1. Department of Computer Science and Technology, Fujian Key Laboratory of Big Data Intelligence and Security, Xiamen Key Laboratory of Computer Vision and Pattern Recognition, Huaqiao University, Xiamen, Fujian, China
2. School of Information Science and Engineering, Xiamen University, Xiamen, Fujian, China
3. Department of Geography and Environmental Management, University of Waterloo, Waterloo, ON, N2L 3G1, Canada