In order to meet the demand of topographic parameters for national soil erosion inventory and mapping, hydrologically correct DEMs (Hc-DEMs in short) with 25m resolution for China have been generated with more than 21000 map sheets of 1:50,000 topographic maps (using contours, spot heights, and part of the streams), primary topographic parameters, including slope, slope length (distributed watershed slope length), and aspect, have been extracted, then a compound parameter, slope length factor (LS), has been calculated based on the fundamental principles of soil erosion and geomorphometry. All the parameters have been analysis, and the results showed that: (1) DEMs representing the terrain shape accurately and correctly, with some preprocessing, and following certain procedures,are base and prerequisite for the extracting of slope, distributed watershed slope length, aspect and calculating of LS (figure 1). (2) Hc-DEMs with 25m resolution generated in this project represent the topographical features of the country, and the parameters, including slope degree, distributed watershed slope length, and aspect, are consistent with general principles and regular understanding of erosional geomorphology (figure 2). (3) It is possible for all the parameters to be extracted and calculated based on the DEM with standard quadrangle DEM datasets (1.5º×1.0º of 1:250,000). But the DEM must be extended the boundary out for some distance for extracting of distributed watershed slope length; the distances at least are 3.8 km and 2.8 km for gentle and steep terrain area respectively. (4) Some local scale characteristics and also large scale differentiation laws for themes of slope and distributed watershed slope length can be identified in China. The slope degree is steeper in the mountainous area, hilly areas (especially in loess hill of northwestern China, red earth hills of southern China), and transitional areas among three terrain ladders. The distributed watershed slope length is longer in gentle hills, mountains in Hengduan mountain area of southwest China, and centre area of Qinghai-Tibet Plateau (figure 3 and figure 4, and table 1). (5) Generally speaking, the calculation result of LS factor is influenced by the slope and distributed watershed slope length synchronously. But it is much more influenced by the slope degree. As result, LS is basically consistent with the slope, and similar to slope, in the geo-spatial pattern and distribution in China (figure 5, figure 6). This paper also proposes some problems to be further researched, including: resolution of DEMs of representative areas, analysis and assessment of the quality of the slope and distributed watershed slope length datasets, scaling effects of slope, distributed watershed slope length and LS factor, analysis of factors influencing on extraction and calculation of LS factor, applicability of the parameters in soil erosion assessment and mapping, methodologies for making thematic maps for slope, distributed watershed slope length, aspect and LS factor for Chin at small map scale (1:4 million).

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