An increasing volume of information is now considered critical to everyday decision making in modern society -- a large portion of this information is essentially related to "place" in the context of position on the Earth. As more information is made available online includes some geographic context, the ability to describe, organise, and access it has become increasingly difficult. The ability to discover and access geographic data resources for use in visualisation, planning, and decision support is a requirement to support societies at the local, regional, national, and international levels. Common solutions have been developed and will be described in this Chapter by evaluating organisational approaches, comparing definitions of community, identifying common architectural solutions, and sharing a base of techniques that are implemented in available non-commercial and commercial standards-based software.
This Chapter presents the concepts, current practices, and designs for geospatial data discovery. It is intended as a guide to those interested in the management, development, and implementation of compatible discovery services in environments where the cross-domain publication of geographic information is desired. Organisational issues and roles are presented that are critical to the understanding and maintenance of the services within a larger spatial data infrastructure. The principles described herein can be interpreted and applied in a range of information management conditions from non-digital collections of map information, through small digital catalogs, to integrated repositories of data and metadata. Relevant standards and software are identified for evaluation and application.
The library has long formed the primary metaphor for accumulation and management of knowledge about people, places, and things. Since the construction of the ancient library in Alexandria, Egypt to its modern day equivalents, libraries have employed classification systems, specialisation, and discipline to information in all forms. A central feature in this virtual library - and a critical part to its navigation and use - is the catalog. In the context of geospatial information management, we use the descriptions of geospatial data, or metadata, as described in Chapter 2 as the common vocabulary to frame the structured fields of information that we seek to manage and to use in search and retrieval. These metadata elements are stored and served through a user-accessible catalog of geospatial information.
Support of a discovery and access service for geospatial information is known variously within the geospatial community as "catalog services" (OpenGIS Consortium), "Spatial Data Directory" (Australian Spatial data Infrastructure), and "Clearinghouse" (U.S. FGDC). Although they have different names, the goals of discovering geospatial data through the metadata properties they report are the same. For the purpose of consistency within this document, these services will be referred to as "catalog services." Further integration of these services with web mapping, live access to spatial data, and additional services can lead to exciting user environments in which data can be discovered, evaluated, fused, and used in problem-solving. Whereas this chapter will focus on finding spatial data, combination of the practices described here with those in other chapters can expand the capabilities of your spatial data infrastructure.
Distributed Catalog Concepts
The Catalog Gateway and its user interface allows a user to query distributed collections of geospatial information through their metadata descriptions. Figure 4.1 shows the basic interactions of various individuals or organisations involved in the advertising and discovery of spatial data. The boxes are identifiable components of the distributed catalog service; the lines that connect the boxes illustrate a specific set of interactions described by the words next to the line.
A user interested in locating geospatial information uses a search user interface, fills out a search form, specifying queries for data with certain properties. The search request is passed to the Catalog Gateway and poses the query of one or more registered catalog servers. Each catalog server manages a collection of metadata entries. Within the metadata entries there are instructions on how to access the spatial data being described. There are a variety of user interfaces available in this type of Catalog search in various national and regional SDIs around the world. Interoperable search across international Catalogs can be achieved through use of a common descriptive vocabulary (metadata), a common search and retrieval protocol, and a registration system for servers of metadata collections.
Context and Rationale
Although the Internet is becoming the world's largest repository of knowledge, its navigation is hindered by the lack of a surrogate and comprehensive catalog. As a result, one is delivered tens of thousands of candidate documents in response to a reasonable query from today's search engines. Fortunately, geographic information frequently has signatures of location in the form of coordinates or place names and even may have a reference date or time associated with the data. These metadata provide a key to a solution that can and does operate in an international context.
Chapter Four | Context and Rationale | Organisational Approach | Implementation Approach