spatial data are data that in some way relate to a position in space, or more specifically, on the earth. The term
infrastructure conveys the idea of an underlying foundation or basic framework on which data, activities or procedures are built.
A formal definition of a SDI which has often been quoted in the SDI literature states that an SDI is "
the technology, policies, standards, human resources, and related activities necessary to acquire, process, distribute, use, maintain, and preserve spatial data."
 A key element of this definition is the scope of issues which are covered by an SDI. It is clearly not just about technology nor even the straightforward provision of certain datasets.
Put simply, an SDI is the 'ground rules' and arrangements which are put in place to enable spatial data from separate digital data bases to be combined seamlessly without undue difficulty and for such data to be widely available and used.
In practical terms, if an ISDI were operational, spatial data would be readily discoverable and available via the Internet, or for some users, on an organisations' Intranet. For example, any device (PDA, mobile phone, laptop, desktop computer, etc.) connected to the Internet could access a GIS client application on a geo-portal which would allow the user to create customised maps from diverse data derived from distributed databases. This could be carried out without the user having to search for and explicitly link to the individual servers. A user could thus create maps consisting of base layers overlain with more specialist thematic data from a variety of fields (e.g., health, demography, transport, education, planning, etc.). GIS users would have a range of options available, from the relatively simple visualisation of data (map viewers) to more sophisticated analyses of spatial data. Spatial data could also be made available for download to local desktop GIS applications.
A range of types of operations could be conducted on the data layers that would depend on the types of functionality which were built into the system. However at its heart the system would allow the viewing of multiple datasets overlaid on each other, and the user being able to perform interactive analysis on these multiple datasets (for instance, to consider possible locations for a new hospital with reference to mapping, demographics, and accident statistics).
Accessing relevant data and using the various functionalities which can be performed on the data should ideally take place with a minimum of delay (seconds in some cases, depending on the type and size of the data files being downloaded into the system).
The databases that would be involved in the ISDI would remain the property and responsibility of the host organisations (termed data custodians).
This is not a full description of a possible future situation but merely an outline which serves to provide a glimpse of what could be involved.
US Federal Government circular issued on 19 August 2002, and available at