A Spatial Data Infrastructure (SDI), also called geospatial data infrastructure, is a data infrastructure implementing a framework of geographic data, metadata, users and tools that are interactively connected in order to use spatial data in an efficient and flexible way. Another definition is "the technology, policies, standards, human resources, and related activities necessary to acquire, process, distribute, use, maintain, and preserve spatial data". A further definition is given in Kuhn (2005): "An SDI is a coordinated series of agreements on technology standards, institutional arrangements, and policies that enable the discovery and use of geospatial information by users and for purposes other than those it was created for." General Some of the main principles are that data and metadata should not be managed centrally, but by the data originator and/or owner, and that tools and services connect via computer networks to the various sources. A GIS is often the platform for deploying an individual node within an SDI. To achieve these objectives, good coordination between all the actors is necessary and the definition of standards is very important. Due to its nature (size, cost, number of t-related. An example of an existing SDI, since 2002, is the National Spatial Data Infrastructure (NSDI) created by the OMB Circular A-16 in the United States. At the European side, since 2007, the INSPIRE is a European Commission initiative to build a European SDI beyond national boundaries and ultimately the United Nations Spatial Data Infrastructure (UNSDI) will do the same for over 30 UN Funds, Programmes, Specialized Agencies and member countries. Software components An SDI should enable the discovery and delivery of spatial data from a data repository, via a spatial service provider, to a user. As mentioned earlier it is often wished that the data provider is able to update spatial data stored in a repository. Hence, the basic software components of an SDI are: Software client - to display, query, and analyse spatial data (this could be a browser or a desktop GIS) Catalogue service - for the discovery, browsing, and querying of metadata or spatial services, spatial datasets and other resources Spatial data service - allowing the delivery of the data via the Internet Processing services - such as datum and projection transformations, or the transformation of cadastral survey observations and owner requests into Cadastral documentation (Spatial) data repository - to store data, e.g., a spatial database GIS software (client or desktop)