Drainage information plays an important role in the presentation of spatial information and is widely used in terrain analysis

resource and environment management and Hydrology. Watershed

which indicates the characteristic of geology

geomorphology and hydrology

is a basic object in the research of earth surface hydrology. And it is very important in soil and water conservation and environmental monitoring. The automatic extraction of valley lines based on grid DEM is the base of the researches mentioned above. And the flow direction determination is one of the most important parts in the automatic ditch extraction. In this paper

we summarized and implemented the existing algorithms of flow direction determination in automatic ditch extraction

and proposed a new algorithm of flow direction determination

in which multiple flow directions and randomness were accounted for

based on the FD8 and Rho8 algorithm. We used two DEM data to validate the advantage of this algorithm in multiple flow direction and randomness.