J. Bao, Y. Zheng, and M. Mokbel. Proceedings of the 20th International Conference on Advances in Geographic Information Systems, page 199--208. New York, NY, USA, ACM, (2012)
L. Li, D. He, W. Jeng, S. Goodwin, and C. Zhang. Proceedings of the 24th International Conference on World Wide Web, page 1453--1458. New York, NY, USA, ACM, (2015)