1. College of Geography and Environment Sciences, Northwest Normal University, Lanzhou 730070, China
2. Key Laboratory of Resource Environment and Sustainable Development of Oasis, Lanzhou 730070, China
3. College of Forestry Engineering, Gansu Forestry Polytechnic, Tianshui 741020, China
4. Surveying and Mapping Engineering Institute of Gansu Province, Lanzhou 730070, China