1. Esri interoperability and standards,
http://www.esri.com/software/opengis
2. Intergraph,
http://www.intergraph.com
3. PostGIS, Geographic objects for PostgreSQL,
http://postgis.refractions.net/
4. Lecture Notes in Computer Science;B. Amann,2002
5. Beech, D., Malhotra, A., Rys, M.: A formal data model and algebra for XML (1999),
http://www-db.stanford.edu/dbseminar/Archive/FallY99/malhotraslides/malhotra.pdf