Digital libraries can be an extremely effective method of extending the services of a traditional library by enabling activities such as access to materials outside the physical confines of the library . The true benefit of a digital library, however, comes not from the replication and enhancement of traditional library functions, but rather in the ability to make possible tasks that would not be possible outside the electronic environment, such as the hypertextual linking of related texts, full text searching of holdings, and the integration of knowledge management, data visualization, and geographic information tools with the texts in the digital library. One of the challenges in building this type of system is the ability to apply these sorts of tools in a scalable manner to a large number of documents tagged according to different levels of specificity, tagging conventions, and document type definitions (DTDs). To address this challenge, we have developed a generalizable toolset to manage XML and SGML documents of varying DTDs for the Perseus Digital Library. These tools extract structural and descriptive metadata from these documents, deliver well formed document fragments on demand to a text display system, and can be extended with other modules that support the sort of advanced applications required to unlock the potential of a digital library.
The Perseus digital library is a heterogeneous collection of texts and images pertaining to the Archaic and Classical Greek world, late Republican and early Imperial Rome, the English Renaissance, and 19th Century London.  The texts are integrated with morphological analysis tools, student and advanced lexica, and sophisticated searching tools that allow users to find all of the inflected instantiations of a particular lexical form.  The current corpus of Greek texts contains approximately four million words by thirty-three different authors. Most of the texts were written in the fifth and fourth centuries B.C.E., with some written as late as the second century C.E. The corpus of Latin texts contains approximately one million five hundred thousand words mostly written by authors from the republican and early imperial periods.  The digital library also contains more than 30,000 images, 1000 maps, and a comprehensive catalog of sculpture. Collections of English language literature from the Renaissance and the 19th century will be added in the fall of 2000.
In developing this collection of SGML and now XML documents, we have benefited from the generality and abstraction of structured markup which has allowed us to deliver our content smoothly on a variety of platforms. The vast majority of our documents are tagged according to the guidelines established by the Text Encoding Initiative (TEI).  While we have had a great deal of success with these guidelines, other digitization projects have found other DTDs more useful for their purposes.  As XML becomes more widely used, more and more specifications for different subject fields and application domains are being created by various industries and user communities; a well known and extensive list of XML applications includes a wide variety of markup standards for different domains ranging from genealogy to astronomy.  Customized DTDs ease the encoding of individual documents and often allow scholars to align their tags with the intellectual conventions of their field. At the same time, they can raise barriers to both basic and advanced applications within a digital library. To take a simple example, a programmer wishing to extract all of the book titles mentioned in a collection of documents marked according to more than one DTD might may have to look for a <cit> tag in some documents and a <title> tag in others, whose DTD might use the <cit> tag to mean a piece of quoted text. More importantly, a document's structural metadata, which is intended to allow access to logical parts of the document, is tied to specific concrete element names. In order to find "chapter 5" in an XML document, the user or system implementor must know something about that document's markup --- whether the DTD and encoding conventions favor counting to the fifth <chapter> tag or searching for <div2 type="chapter" n="5">. 
To address this problem, our system allows digital librarians to create partial mappings between the elements in a DTD and abstract structural elements and then produces an index of the elements so mapped. What is encoded as <div1 type="scene"> in one document and as <scene> in another is presented to the system as an abstract, structural "scene". Identifier attributes on XML elements, such as 'ID' or 'N', are also indexed, and occurrences of each structure within the document are sequentially numbered. A mapping may also specify that some of the document's content, such as section titles or dates, be recorded and passed on to other resource discovery or visualization modules within the system. Finally, not every element in the DTD need be mapped. This allows the digital librarians to focus attention on the structural features of the texts. After this structural element map is created for the DTD, an unlimited number of documents written in conformance with this DTD can be parsed and indexed by the system.
