Lars Kulik Carola Eschenbach Christopher Habel Hedda Rahel Schmidtke A Graded Approach to Directions between Extended Objects. 119-131 2002 conf/giscience/2002 GIScience http://dx.doi.org/10.1007/3-540-45799-2_9 db/conf/giscience/giscience2002.html#KulikEHS02