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A review of metadata: a survey of current resource description formats
Work Package 3 of Telematics for Research project DESIRE (RE 1004) |
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The Federal Geographic Data Committee (FGDC) initiated work in June 1992 on a common set of terminology and definitions for the documentation of geospatial data. The resulting standard was approved by the committee in June 1994 as the Content Standards for Digital Geospatial Metadata. The name of the format is strictly speaking the Content Standards for Digital Geospatial Metadata (CSDGM) however it is more commonly referred to as the FGDC standard and will be referred to as such throughout this review.
The collection of geospatial metadata was mandated in the US by Executive Order 12906, Co-ordinating Geographic Data Acquisition and Access: The National Spatial Data Infrastructure in April 1994 (Executive Order 12906. <URL:http://www.fdgc.gov/execord.html>). The order instructs federal agencies to document new geospatial data beginning in 1995 and to provide the metadata to the public through a National Geospatial Clearinghouse. Geospatial data prepared before January 1995 only requires identification and contact information. Responsibility for the preparation of metadata lies with the agency or the source of data.
The National Geospatial Data Clearinghouse is comprised of software and institutions to facilitate the discovery, evaluation and downloading of digital geospatial data and to implement the FGDC standard.
Documentation details for the standard can be found at the FGDC web site <URL:http://www.fgdc.gov/Metadata/metahome.html>. The current approving authority for the standard is the FGDC although there are plans to submit it to the Department of Commerce for approval as a Federal Information Processing Standard (FIPS).
The FGDC standard is a complex format with over 300 data elements. A number of tools have been developed by different agencies to assist with the creation of metadata records, a selection of them are available from <URL:http://www.fgdc.gov/clearinghouse/mitre/task2/tools.html>. The FGDC Clearinghouse Working Group sponsor orientation and training sessions on implementation of the metadata standard and clearinghouse procedures.
The FGDC standard was put forward at the first meeting of the International Standards Organization Technical Committee 211 (ISO/TC211) in December 1995. The committee are working on creating an international standard for geographic information (due late 1998). Modifications to the FGDC metadata standard are being run in co-ordination with ISO and FGDC Standards Processes and will enter public review in the next few months, to be implemented by July 1997. (Summary of Actions - FDGC Clearinghouse Working Group, April 4 1996. <URL:http://fgdc.er.usgs.gov/clearinghouse/496/summary.html>).
The FGDC standard has also been mapped to many other existing standards e.g. DIF (the NASA Directory Interchange Format), GILS (Government Information Locator Service), USMARC and the Dublin Core.
The standard provides specifications for the information content of metadata for digital geospatial metadata. It is designed to allow a prospective user to determine the availability, fitness for use and means to access the data. However it does not specify "the means by which this information is organised in a computer system or in a data transfer, nor the means by which this information is transmitted, communicated or presented to the user" (Overview: FGDC metadata content standard, June 8 1994. <URL:http://geochange.er.usgs.gov/pub/tools/metadata/standard/overview.html>). All implementation details for the standard are part of the responsibilities of the National Clearinghouse and are dealt with in Draft Implementation Methods for Access to Digital Geospatial Metadata - Normative Annex D (ISO/TC211/WG3/WI15/N001 <URL:http://www.fgdc.gov/clearinghouse/annex.html>).
The standard consists of 7 main sections of metadata and 3 'support' sections:
Support Sections
Within these sections elements are specified as mandatory, mandatory if applicable and optional. The only two sections to be deemed mandatory as a whole are Section 1 (the identification information) and section seven (the metadata reference information). The support sections do not stand alone but contain information that is referenced more than once in the seven main sections.
Basic bibliographic elements of a data set are recorded in Section 1 (Identification information). These include:
The subject coverage of a data set is described in Section 1 (Identification information) under 'theme keywords'. A draft on-line thesaurus of geospatial keywords from the NASA Master Directory is being provided by the Clearinghouse.
Annex D specifies an element 'on-line linkage' in the format of a Uniform Resource Locator.
The format and technical characteristics of a data set are recorded in Section 6 (Distribution information):
These are provided in section 6 (Distribution information):
Section 7 is devoted entirely to administrative metadata:
The source of a data set is provided in the support Section 8 (Citation information):
The terms of availability are covered in Section 6 (Distribution information) these include:
Uses attribute/value scheme. With the adoption of SGML the attribute naming will use an 8 character tag scheme <URL:http://www.fgdc.gov/clearinghouse/tag8names.txt>.
The standard specifies the information content of metadata for a set of digital geospatial data, it does not specify how this metadata should be encoded. The Clearinghouse however is proposing that the standard uses SGML to support metadata loading, exchange and presentation. A reference DTD is being developed to reflect the FGDC content element (draft versions are available from <URL:http://www.fgdc.gov/clearinghouse/dtds.html>). The General Record Syntax (GRS-1) will be used to encapsulate the metadata entry written in SGML.
There are no provisions made within the standard for the description/use of other languages.
Within the implementation details (Annex D) there is a concept of a larger work citation that allows a data set to identify a parent metadata entry. These elements include:
These are internal elements that can be used to generate links between metadata objects. This model only supports 'single inheritance' e.g. a data set can only belong to one data series.
Full - provides a very detailed content description for digital geospatial data sets with various specialised descriptive elements e.g. percentage cloud cover.
Protocol issues are not dealt with in the content standard but Annex D specifies TCP/IP and OSI protocols.
The Clearinghouse have implemented variants of the Z39.50 protocol, this was originally with the NISO/ANSI Z39.50 - 1988 standard using freeWAIS. The draft implementation document (Annex D) proposes the use of Z39.50 - 1995 compliant software and the GEO profile as a set of the FGDC metadata elements. The GEO profile is "is a draft specification for Z39.50 programmers that formalizes the data elements within the FGDC Metadata Standard as registered or well-known attributes in a Z39.50 server"(FGDC Newsletter, March 1996 <URL:http://www.fgdc.gov/News/fgdcnl0396.html>) , this will eventually allow queries by polygons and other spatial elements. The ISite ISearch software is being revised by CNIDR to support spatial search and to index SGML entries. There are also plans to support other database management systems such as mSQL.
Ten sites are using the freeWAIS-sf software to implement the content standard. These sites are:
A prototype spatial data discovery system for Web clients is available through the National Geospatial Data Clearinghouse. This is providing a WWW gateway for the sites above who are using the freeWAIS implementation <URL:http://nsdi.usgs.gov/public/fgdcquery.html>. In addition the Clearinghouse with assistance from the US Geological Survey has developed some services in Brazil and Costa Rica and the Australian Environmental Resource Information Network (ERIN) has implemented a searchable Clearinghouse node for federal holdings in Australia.
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