The USGS Seamless Datasets service is a collection of three API's that work in tandem to provide developers with a programmatic portal to the USGS Seamless and Tiled Server's data holdings. The Seamless Data Warehouse contains massive amounts of data compiled by the U.S. Geological Survey (USGS) and Earth Resources Observation and Science (EROS) - two projects backed by the Department of the Interior to create, maintain, and allow access to geospatial data of the United States.
The first of these API's is the Inventory Service, a data discovery service that provides information about what data is available in the server over a particular area of interest.
The Request Validation Service uses dataset information generated by previous calls to the Inventory Service (including the user-defined area of interest) to verify and validate the informaiton, resulting in a fully parameterized URL or set of URL's that can be used by the download service.
The download service takes the parameterized URL(s) created by the Request Validation Service to initiate a request for data, query the system for request status updates, and finally return the requested data to the user in XML format.