GIS has long been recognized as an integrating technology that can bring highly disparate data into a common map display. Overlaying layers of different data in a common geographic space allows you to integrate and derive new information to help you solve problems and to work more efficiently and effectively.

GIS has become an essential tool when information integration is critical. It works with geographic information from many sources to support a broad range of applications. GIS can repurpose information for new applications that go beyond the data's original intent. For example, information compiled for land records management can be used for environmental applications, utilities, emergency response (E911), homeland security, and many other applications.

There is widespread recognition that the data layers and tables in most GIS implementations come from multiple organizations. An organization typically develops some, but not all, of its own spatial data content. At least some of the layers will be from external sources. Thus GIS data management, by its very nature, is distributed among many users. GIS requires a distributed information system concept to manage and share geographic data.

Because GIS users are hungry for quality geographic information, there is a fundamental need for users to share their data. Today, thousands of organizations worldwide invest billions of dollars annually automating and integrating map information for their focused GIS projects. Their need for up-to-date geographic information drives these efforts.

While the common vision is to share this information, most geographic data sets remain inaccessible to external users for a variety of reasons. One of the main reasons is the lack of a common framework for sharing information about spatial data.

Imagine a large public library without a card catalog or any systematic organization of the materials. It would just be a pile of books--a collection of immeasurable value with no means for readily finding the information that you need.

Modern GIS requires a mechanism for sharing geographic information much like libraries do. This requires tools and methods for cataloging GIS information as well as tools to publish the catalog and search it for relevant content.

This catalog function is implemented in the new ArcIMS Metadata Server, enabling GIS users to manage metadata in a catalog and to search for geographic information. The Metadata Server is available in the ArcIMS 4 release, which was released in spring 2002 (see ArcIMS 4 Offers Significant New Capabilities and Extensions and ArcIMS and ArcGIS Combine to Bring Spatial Metadata to the Internet).

Many people are familiar with the term NSDI, or National Spatial Data Infrastructure, a concept defined as the technologies, policies, and people necessary to promote the sharing of geospatial data throughout all levels of government, the private and nonprofit sectors, and the academic community.

Recent discussions have revolved around the GSDI, or Global Spatial Data Infrastructure, which describes a framework for data sharing at the global level.

A Spatial Data Infrastructure is built from three fundamental building blocks.

• Users--A wide range of users who connect to the catalogs, search for useful geographic information, and then connect to GIS portals that provide the information access
• Catalogs--A series of well-organized catalogs that describe and reference geographic information sets at each node
• Portal--A GIS portal that provides access to FTP download sites, maps, reports, Internet GIS services, and so on

In practice, each node in an SDI would be built to provide one or more of these three capabilities. For example, a city might provide a catalog referencing the available data and services at its Web site along with a series of data and mapping services. A small GIS shop would be able to access and use those services in their ArcView seat across the Internet. In another case, a state government might provide the metadata catalog describing its services as well as data and services at other GIS portals within the state. The state might also publish GIS Web services with a set of HTML clients to access various Internet map and data services.

At its most basic level, a Spatial Data Infrastructure is realized through a catalog holding metadata about available data and services. As the number of participating organizations and users grow, so will the numbers of entries and complexity of the metadata catalog. The role of a metadata server and search tools becomes more critical.

g.net and SDI concepts play important roles at a number of levels in the GIS community. g.net is flexible. The building blocks can be used interchangeably. Following are some of the possibilities for deploying g.net.

• Be a User--Individual users can enhance their GIS application by searching for and accessing Web sites containing up-to-date information and GIS services. Users will discover and connect to GIS portals providing critical data and services.



• Build a GIS Enterprise--GIS enterprises can be built from the SDI building blocks in g.net. Many larger enterprises will apply the concepts of a metadata portal and GIS services.
  
  The g.net architecture, like standard Web services, is flexible enough to fit the computing framework of the organization. Computer services might be centralized or distributed among departments, and g.net can be adapted to fit the organization. For example, each department might provide access to a set of GIS services for the department's information. Enterprise users could integrate these information sets with data and services from other departments.



• Publish and Advertise a Set of GIS Data and Services--A GIS shop could publish a Web site with access to data files (e.g., FTP), online maps, and other GIS services. g.net could be used to implement these portals and provide a metadata service to advertise the GIS data and services at each site.



• Build a Clearinghouse Node--With g.net, the Federal Geographic Data Committee (FGDC) and other clearinghouses can be built using open technology to serve all GIS users.
  
  Many GIS users want to provide information portals to publish a metadata catalog for their collection of GIS data and services. A few clearinghouse sites want to provide a catalog of information sets published by many users. Sometimes the data and services will be published for a user's own GIS node, while at other times the catalog will include data and services at many other GIS sites. Access to such a clearinghouse node could be open on the World Wide Web, while at other times it can be restricted and secure for a select audience.



• Participate in the Geography Network--Users who wish to provide a GIS portal can register their data and services with the Geography Network at www.geographynetwork.com



• Build a GIS Community by Creating Your Own Geography Network--Many focused GIS user groups want to collaborate on information sharing between the members in the community. These user groups can create community-based portals to publish and share geographic information with one another. This may take the form of a loosely coupled collection of GIS portals or a single central portal.
  
  Using the new metadata server in ArcIMS 4, users can collaborate and implement the same building blocks Esri has to build their own GIS search portal like the Geography Network. The goal of each Geography Network implementation is to encourage the sharing of GIS information and services by providing easy-to-use frameworks for finding and accessing information.