There are different levels that metadata may be used for:
This is not to say that these levels of metadata are unique. There is a high degree of reuse of the metadata for each level and an organisation will design its metadata schema and implementation based on its business needs to accommodate these three requirements.
Discovery Metadata is the minimum amount of information that needs to be provided to convey to the inquirer the nature and content of the data resource. This falls into broad categories to answer the "what, why when who, where and how" questions about geospatial data.
Online systems for handling metadata need to rely on their (metadata is plural, like data) being predictable in both form and content. The level of metadata detail that will be documented is dependent on the type of data held and the methods that it is being accessed and used. Different types of data (e.g. vector, raster, textual, imagery, thematic, boundary, polygon, attribute, point, etc.) will require different levels and forms of metadata to be collected. However there is still a high degree of compatibility between most of the metadata elements required.
Similarly, organisations will manage their data in mission-defined ways. Some organisations manage information as a data set, tiles of data sets, series of data sets, or manage the information down to the feature level. Again there is still a high level of compatibility between the levels of metadata required, particularly as the data is cascaded from the feature level to the data set or data series level.
Thus, not only can metadata content vary according to purpose; it can also vary according to scope of the data being defined. Discovery metadata usually, but not exclusively, relates to collections of data resources or data set series that have similar characteristics but relate to different geographic extents or times. A map series is the commonest example but it can equally be applied to statistical surveys. More detailed metadata may be applied to a collection or series but may apply to an individual data set (e.g. one map tile). Transfer metadata applies exclusively to that transfer.
Exploration metadata provides sufficient information enable an inquirer to ascertain that data fit for a given purpose exists, to evaluate its properties, and to reference some point of contact for more information. Thus, after discovery, more detail is needed about individual data sets, and more comprehensive and more specific metadata is required. If the data are transferred as a single data set then quite specific and detailed metadata is needed possibly down to the feature, object or record level. Exploration metadata include those properties required to allow the prospective end user know whether the data will meet general requirements of a given problem.
Exploitation metadata include those properties required to access, transfer, load, interpret, and apply the data in the end application where it is exploited. This class of metadata often includes the details of a data dictionary, the data organization or schema, projection and geometric characteristics, and other parameters that are useful to human and machine in the proper use of the geospatial data.
These roles form a continuum in which a user cascades through a pyramid of choices to determine what data are available, to evaluate the fitness of the data for use, to access the data, and to transfer and process the data. The exact order in which data elements are evaluated, and the relative importance of data elements, will not be the same for all users.
Linkages between geospatial data and metadata
Until recently, metadata have been created or derived with little or no automation. In fact, it is only with the recent development of metadata standards, and the development of metadata software based on these standards, has the consistent management of metadata been given any consideration by those collecting geospatial data. With an increased focus of incorporating geospatial data into corporate information systems, the development of an international standard for metadata, and the OpenGIS catalogue service specifications, new versions of commercial GIS software are now facilitating a close linkage between geospatial data and metadata.
Regardless of style of metadata, there is nominally one collection of properties or metadata associated with a given data set or feature collection. The 1:1 rule expresses the notion that a discrete resource should have a discrete metadata record. Although it seems simple enough, it isn't always so neat because resources are often not so discrete. For example, should each photograph in an article have its own record? How do you manage collections of articles? Can the collection be thought of as a resource? What about multi-media objects? Thus, one of the first tasks in metadata management is the identification of the data product or entity to be documented.
etadata may exist at the collection level (e.g. satellite series), at a data product level (an image mosaic), at a data unit level (a vector data set), a group of features of a given type (certain roads), or even at a specific feature instance (a single road). Regardless of the level of abstraction, these associations of metadata to data objects should be maintained.
In practice, most metadata are currently collected at the data set level, and a metadata entry in a catalogue refers the user to its location for access. Increasingly sophisticated providers of geospatial data are including metadata at other levels of detail so as to preserve information richness. Metadata standards such as ISO 19115 allow different levels of metadata abstraction, and catalogue services will also need to accommodate this richness without confusing the user in its complexity.
Ideally, metadata structures and definitions should be referenced to a standard. One benefit of standards is that they have been developed through a consultative process (with other "experts") and provide a basis from which to develop national or discipline-oriented profiles. As standards become adopted within the wider community, software programs will be developed to assist the industry in implementing the standard. The consistency in metadata content and style is recommended to ensure that comparisons can be made quickly by data users as to the suitability of data from different sources. This means for example when comparing metadata about property or hazardous waste there is an indication of the dates to which the information refers or if comparing metadata about different map sources the relevant scales are shown. Without standardization, meaningful comparisons are more difficult to derive without reading and learning many metadata management styles.
Predictability is also encouraged through conformance to standards. However the problem has been that there are a number of "standards" in use or development. Detailed metadata standards that provide for an exhaustive definition of all aspects of various types of geospatial data are currently under preparation by a number of bodies, as are profiles of these standards as reference models to be adopted for international use.
Geospatial Metadata Standards
Considerable debate across the world centres on metadata and those characteristics that should be chosen to best describe the data set. There are discussion groups, seminars and conferences and quantities of paper generated in the debate about the subject. Standards have been generated by a number of organisations all designed to ensure that a degree of consistency exists within a given application community.
