Overview

Keywords are words or phrases that describe the metadata resource.  They facilitate search, indexing and discovery of metadata, and are the largest single component of many metadata collections, regardless of the dialect.

Conceptual Model (UML)

The UML for the MD_Keywords object includes keyword, type, and thesaurusName fields.  A minimum of one keyword field is mandatory.  The type field groups keywords into like categories using KeywordTypeCodes, and the thesaurusName field provides a citation to the authoritative source of keywords. Using common types to classify keywords is critical for consistent discovery, particularly using faceted searches.

The MD_KeywordClass object is new with ISO 19115-1.  This object was added in order to allow unambiguous specification of keywords from ontologies. The MD_KeywordClass enables users to define categories of keywords that extend or are orthogonal to the standardized KeywordTypeCodes, and are associated with an ontology that allows additional semantic query processing.

In contrast the thesaurus citation specifies a collection of instances from some ontology, but is not an ontology. It might be a list of places that include rivers, mountains, counties and cities. There might be a Laconte county, the city of Laconte, the Laconte River, and Mt. Laconte; when searching it is useful for the user to be able to restrict the search to only rivers.

Note:  ISO 19115-1 extends the KeywordType codelist to include keyword types supported by the NASA Global Change Master Directory (GCMD). The 10 additional GCMD keyword types are used in many existing metadata collections. In the figure, the new keyword type values are highlighted in red.

Implementation (XML)

<gmd:MD_Keywords>
    <gmd:keyword>
        <gco:CharacterString>Alaska Satellite Facility</gco:CharacterString>
    </gmd:keyword>
    <gmd:type>
        <gmd:MD_KeywordTypeCode codeList="http://www.ngdc.noaa.gov/metadata/published/xsd/schema/resources/Codelist/gmxCodelists.xml#MD_KeywordTypeCode"
        codeListValue="dataCenter">dataCenter</gmd:MD_KeywordTypeCode>
    </gmd:type>
    <gmd:thesaurusName>
        <gmd:CI_Citation>
            <gmd:title>
                <gco:CharacterString>NASA/Global Change Master Directory (GCMD) Data Center Keywords</gco:CharacterString>
            </gmd:title>
            <gmd:date>
                <gmd:CI_Date>
                    <gmd:date>
                        <gco:Date>2008-02-07</gco:Date>
                    </gmd:date>
                    <gmd:dateType>
                        <gmd:CI_DateTypeCode codeList="http://www.isotc211.org/2005/resources/Codelist/gmxCodelists.xml#CI_DateTypeCode" codeListValue="publication">publication</gmd:CI_DateTypeCode>
                    </gmd:dateType>
                </gmd:CI_Date>
            </gmd:date>
            <gmd:identifier />
        </gmd:CI_Citation>
    </gmd:thesaurusName>
</gmd:MD_Keywords>

Usage

ECHO to ISO Keyword Mapping