River classification and river network structuration in river auto-selection

River auto-selection is an essential part of the automatic generalization of thematic maps. To complete the task of river selection, the key is to reasonably evaluate the importance of various rivers on the global structure. A grading system is needed here to quantify the importance. In this system, the importance of river differs among different levels. Though there have been already several grading systems (Horton, Strahler, etc.), each with its strengths, they still cannot meet the requirements of river auto-selection. Meanwhile, as for river auto-selection, river classification is extremely crucial and indispensable. And, when taking river classification into consideration, it is mainly based on the following two points: (1) high-level river should be chosen to ensure the connectivity of its network, with the assistant of the grading system; (2) rivers in the same level can be selected by length, density and other indicators. This study proposed river classification based on basin, starting from the importance of the rivers in local areas, can choose according to the importance of the rivers, that can help keep the drainage density unchanged before and after selection.