基于GIS的斜坡单元划分方法改进与实现

An Approach to Improving Slope Unit Division Using GIS Technique

斜坡单元已广泛应用于滑坡易发性制图和地质灾害评价。然而在山间盆地或大型宽谷处,常规方法划分出的斜坡单元与地貌背景难以匹配。依据高程及其衍生变量的基本形态系统和曲率的流域分割原理,基于ArcGIS技术,通过叠加曲率和反转曲率的流域边界,改进了斜坡单元划分方法。结果表明:与常规方法相比,改进方法不仅能够使用山脊线和山谷线以划分斜坡单元,还能利用台地边界和宽谷边界以分割水平地表与倾斜地表;划分出的单元大小相对均匀,单元形状总体介于圆形和正三角形之间。对于水平成分较多的地区,如黄土塬区和水库库区,该方法与传统方法相比,具有较好的应用前景。

In recent years, slope units have been widely utilized in landslide susceptibility mapping and geological hazard assessment. Slope units automatically derived from high-quality DTMs, partition the territory into hydrological regions between drainage and divide lines. The division method of slope units conducts subwatershed segmentation on elevation and the reversal to extract ridge lines and valley lines, then overlays these terrain feature lines. However, these resultant units are unable to match with geomorphology background where intermountain basins or large open valleys exist. In this article, according to the basic morphometic system of elevation and the derived variables, slope is derivative of elevation while curvature is that of slope, thus the break of slope should be indicated by curvature instead of elevation. The disability of indicating variation of slope is regarded as the main reason for the limitation in the conventional method. According to the theory of watershed segmentation of mean curvature, the division method of slope units is improved by overlaying watershed boundaries on curvature and the reversal in Arc GIS. Firstly, the DEM should be smoothed twice with a5×5 average filter in order to reduce the effect of noise and small scale variation in the DEM. The curvature is then calculated from the filtered DEM using the Arctool named Curvature. After that, watershed boundaries are generated mainly through computing flow direction, scouting sink and dividing watershed in the curvature image, and this image should be reversed for watershed boundaries on the reversal curvature. Finally, slope units can be separated from each other by these two kinds of boundaries which are further combined into the boundaries of slope units. Giving an example from Huachi County of Gansu Province, elevation layer in the resolution of 20 m was used as input data, and both two methods were then applied for slope unit division.Compared with the conventional method, the revised approach not only uses ridge lines and valley lines to segment slope units, but also utilizes tableland boundaries and open valley boundaries to separate horizontal surface from the inclined. The revised approach may give a new definition that slope units should be divided by ridge lines, valley lines, tableland boundaries and open valley boundaries. Also the revised method utilizes watershed segmentation on curvature instead of subwatershed division on DEM where filling sinks perhaps causes the serious change of reversal elevation. Furthermore, the resultant units have relatively uniform size and shape, more than 80% of them with the area from 0.1×104 to 6×104 m2 and about 60% between circle and triangle shape, which are more suitable for landslide hazard evaluation. It is worth mentioning that the revised method would be more competitive than the original in some regions with large area of horizontal surface, just like Loess Tableland and reservoir surface.