 All registered data clearinghouse nodes are polled periodically (hourly) to determine their status. All entry points or gateways shown have exactly the same lists of servers (nearly 400 in February 2006) The archive of global geospatial data is so voluminous that no single site can house the data. Digital geospatial data is archived both offline (eg in DVDs or tapes that are stacked in silos) and online at distributed data nodes (agencies/organizations that provide data).
For easy data discovery and usability the GIS data user community has developed certain standards/guidelines/best practices on geospatial data formats and data cataloging. As good practice all datasets should have an associated data descriptor or metadata. In simplest terms, metadata is data about data.
In the last two decades there has been considerable effort put in establishing and conforming to metadata standards to enable building large repositories / distribution sites / clearinghouses which would house the metadata for geospatial data collected by many organizations. The most significant effort comes from the Federal Geographic Data Committee (FGDC), which is an interagency committee that coordinates the sharing of geographic data, maps, and online services through an online portal, geodata.gov, that searches metadata held within the National Spatial Data Infrastructure (NSDI) Clearinghouse Network. The National Geospatial Data Clearinghouse is a collection of nearly 400 spatial data servers that have digital geographic data primarily for use within a GIS framework.
Most of the federal and state agencies are already a part of this national infrastructure. However, there are several smaller research groups / universities / that also contain regional and local GIS data sets. Maintaining an updated list of such resources is not easy. A slightly outdated, yet useful, start page on GIS resources (data, tools, tutorials, etc) can be found at the GIS WWW Resource List
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