Three main metadata standards exist or are in development that are of broad international scope and usage and provide detail for all levels of metadata mentioned earlier:
etadata also forms an important part of the OpenGIS Abstract Specification. The OpenGIS Consortium (OGC) http://www.opengis.org is an international membership organisation engaged in a co-operative effort to create open computing specifications in the area of geoprocessing. As part of its draft 'OpenGIS Abstract Specification' OGC has a topic on recording metadata for spatial data. OGC are working closely with FGDC and ISO/TC 211 to develop formal, global spatial metadata standards. At their plenary meeting in Vienna, Austria in March 1999, ISO/TC 211 welcomed the satisfactory completion of the co-operative agreement between the OpenGIS Consortium and ISO/TC 211 and endorsed the terms of reference for an ISO/TC 211 / OGC co-ordination group.
These standards have had different ideas about what characteristics should be included. To derive all these elements the data provider requires spending considerable time and resources collecting this information and for the data user this detail might be greater than required for an initial investigation. In many situations therefore different levels of metadata need to be defined with the ability to "drill down" into increasing levels of detail. Metadata should therefore vary according to purpose.
A number of national and regional initiatives have also developed metadata standards. These include initiatives managed by The Australian and New Zealand Land Information Council (ANZLIC) and two European Commission financed projects (LaCLEF and ESMI). These initiatives have taken similar approaches in promoting a limited set of metadata (described as "Core Metadata" or "Discovery Metadata" that organisations should use, as a minimum, to improve the knowledge, awareness and accessibility of the available geospatial data resources.
Each of the initiatives is promoting the standards and use of discovery metadata as a foundation of their respective metadata directory initiatives. This discovery metadata provides sufficient information to enable an inquirer to ascertain that existence of data fit for purpose exists and to reference some point of contact for more information. If, after discovery, more detail is needed about individual data sets then more comprehensive and more specific metadata is required. It is possible that organisations may wish to develop metadata at different but complementary levels - at one level discovery metadata for external use and for in-house / internal use more detailed metadata. And to avoid duplication of effort those elements common to both are flagged. These guidelines have been developed with recognition of the importance of more extensive metadata required for data management and each of the organisations is promoting the adoption of ISO Metadata Standard.
General Metadata Standards
Other standards exist in the broader topic of metadata that do not specifically apply to geospatial information. These conventions are listed here for informational purposes. They may be useful references for linking or integrating non-geospatial resources into a geospatial framework.
The Dublin Core is a metadata element set intended to facilitate discovery of electronic resources. Originally conceived for author-generated description of Web resources, it has attracted the attention of formal resource description communities such as museums, libraries, government agencies, and commercial organisations.
The Dublin Core Workshop Series has gathered participants from the library world, the networking and digital library research communities, and a variety of content specialists in a series of invitational workshops. The building of an interdisciplinary, international consensus around a core element set is the central feature of the Dublin Core. The progress represents the emergent wisdom and collective experience of many stakeholders in the resource description arena. Dublin Core metadata is specifically intended to support general-purpose resource discovery. The elements represent one community's concepts of core elements that are likely to be useful to support resource discovery. Unfortunately, the formal use of the Dublin Core metadata model does not recognize the inclusion of qualified elements such as "Coverage." This metadata element thus may contain text that represents a date or time, a description of a place name or time period, or coordinates, without a means to declare what type of content is
present in the text element. As such, the Dublin Core unqualified elements are inadequate for even basic geospatial resource description and discovery, though they may be applied to web and library resources with a loose geospatial definition.
The Spatial Data Transfer Standard (SDTS) and the Vector Product Format (VPF) Digital Exchange Standards (DIGEST) were developed to allow the encoding of digital spatial data sets for transfer between spatial data software. Both of these standards support the inclusion of metadata elements in an exchange, but remarkably have not until recently considered support for standardised the encoding of relevant geospatial metadata standards in their export or archival formats.
While other general-purpose metadata standards exist, it is recommended that a comprehensive geospatial metadata standard should be used to document geospatial data. It is easier to produce simplified metadata from a more robust collection of metadata, but it is impossible to do the opposite. Eventually, the integration of data content and exchange standards will converge with those in metadata content and exchange so that spatial data encoding efforts will provide a comprehensive solution for archive and documentation.
Metadata Standards
Why use Standards?
In the USA the Federal Geographic Data Committee (FGDC) approved their Content Standard for Digital Geospatial Metadata in 1994. This is a national spatial metadata standard developed to support the development of the National Spatial Data Infrastructure. The standard has also been adopted and implemented in the United States, Canada, and the United Kingdom through the National Geographic Data Framework (NGDF). It is also in use by the South African Spatial Data Discovery Facility, the Inter-American Geospatial Data Network of twelve Latin American countries, and elsewhere in Asia.
In 1992 the Comité Européen de Normalisation (CEN) created technical committee 287 with responsibility for geographic information standards. A family of European Pre-standards have now been adopted including 'ENV (Euro-Norme Voluntaire) 12657 Geographic information - Data description - Metadata'.
An ISO standard is now at a drafting stage and expected to be ratified in early 2001 (http://www.statkart.no/isotc211/welcome.html ). In 1994 the International Standards Organisation created technical committee 211 (ISO/TC 211) with responsibility for Geoinformation/Geomatics. They are preparing a family of standards; this process involves a working group, a committee draft, a draft international standard and finally the international standard. ISO have now released the committee draft of 'ISO 19115 - GI - Metadata. It is hoped that all the existing standards will converge through the ISO initiative. Indeed, most of the existing standards already have a great deal in common and a robust international discussion has ensured that the ISO standard has accommodated most of the various international requirements. The ISO standard has equally benefited from the experiences of the various national bodies and their implementations of the respective metadata standards assisted by metadata software.
Chapter Three | Context and Rationale | Organisational Approach | Implementation Approach