Metadata and metadata catalogues
Metadata is data about data. Metadata describes how and when and by whom a particular set of data or a service was collected or prepared, and how the data is formatted or the service is available. Metadata is essential for understanding information stored in and has become increasingly important.
Metadata is structured information that describes, explains, locates, or otherwise makes it easier to retrieve, use, or manage an information resource. Metadata is often called data about data or information about information.
Metadata is also data about services. Metadata describes the content, quality, condition, and other characteristics of a data set or the capabilities of a service. Creating metadata or data documentation for geospatial data sets are crucial to the data development process. Metadata is a valuable part of a data set and can be used to:
- Organize your data holdings (Do you know what you have?).
- Provide information about your data holdings (Can you describe to someone else what you have?).
- Provide information to data users (Can they figure out if your data are useful to them?).
- Maintain the value of your data (Can they figure out if your data are useful 20 years from now?).
In the geographical domain we can have a description of spatial data (spatial data metadata), a service (service metadata) or a special analysis process (process metadata). Most for the standardization work is done for data metadata, however service and process metadata is becoming increasingly important. Metadata is used in discovery mechanisms to bring spatial information providers and users together. The following mechanisms are recognized:
- Discovery: which data source contains the information that I am looking for?
- Exploration (or evaluation): do I find within the data sources the right information to suit my information needs?
- Exploitation (use and access): how can I obtain and use the data sources?
Each mechanism has its own use of metadata. The selected standards should fulfil the needs to carry out services using these mechanisms. Metadata is required to provide information about an organisation’s data holdings. Data resources are a major national asset, and information of what datasets exist within different organisations, particularly in the public sector, is required to improve efficiencies and reduce data duplication. Data catalogues and data discovery services enable potential users to find, evaluate and use that data, thereby increasing its value. This is also becoming important at the European level. In addition, metadata received from an external source requires further information supplied by metadata in order to process and interpret it.
Metadata and Catalogue Standards
The International Organisation for Standardisation (ISO) includes ISO/TC 2112, which is an international, technical committee for geographic information. TC 211 has created a strong, globally implemented set of standards for geospatial metadata: the baseline ISO 19115; ISO 19139 for implementation of data metadata and the ISO 19119 for services metadata.
These open standards define the structure and content of metadata records and are essential for any catalogue implementation.
- ISO 19115: describes all aspects of geospatial metadata and provides a comprehensive set of metadata elements. It is designed for electronic metadata services, and the elements are designed to be searchable wherever possible. It is widely used as the basis for geospatial metadata services. However, because of the large number of metadata elements and the complexity of its data model, it is difficult to implement.
The INSPIRE DIRECTIVE applies these standards and specifications in its implementation.
A Metadata Catalogue Service is a mechanism for storing and accessing descriptive metadata and allows users to query, for data items based on desired attribute, the catalogue service that stores descriptive information (metadata) about logical data items. The Open Geospatial Consortium (OGC) has created the Catalogue Service for Web (CS-W) standard to enable discovery from a catalogue node. Catalogue services support the ability to publish and search metadata for data, services, and related information. Metadata in catalogues can be queried and presented for evaluation and further processing by both humans and software. Catalogue services are required to support the discovery and binding to published web map services.
The CS-W standard is extremely rich. In addition to supporting a query from a user, it can support distributed queries (one query that searches many catalogues) and the harvesting of metadata from node to node.