How to decide the units of drainage pattern of generalization

It is undoubted that there is an affinity between the river network pattern and the terrain. The hierarchically structured river networks and associated drainage basins are important basis for international modeling and monitoring activities. In fact, drainage basins are important reference units for many biophysical processes and rivers represent major transport networks not only for water, but also for sediments, nutrients and pollutants. We know that drainage abstraction is also an important part in cartographic generalization, and drainage pattern is definitely an important factor which should be considered in drainage abstraction. But how to decide the drainage pattern is really a complicated job. From all these, deciding the unit of drainage abstraction is the first step and unit should simple which means that there is only one type of pattern in this unit. The generalization operation should first make decision of object importance at geographic level, which relates not only to the geometric properties of independent object but also the other context objects. For the generalization of river network, we try to get the simple representation but remaining the main properties of hydrographic meaning. In this paper, we choose basin as the selection unit of 1:25 000 to 1:1000 000 which get the better results.