Resources Cataloguing: Key to Valuable SDI

Resources Cataloguing: Key to Valuable SDI

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R. Penneman, F. Houbie, N. Vanraes
IONIC Software, Liège, Belgium

To satisfy data exchanges between users, the EC INSPIRE initiative (INfrastructure for Spatial InfoRmation in Europe), intend to trigger the creation of Spatial Data Infrastructure (SDI) that delivers to the users integrated spatial information services. It also means observing ISO TC/211 and OGC standards.

These services should allow the users to identify and access spatial or geographical nformation from a wide range of sources, from the local level to the global level, in an interoperable way for a variety of uses. To implement SDI, the users will have to face several challenges:

  • Heterogeneous sources of information
    – Data should be collected once and maintained at the level where this can be done most effectively
    – Construct semantics to allow the exchange of information
  • Combine seamlessly spatial data from different sources
  • Share spatial data between many users and applications
  • Build a discovery mechanism
    – which spatial data is available evaluate its fitness for purpose which conditions apply for its use

The implementation of ISO/OGC standards will facilitate the discovery, access, use and extraction of geospatial information from several different architectures, such as different vendor formats, in one common environment. This article aims to focus on the standardisation of discovery mechanism.

SERVICE ORIENTED ARCHITECTURE
A service-oriented architecture is essentially a collection of services that communicate with each other. The communication can involve either simple data passing or it could involve two or more services coordinating some activity. Some means of connecting services to each other is needed. Service Oriented Architecture (SOA), implemented by means of OGC Web Services, is particularly useful for setting up SDI for GIS. It enables to combine seamlessly spatial information from different sources through Web Services and share it among users & applications, for many purposes.

SOA adds strong internal consistency to “loosely coupled” configurations. The most important benefit is the separation of data from applications, by means of open but secured Service Oriented Architecture (SOA).

CATALOGUE SERVICES
One of the key elements of an SOA is the Service Registry : a record and description of all available services. It allows users not only to find and discover which services could be useful in providing solutions to their needs but also to integrate them into their business applications.

The methodology can thus be described as follows :

  • service providers ” set up Web Services
  • they then publish these Web Services in the service registry : ” publish “
  • consumers ” then discover these services by means of queries… ” find “
  • and integrate them into their applications ” bind “

The OpenGIS® Catalogue Service Implementation Specification defines a common interface that enables diverse but conformant applications to perform discovery, browse and query operations against distributed heterogeneous catalog servers.

Catalogue services support the ability to publish and search collections of descriptive information (metadata) for data, services, and related information objects. Metadata in catalogues describe resource characteristics that 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 registered information resources within an information community.

This is the essence of the publish-findbind model, but transposed to new IT architecture that are SOA. As illustrated here after, IONIC software is totally in line with the INSPIRE initiative, the OGC and ISO standards.

APPLICATION SCHEMAS & PROFILES FOR CATALOGUE SERVICES
Specifications exist for interfaces, bindings, and frameworks for defining application profiles required to publish and access digital catalogues of metadata for geospatial data, services, and related resources. Metadata act as generalized properties that can be queried and returned through catalogue services for resource evaluation and, in many cases, invocation or retrieval of the referenced resource. Catalogue services support the use of one of several identified query languages to find and return results using well-known content models (metadata schemas) and encodings.

The definition of application profiles according to ISO 19106 (Geographic information – Profiles) has an overall goal to improve interoperability between systems conforming to a specific profile.

Experience has shown that need for application profiles results from the fact that in practice, there is no single solution for catalogue services that fits every user’s needs. As stated in CS 2.0, a base profile that provides a basic set of information objects has to be supported by each catalogue instance; in addition, application profiles for different information communities should be specified.

Catalogue Services based on the ISO19115/ISO19119 Application Profile
This catalogue implementation is based an application profile for ISO 19115/ISO 19119 metadata with support for XML encoding per ISO 19139 and HTTP protocol binding. It relies on requirements coming from the CS/CSW 2.0 specification. A catalogue implementation that conforms to this application profile can serve OGC web services and their ISO MD descriptions.

Catalogue Services based on ebRIM Application profile
A catalogue implementation that conforms to this applicationprofile can serve many purposes in a variety of domains; it provides facilities for discovering and advertising shared resources. The catalogue information model is a general and flexible one that can be employed to handle many kinds of resources including but not limited to: service offers, interface definitions, dataset descriptions, application schemas, and classification schemes. The service may be used to catalogue resources located in both local and remote repositories. Representations of these resources are exchanged using the HTTP protocol.

CONCLUSION
“Alongside the existing OGC Open Standards, specialised Software Technology for cataloguing resources is available and offers a pre-configured service enabling the automatic registration of a wide number of resources such as WFS, WMS and WCS, Contexts (WMC) using metadata. Once a resource has been registered, the automatic update procedures mean that the information is kept up to date.

Such product is accompanied by a “client” who allows the discovery of resources online. Using the client allows the user to discover the different services available and to show their use in a data visualisation client.

Finally, existing Software Technology or Cataloguing resources according to OGC Open Standards allows users to build harmoniously Spatial ata Infrastructure (SDI), from local to global level, so that users can identify and access spatial or geographical information from a wide range of sources in an interoperable way for a variety of uses.”