One of the primary benefits that we derive from this system is the ability to display any number of documents tagged according to the same DTD very quickly. In previous instantiations of the Perseus Digital Library, after a text was tagged, it was necessary to write a custom specification to transform the archival SGML text into a form that could be delivered to end users to read. We also were required to add the document to both our catalog and our searching programs by hand. This process not only added extra time to the process of publishing a document, it also was prone to error. It was possible, for example, to publish a document that was not indexed or included in the searching interface. Likewise, it was possible for an uncataloged document to exist in the digital library and only be accessible by knowing and typing in the exact URL required to display the document. In the new system, cataloging information is automatically extracted from the document headers and included in the table of contents and other searching interfaces. This has enabled us to cut dramatically the time required to include new documents in the Perseus digital library; in fact, since we released this software on our production servers in the spring of 2000, we have been able to add substantial collections of Latin texts and commentaries on Greek texts to the digital library.  In addition to facilitating our own work, this system has also allowed us to collaborate more effectively with other projects. For example, we have been conducting experiments with the SGML documents created by the Library of Congress American Memory project and the documents on the Thesaurus Linguae Graecae CD ROM.  In internal experiments, we have been able to integrate large numbers of texts from these collections into the Perseus Digital Library with very little custom programming and gain the benefits of all of the advanced modules in this system.
One of the modules in our system allows us to create custom display templates for different documents or collections. When a user requests a document, our display system locates the XML file and its associated metadata, uses the abstracted mappings to determine which portion of the file contains the desired text section, applies appropriate styling rules, and presents the text to the end user in HTML. The styling rules are controlled by templates that allow us to create a custom look and feel for different texts or collections of texts within the digital library. Currently, this template is written in HTML with place-holders for variable display elements and the text itself. We can also apply these templates to objects in the digital library: images, maps, or searching tools. We produce HTML output because our current primary delivery mechanism is the World Wide Web. Our display system can produce other formats, notably Adobe's Portable Document Format and raw XML.  As browser support for XML becomes more robust, we expect to exploit XSL (Extensible Stylesheet Language) or other XML styling tools to produce attractive direct XML displays in this module.
The advantages of this toolset extend far beyond the practical issues related to displaying, cataloging, and indexing documents for end users. Because this system abstracts the metadata from the text, we are able to develop scalable tools for linguistic analysis, knowledge management, and information retrieval within the digital library, thereby allowing users to work with documents in ways that would simply not be possible outside the digital environment. The tools in the currently existing system include sophisticated full-text searching tools, the creation of live links among documents in the system, extraction of toponyms and the automatic generation of maps, discovery of dates and the dynamic display of timelines, the automatic implicit searching, discovery of word co-occurrence patterns, and linkages to morphological analysis.
A great deal of traditional scholarship involves tracking down footnotes and discovering what others have said about a text. Our toolset allows us to display links to other texts that cite the document currently being displayed. A simple example is a commentary, which explicitly talks about another text. For example, when a reader views the text of Thucydides or Homer's Iliad, we are able to show notes from several commentaries about these texts. Much more exciting, however, is the ability to display citations from texts that are not explicitly related to each other. For example, a reader of Andocides 1.57 might be interested to know that this passage is cited in section 2017 of Smyth's Greek Grammar. The reader can follow an active link to this Smyth passage and read the discussion there about the usage of the word eipon and compare other passages also cited by Smyth to illustrate this point. Because this 'citation reversal' happens automatically each time a text is added to the system, it becomes even more valuable as the digital library expands. This system also reveals unexpected links among texts, which scholars might not have been aware of. A reader of Homer might be surprised to find that Iliad 8.442 is cited in the Variorum Edition of Shakespeare's Coriolanus, V.i.
Another module automatically scans the English language texts in the digital library for place names. These names are linked to a Geographic Information System (GIS) that allows users to generate a map of the places mentioned in the entire document, in the section currently being displayed, or in a larger logical unit, for example a book or an act.  Moreover, the map serves as a gateway into the collection, since users can move from a spot on the map to a document that refers to it. The tool can also be used to map all of the places mentioned in a collection of documents. For example, a map of the American Memory texts shows that most of the documents in this collection describe places in North America, while a map of our collection of documents about 19th century London shows very few discussions of places in North America. While these examples are relatively obvious, they illustrate how this tool might be used to show a new user the characteristics of the collections within the digital library. At the same time, the digital librarian need not add metadata describing the geographic and temporal extent of a collection. For this tool, our abstraction of structural and descriptive metadata is more important than the markup of the individual documents: place names are discovered by an information extraction system after the documents have been indexed.  Because the XML back end described above presents all documents to the information discovery module in a uniform way, the GIS programmer need not be aware of the details of the DTDs or of the markup conventions used in the texts.
