Since 1994, the Alexandria Digital Library Project has developed three prototype digital libraries for georeferenced information. This paper describes the most recent of these efforts, a three-tier client-server architecture that relies heavily on a middleware layer to present a single uniform set of interfaces to multiple heterogeneous servers. These standard interfaces, all of which are implemented in HTTP, support session management, collection discovery and evaluation, metadata searching,

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... tadata retrieval, and online holding retrieval. An XML-based metadata encoding scheme and a simple boolean query language have also been developed. The architecture described by these interfaces has been implemented at UCSB.