2.2 The extraction of alluvial fans
The alluvial fans were drawled by visual interoperation in Google Earth Pro (7.3.3.7692). The standard about visual interoperation was the three obvious characteristics of alluvial fans formed by flooding process in LRB: (1) fan-shaped landform, (2) braided flow channels, and (3) none consistent flow, as shown Fig2.. According those characteristics, the outline data is got in Google Earth Pro. Then added outline to ArcGIS to get the data about area, position and distribution of alluvial fans.