1. School of Urban Railway Engineering, Shaanxi Railway Institute, Weinan 714000, P. R. China.
2. School of Highway, Chang’an Univ., Xi’an 710064, P. R. China; Wuhu Surveying and Mapping Design Institute Co., Ltd., Wuhu 241000, P. R. China (corresponding author).
3. Wuhu Surveying and Mapping Design Institute Co., Ltd., Wuhu 241000, P. R. China.