基于高分辨率DEM的黄土地貌正负地形自动分割技术研究

Automatic Segmentation of Loess Positive and Negative Terrains Based on High Resolution Grid DEMs

黄土地貌正负地形自动分割是构建地表空间分布式机理-过程模型的基础。在分析黄土高原地区典型地貌坡面形态及汇流过程特征的基础上,提出了基于5m分辨率栅格DEM自动分割黄土正、负地形的技术方案。该方案首先利用坡面上下游栅格点的坡度对比识别沟沿线点,然后利用汇水模型提取沟沿线点约束的上游汇水区域,从而实现正、负地形的自动分割。在黄土塬区及丘陵沟壑区的实验结果表明,该方法的优点是提取精度高,人工干预少,在不同地貌类型区域内有很好的应用适宜性。

China Loess Plateau is world-wide famous for its peculiar and unique landscape, in which the loess shoulder-lines zigzag on the surface making a distinct separation between loess positive terrain and the negative one. The automatic segmentation of loess positive and negative terrains （P-N terrains） with precise location and high efficiency, which is a crucial technique in constructing the mechanism-process models of loess landform drainage systems, is still need to be further improved. Nowadays, with the increasing availability of digital topographic data, the Digital Elevation Model （DEM） can now be applied to better description of natural topography, and it is also regarded as the most proper data source for automatic extraction of loess positive and negative terrains. In this paper, the conformation and flow confluence characteristic of the hill-slope in representative loess landforms is discussed. Based on this, an auto-segmentation method of P-N terrains from 5 m resolution DEMs is proposed. There are three key procedures in the operation. The first step is the identification of grid points on the shoulder-line position by considering the slope difference up and down. Slope gradient 30°,25°and 20°are used as the thresholds in loess platform area, loess ridge-hill area and loess hill area respectively in the extraction model. The second stage is to expand shoulder-lines’ candidate cells directionally by considering the spatial direction of the local hill slope aspect and the trend of the shoulder-lines. This step is helpful for deriving more consecutive and detailed shoulder-lines on DEMs. The third step is the generation of positive terrain which can be regarded as the extraction of the upstream area of the shoulder-line points by using the hydrologic analysis model. All the shoulder-lines’ cells derived above are imported into the model as the pour point data. This is an alternative method for evading the difficulty of converting shoulder-line grid cells into consecutive vector lines. Validation tests are took out by contrasting the auto-extraction results from DEMs and the delineating results from 1 m resolution DOMs in six drainages. Results show that maximum area difference between the positive terrain area derived above and actural area is 1.15 km2, and the percentage of the cells distance offset values less than 10 m exceed 95%. So the main advantages of this approach are high accuracy, lower demands on manual intervention and ready availability of required data for many regions on the Loess Plateau. The morphology of the earth surface is the interactional production of the hydrosphere, lithosphere, atmosphere and the biosphere, thus the morphology is the external representation, and the interaction process is the inner dynamical force. The extraction of the topographic feature lines not only need considering the characteristic of the terrain morphology, but also need considering the inner mechanism of land surface process.