Abstract: Valley extraction from grid-based DEM depends on the geometrical shape, spatial tessellation and placement of grids. Traditional researches ignore that the shape of grid can influence the neighborhood relationship, impacting on the result of valley extraction. The traditional D8 algorithm is based on eight-neighborhood relationship, where the distance difference between node neighborhoods and edge neighborhoods results in the imbalance of topographic change expression in these two directions and further affects the extraction results of the valley lines. Hexagonal grid has been proved to be advantageous over square grid due to its consistent connectivity, isotropy of local neighborhoods, higher symmetry, more compact, and so on. Taking these merits into consideration, this paper tries to explore the advantage of hexagonal grid-based DEM on the results of valley extraction in comparison with traditional square structure. First, preprocess for valley extraction with depression filling, flow direction calculation and the flow direction in flat area based on the six-neighborhood relationship. Then, connect the flow lines according to the estimated flow direction. From the comparison between the results of hexagonal grid-based DEM and square grid-based DEM, we conclude that the hexagonal grid-based DEM has a superior capability in maintaining the detailed shape and the characteristics of extracted valley networks. That is to say, with the decrease of resolution, the valley lines extracted by hexagonal DEM have better spatial agreement with real river systems, and the shape loss characteristic of them is lower. In conclusion, the hexagonal grid-based DEM has higher data accuracy, and it can extract a finer valley networks with same data volume.

Key words: Hexagonal grids, DEM analysis, D6 algorithm, valley extraction