NGDF Metadata Gateway

Architecture V1.1

7 Metadata Specifications
The Metadata Guidelines need to be reviewed for both the data collection (metadata documentation) guidelines and the data portrayal (XML documentation) requirements and how they are used by the NGDF communications protocol (Z39.50). While there is a high degree of dependency between the data guidelines and data portrayal requirements, it is possible to address each issue separately to ensure that the NGDF communications protocol can be utilised to realise the goals of the NGDF Gateway Project.

7.1 NGDF Discovery Metadata
It is anticipated that many agencies have commenced or will shortly commence the collection and maintenance of metadata for their datasets. In the first phase of the Gateway, there is an expectation that only a few of these agencies (or service providers) will provide a distributed directory service (node) for use with the Gateway; and that other agencies will provide their metadata to one of these nodes.

It is the focus for NGDF to encourage agencies to collect the minimum metadata identified by the NGDF Discovery Metadata Guidelines. The Guidelines are seen "as interim pending the development of standards at the international level such as those being developed by Technical Committee 211 of the International Organisation for Standardisation (ISO)" (NGDF Discovery Metadata Guidelines version 1.1). In effect the Guidelines do not specify how to collect, maintain or even present metadata, but provide a list of minimum required elements to undertake these processes.

Agencies will decide on their individual specifications to ensure that they collect relevant metadata for their data management needs and subsequently for the NGDF Gateway needs. The metadata requirements for an agency to manage and document their data should be significantly more extensive than the NGDF discovery metadata guidelines. Therefore it is important that users are able to collect and maintain metadata to a level and by a method that meets their business requirement.

The NGDF Discovery Metadata Guidelines provide a valuable overview in establishing the requirements for the Gateway. Agencies can use the Guidelines when establishing their own metadata processes (whether commercial or in-house) to ensure they collect metadata required for the Gateway. In fact a number of agencies have developed, or are developing NGDF compliant metadata applications.

There are now many metadata tools that have been developed for local or network use based on text (documents), tabular (spreadsheet) or database structures and enable the collection of extensive metadata information. Most of these tools have been developed for the USA market and use the USA Federal Geographic Data Committee metadata standard. It is not important that users follow a specific schema in collecting metadata, rather than be able to provide metadata output to a specification for the Gateway requirements.

As such there are various opportunities for users to collect and maintain metadata for their business requirements that are based on two quite complementary specifications (i.e. NGDF and FGDC).

7.2 NGDF Metadata Transfer Format
The NGDF Metadata Transfer Format provides a specification to "enable the conveyance of NGDF compliant metadata to take place between data suppliers and service providers" (NGDF Guidelines). The NGDF transfer format specification uses Extendable Markup Language (XML) based on the NGDF Document Type Definition (DTD). The DTD defines the makeup of the XML document, which elements are compulsory, the repeatability of elements and what data can be entered against each element. XML is an ASCII text format (similar in nature to HTML) and can be created as a simple report from any data management system being used by an agency.

Agencies utilising either the NGDF or FGDC tools will be expected to provide XML output based on the NGDF or FGDC specifications (DTD), respectively. Both FGDC and NGDF provide a formal mechanism to manage their respective metadata guidelines (including the DTD).


7.3 NGDF Communications Protocol
The NGDF Communications protocol provides a mechanism to exchange data between different IT infrastructures (hardware, software and databases). Z39.50 has been chosen by NGDF, as it was the only current protocol that provided this ability in a web-based environment.

NGDF has adopted the use of the GEO Profile, in conjunction with the Z39.50 protocol The GEO Profile has been developed for use in searching distributed databases of geospatial data collections. The NGDF Guidelines notes that while the Geo Profile does not completely support the NGDF metadata content, the GEO profile should be adopted as a short term solution for the implementation of the Gateway. It is likely that the work currently being undertaken by ISO TC211 and the OpenGIS Consortium will dictate the developments of the communications protocol and the GEO Profile. Therefore, it is not sensible for NGDF to develop any specific alterations to the GEO Profile for UK requirements, particularly as the metadata content is also seen as interim pending the ISO TC211 outcomes.

Appendix C provides a revised mapping of the NGDF Discovery Metadata Elements with their corresponding GEO Profile attribute. This revised list provides an improved mapping from that portrayed in version 1.1 of the guidelines and will enable an improved implementation within current GEO compliant Z39.50 applications. There are only a few elements that can not be accurately portrayed in the GEO profile, namely:

7.4 Implementation of NGDF Guidelines
Note that in current GEO compliant Z39.50 applications, the use of the geographic extent is significantly different to that described in the NGDF Guidelines. The NGDF guidelines permits the description of the geographic extent by either a geographic identifier (i.e. name of an administrative zone) or by minimum bounding co-ordinates stored in latitude/longitude or national grid (British or Irish). Similarly, the NGDF guidelines permit multiple minimum bounding co-ordinates to cater for non contiguous footprints of a single dataset. The next version of the GEO profile is expected to cater for multiple footprints but will still require at least one set of minimum bounding co-ordinates to permit spatial searching of the metadata. Organisations providing distributed directory nodes to the NGDF Gateway will need to carefully assess how they apply the geographic extent elements. Organisations can successfully approach the use of the geographic extent elements by a few different options:
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