The Global Spatial Data Infrastructure Association's (GSDI) Spatial Data Infrastructure (SDI) Cookbook (www.gsdi.org) defines SDI as "the relevant base collection of technologies, policies and institutional arrangements that facilitate the availability of and access to spatial data." Those who advocate SDI promote SDI as public infrastructure, like roads and telephone systems. Like other public infrastructure, SDI provides a reliable, shared, supporting environment that makes individuals more effective in the world, businesses more profitable and governments more efficient.

The concept of "National Spatial Data Infrastructure" (NSDI) has been widely adopted by nations around the world, but there are also enterprise SDIs, municipal SDIs, regional SDIs (for multi-city or multi-nation regions), application domain SDIs and the Global SDI (GSDI).
The World Wide Web has provided a critical foundation for SDI development. The importance of the Web became obvious to members of the OGC in the late 1990s. The emergence at that time of the eXtensible Markup Language (XML) as the basic information encoding system of the Web (html became a subset of XML) provided an opportunity to create a standard way of encoding geospatial information on the Web. This standard was developed in the OGC and it is called the OpenGIS® Geography Markup Language™ (GML™). GML is an important part of many other OGC standards.

GML is a "namespace," or more formally, an “XML grammar.” Namespaces are identified on the Web via a URL (in this case, http://www.opengis.net/gml) and thus, they are globally accessible. They are a mechanism to prevent naming conflicts, that is to say, to prevent non-interoperability, in Web applications.

The current version, GML 3.1.1, is heavily used and users have created dozens of application schemas (domain-specific GML schemas) and several GML profiles (agreed-upon subsets). All the major GIS vendors provide GML support and many have made GML an integral part of their offerings. The OGC and ISO TC/211 have worked closely together on GML. GML 3.2.1 is up for a vote in the OGC and it has been approved by TC/211 and sent to ISO Central Secretariat for publication.