Author: Lv Guonian, Professor, specialized in GIS and virtual geographic erivironment. E-mail: gnlu@njnu.edu.cn Corresponding author: Xiong Liyang, Associate Professor, specialized in digital terrain modelling and analysis. E-mail: xiongliyang@nj nu.edu.cn Corresponding author: Chen Min, Professor, specialized in geographic modeling. E-mail: chenmin0902@ 163.com