Fig. 1. The schematic diagram of the topological relationship between saddle point and contour

Fig. 2. Process of saddle point extraction

Fig. 3. The schematic diagram of contour plane from contour and outermost contour

Fig. 4. Flowchart of the saddle point extraction based on topological relationship

Fig. 5. The schematic diagram of determination of saddle point spatial location

Fig. 6. Locations of study areas and study data

Fig. 7. Comparison of the results of saddle point extraction in study sample area 1

Fig. 8. Relationships between the extraction quantity of saddle points and contour interval

Fig. 9. The schematic diagrams of contour data with different contour interval containing different numbers of peaks

Fig. 10. Comparison of spatial positions of saddle points extracted by contour data with different contour interval

Fig. 11. Comparison of extraction results of different methods in study area 1

Fig. 12. Comparison of extraction results of different methods in study area 1