H. Zhang, M. Korayem, E. You, and D. Crandall. Proceedings of the fifth ACM international conference on Web search and data mining, page 33--42. ACM, (2012)
M. Fornefeld, P. Oefinger, and U. Rausch. Report by Micus Management Consulting GmbH on behalf of the Federal Ministry of Economics and Labour, Germany, URL:< http://www. bmwi. de/Homepage/download/infogesellschaft/Geoinformationen. pdf, (2003)