A third tool automatically extracts date information and uses this information to generate timelines of the dates mentioned in texts. The dates in this timeline are active links, leading to the passage in the text that contains this link. This means that the reader of a text who is interested in events that happened in 1666 can quickly locate these passages on the timeline. Likewise, a reader can use this timeline to locate other passages that describe the date in question. This tool, like the mapping tool, can also be applied to collections of documents, allowing users to gain a sense of what time frame the documents cover. For example, when this tool is applied to our Greco-Roman collection, users can quickly see that we have much greater coverage of the Classical period than of the Hellenistic period.
We have exploited this abstraction in other information extraction systems we have written, one of which is the creation of automatic hypertexts for implicit searching. Important subject terms within the domain area are recognized and linked to other documents within the digital library that contain the term. These links are provided automatically for every document when it is displayed. This makes it easy for readers to get fuller information about important ideas, to contextualize unfamiliar vocabulary, and to explore related documents in the digital library. Because subject terms are linked to dynamic hypertexts, not to simple glossary or dictionary entries, readers can explore types of material they might not have thought relevant (or even have known of): historical texts, site plans, art works, or the like. Further, as the library expands, new documents appear on these pages without any additional programming. In addition to helping users explore large domains, we also have modules that help users explore the smallest details of the language. These details include morphology and word co-occurrence. Every word of Latin or Greek is passed through a morphological analyzer and the resulting analyses are placed in a database. When Greek or Latin text is displayed, each word is checked against this database, and links are generated between words and their analyses. These analyses, in turn, are linked to a suite of dictionaries and grammars, allowing users to read texts in languages they do not yet know well. Another module that operates at the level of the word identifies words that regularly co-occur. Abstracted indices are used to scan texts and calculate word frequencies and co-occurrence ratios. Highly significant word pairs can be presented along with lexical information, or in independent tabular displays.
While it certainly would be possible to develop many of these tools without the document management system described here, this document management system makes this task much easier. The abstraction of the varying tagging systems in these documents allows programmers to focus on interesting tasks instead of dealing with different tagging systems on an ad hoc basis. Further, because these tools function as 'modules' within the Perseus digital library rather than stand-alone programs, we can apply these tools to every text that is added to the library without any additional programming. This allows us to begin to meet the challenges involved in the scalable creation of a digital library that does not simply replicate and enhance the tasks of a traditional library, but rather renders it possible for a large number of people to study texts in ways that would not be possible outside the electronic environment.
 A version of this article that focuses much more on the technical details of the system described here will appear in Smith (2000). The work described here was supported by the Digital Library Initiative, with primary support from the NEH and the NSF.
 Various aspects of the Perseus Digital Library are described in Crane (2000), Smith, Crane, and Rydberg-Cox (2000), Crane (1998b), and Crane (1998a).
 The Greek parser is described in Crane (1991).
 Yale University Press has published the Greek materials and morphological analysis tools on several CD ROMs and all of the most current materials are freely available on the World Wide Web at http://www.perseus.tufts.edu. We also have two European mirrors, one at Oxford University at http://perseus.csad.ox.ac.uk and one in Berlin at http://perseus.mpiwg-berlin.mpg.de. The audience for these texts and tools in an integrated digital library does exist and it is larger than one might expect. The past four years have seen remarkable increase in the use of the Perseus web site. Usage has grown from just under three thousand hits on its first day in July of 1996 to two hundred fifty thousand hits on peak days during the current academic year. From July 1996 until May 2000, Perseus servers have delivered more than one hundred ten million pages of primary source material.
 Sperberg-McQueen and Burnard (1994).
 Usdin and Graham (1998).
 Although much work has been done on XML Namespaces to encourage markup reuse and minimize duplication of semantic structures, it is unlikely that all marked up documents will eventually use tags such as <ns:title> for book titles.
 Our current production system does not, however, allow users to request outputs other than HTML.
 See Chavez 2000.
 Editors may also choose to mark place names.
 See Mahoney 2000.
|Jeffrey A. Rydberg-Cox
Department of English
University of Missouri at Kansas City
|Robert F. Chavez
Perseus Project, Tufts University
Perseus Project, Tufts University
|David A. Smith
Perseus Project, Tufts University
|Gregory R. Crane
Professor of Classics
Winnick Family Chair in Technology and Entrepreneurship
Editor-in-Chief, Perseus Project Tufts University