SEMANTIC-BASED SPATIAL INFORMATION INFRASTRUCTURES: INTEGRATING DATA AND SERVICES INTO A SINGLE COLLECTION
E. Stefanakis, P. Prastacos
Nowadays special attention is given towards the development of effective spatial data infrastructures (SDIs) at regional, national or international level. SDIs are frameworks of policies, institutional arrangements, data, services, technologies, and people with a common scope, i.e., to promote the accessibility and usability of geospatial content (data and services).
The construction of an SDI presumes that the participating organizations have agreed on the adoption of common vocabularies, practices, standards, technical specifications and operational components. Hence, an SDI is not a simple data set or repository. Instead, an SDI hosts: (a) geographic content (data and services); (b) sufficient description of this content (metadata); (c) effective methods to discover and evaluate this content (data catalogues); (d) tools to visualize the data (e.g., web mapping); and (e) services and software tools to support specific application domains.
An SDI architecture is usually conceptualized as a three-tier architecture. At the top layer (the client) reside the users and applications; at the bottom layer (the server) reside the geospatial data; and at the middle layer (the middleware) reside all services that assist the accessibility to the data repositories. Obviously, the middle layer is the one that facilitates the worldwide access to and the use of online geospatial data.
Currently, SDI paradigm focuses on the exchange of geospatial data; i.e., the middle layer supports the discovery and retrieval of data available at the bottom layer; and this is consistent to the term spatial data infrastructure.
On the other hand, it is widely recognized, that future SDI research must focus more on the users’ needs and the design of appropriate geoinformation services and service architectures to satisfy these needs. In other words, the development of service-driven infrastructures; or spatial information infrastructures will enhance infrastructures capabilities.
This may be accomplished by enriching both the bottom and middle layers. The bottom layer will also accommodate geospatial services (i.e., analytical software tools) along with data. As regards to the middle layer, existing discovery and retrieval services will be extended to apply to both the data and services available at the bottom layer. Additionally, the middle layer will incorporate software tools and services that may be used by the top layer (users and applications) and hence enhance the whole SDI functionality.
In both architectures the middle layer plays the role of the middleware between the clients (users and applications) and the servers (data and services). Its role is supported by a set of mechanisms, as follows: (a) sufficient descriptions of the geospatial content (both data and services), i.e., the metadata; (b) discovery of distributed collections of geospatial content through their metadata descriptions, i.e., the catalogue gateway; and (c) metaphors to discover and access the geospatial content, i.e., the interfaces.
All mechanisms above must be constructed in such a way that enable the effective sharing and use of geospatial content. The incorporation of rich semantics at all these components affects definitely the efficiency of the middleware. Semantics enable the interoperability between different data collections and services. The scope of this paper is to highlight the presence of semantics into these components, and discuss the alternative architectures of the middleware.
In the sequel, we use the single acronym SDI to represent both spatial data and information infrastructures. Although, several other acronyms can be found in the literature (such as SII standing for spatial information infrastructures) we maintain the widely used acronym SDI, which can also stand for service-driven infrastructures.
The discussion is organized as follows. Sections 2 to 4 comment the role of semantics and related methods in the components of an SDI and how they assist the role of the middleware. Specifically, Section 2 focuses on metadata; Section 3 on the catalogue gateway; and Section 4 on the interfaces. Section 5 presents briefly the efforts and products of the organizations developing standards for SDIs. Then Section 6 shows the alternative architectures for SDIs. Finally, Section 7 concludes the discussion.