ALTO (Analyzed Layout and Text Object) is a XML Schema that details technical metadata for describing the layout and content of physical text resources, such as pages of a book or a newspaper. It most commonly serves as an extension schema used within the Metadata Encoding and Transmission Schema (METS) administrative metadata section. However, ALTO instances can also exist as a standalone document used independently of METS.
D. Shen, J. Sun, Q. Yang, and Z. Chen. WWW '06: Proceedings of the 15th international conference on World Wide Web, page 643--650. New York, NY, USA, ACM Press, (2006)
Y. Yang, and J. Pedersen. Proceedings of the Fourteenth International Conference on Machine Learning, page 412--420. San Francisco, CA, USA, Morgan Kaufmann Publishers Inc., (1997)