Q. Li, Y. Zheng, X. Xie, Y. Chen, W. Liu, and W. Ma. Proceedings of the 16th ACM SIGSPATIAL international conference on Advances in geographic information systems, page 34. ACM, (2008)
Z. Chen, H. Shen, X. Zhou, Y. Zheng, and X. Xie. Proceedings of the 2010 ACM SIGMOD International Conference on Management of data, page 255--266. ACM, (2010)