2.1 Pilot Catalog Service Systems
The Federal Geographic Data Committee (FGDC) Clearinghouse is a virtual collection of digital spatial data distributed over many servers in the United States and abroad. The primary intention of the Clearinghouse is to provide discovery services for digital data, allowing users to evaluate its quality through metadata. Most metadata provide information on how to acquire the data; in many cases, links to the data or an order form are available online.
The NASA Earth Observing System ClearingHOuse (ECHO) is a clearinghouse of spatial and temporal metadata that enables the science community to exchange data and information. ECHO technology can provide metadata discovery services and serve as an order broker for clients and data partners. All the NASA Distributed Active Archive Centers (DAACs), as data providers, generate and ingest metadata information into ECHO.
The Open Geospatial Consortium (OGC) has promoted standardization and interoperability among the geospatial communities. In catalogue service aspect, OGC has defined the Catalog Service implementation standard (OpenGIS, 2004) and published two recommendation papers (OpenGIS, 2005a; OpenGIS 2005b). The George Mason University (GMU) CSISS Catalog service for Web (CSW) system is an OGC-compliant catalog service, which demonstrates how the earth science community can publish geospatial resources by searching pre-registered spatial and temporal metadata information. In particular, the GMU CSISS CSW catalog service is based on the OpenGIS implementation standard, and the ebRIM application profile (OpenGIS, 2005). It provides users with an open and standard means to access more than 15 Terabytes global Landsat datasets.
2.2 Conceptual System Architecture
Since these geospatial catalog services address similar needs, it is not surprising that they have almost the same conceptual system architecture, as shown in Figure 1.
Conceptual Architecture of Catalog Service
From the point of view of metadata circulation, a catalog service usually consists of three components: metadata generation and ingestion, a conceptual schema for catalog service, and a query interface for catalog service.
Metadata generation and ingestion is always based on applicable metadata standards, such as the Dublin Core (DCMI, 2003), Geographic information – Metadata (19115) from International Organization for Standard (ISO, 2003), Content Standard for Digital Geospatial Metadata (CSDGM) from Federal Geographic Data Committee (FGDC, 1998), or the ECS Earth Science Information Model from National Aeronautics and Space Administration (NASA, 2006).
Metadata structures, relationships and definitions, known as conceptual schemas, play a key role in catalog services. They define what kind of metadata information can be provided and how the metadata are organized. The conceptual schemas are closely related to those of the pre-ingested metadata information, but are not necessarily identical. Catalog service conceptual schemas are always oriented toward the field of application and may be tailored to particular application profiles.
The query interface for a catalog service defines the necessary operations, the syntax of each operation, and the binding protocol. To facilitate access and promote interoperability among catalog services, the interface definition may be kept open.