Michael F. Worboys Kathleen Hornsby From Objects to Events: GEM, the Geospatial Event Model. 327-344 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3234&spage=327 2004 conf/giscience/2004 GIScience db/conf/giscience/giscience2004.html#WorboysH04