be described with a datetime description having a periodo:earliestYear value of -0799 and a periodo:latestYear value of -0700.

The datatype for values of time:year, periodo:earliestYear, and periodo:latestYear is xsd:gYear. Note that:

  1. xsd:gYear values can have any number of digits.

  2. xsd:gYear values may be zero. The value 0000 is interpreted as 1 BCE.

  3. xsd:gYear values represent Gregorian calendar years and “are not, in general, convertible to simple values corresponding to years in other calendars.” We are comfortable with this limitation because we use these values only for the purposes of ordering and visualizing temporal extents of intervals. The skos:prefLabel of an interval should be considered the authoritative description.

We may use additional properties in our datetime descriptions in the future, for example to describe intervals at a finer temporal granularity than a year.

Spatial extent

We use the following properties to describe the spatial extent of period definitions:

  • periodo:spatialCoverageDescription is used to textually describe the spatial extent exactly as given in the original source, for example “Near East and Greece”. The value of this property is a simple literal xsd:string, with no language tag.

  • dcterms:spatial is used to link a period definition to descriptions of locations in gazetteers such as DBpedia/Wikidata, GeoNames, or Pleiades.

Period collections

A period collection is simply a set of period definitions that share a source. We use dcterms:source to link period collections to bibliographic descriptions of their sources. Where possible we rely on external bibliographic databases such as WorldCat and CrossRef for bibliographic metadata.

A period collection is a skos:ConceptScheme, “an aggregation of one or more SKOS concepts”. Belonging to the same period collection does not imply any semantic relationship between period definitions, other than sharing a source. In particular, the period definitions belonging to a period collection do not constitute a periodization, meaning a single coherent, continuous division of historical time. In the future we plan to add additional properties for indicating when a set of period definitions constitute a periodization.

The root resource of the PeriodO dataset is an rdf:Bag (unordered collection) of period collections.

Identifiers

Each period definition is given its own Web-based, resolvable Uniform Resource Identifier (URI) in the form of a Archival Resource Key (ARK), minted through the EZID system of the California Digital Library. These ARKs are resolvable to structured, machine-readable representations of individual period definitions.

Wherever possible bibliographic sources are identified with WorldCat URIs or CrossRef DOIs, and creators are identified with Virtual International Authority File URIs.

Data Serialization

Period definitions in the PeriodO gazetteer are published as JSON-LD, a serialized form of the Resource Description Format (RDF) used to describe Linked Data. The entire dataset is available for download as a single JSON file, so that it can be more easily reused by other projects.

User Interface

A browser-based client provides search, visualization, and data entry tools. It also provides an interface for managing distributed versioning and collaborative development of the PeriodO dataset. The entire editorial history of the PeriodO gazetteer is published as a separate dataset using the PROV ontology.