ArcGIS环境下DEM的坡长计算与误差分析

坡长是水土保持、土壤侵蚀和环境评价等研究中的基本因子。在GIS环境下,坡长的提取是基于格网DEM进行的,其结果受到DEM精度、DEM结构、流向提取算法和栅格距离计算方式的影响。在确定分辨率和精度的DEM下,坡长计算误差主要来自流向提取算法和栅格距离计算方式。本文对这两种DEM坡长计算误差源进行了详细的分析,指出集成到ArcGIS软件中的坡长计算模型D8存在较大缺陷,尽管栅格距离计算方式对坡长影响最大误差为8.9%,并可通过统计方式实现整体坡长的修正,但D8算法的单流向特性使得水流方向变得确定和不连续,进而导致坡面单元坡长产生较大误差,因此,利用D8算法进行数值模拟计算需要考虑其精度问题。同时应提高坡长计算的准确性,需要发展更为完善的流向计算方法和坡长计算模型。

Analysis of Slope Length Extracted from Grid-based Digital Elevation Model in ArcGIS Environment

Slope length,which describes the distance of flow path from origin point,is often a vital terrain parameter in soil and water conservation,soil erosion and environmental assessment studies.A common method of obtaining estimates of slope length is based on flow directions from grid-based Digital Elevation Model（DEM） in Geographical Information System（GIS） software.This method leads to slope length estimates with variable accuracy,which depends on DEM error,DEM resolution,algorithms of flow direction and distance measurement.Error and resolution are determined for a given DEM,leaving only algorithms of flow direction and distance measurement.This thesis focuses on the effect of these two uncertain factors on slope length which is derived from DEM.Based on a series of rigorous calculation on a DEM of simulated gully network,the paper finds that the model of slope length in ArcGIS software is not suitable to calculate the slope length because the flow direction derived from D8 is deterministic and this makes the flow route become discontinuous.Although the error by which distances are calculated over raster structures is less than 8.9% and can be improved by statistical methods such as distance transforms,any method that used to low this error is meaningless if the flow direction is wrong.In order to test these conclusions,we applied non-cumulative slope length algorithm in the platform of ArcGIS for two different typical DEMs,including a sample data in ArcGIS and a 5m grid cell DEM of Jiuyuangou drainage basin in North Shaanxi Loess Plateau.Comparing with the lengths obtained from contour map,the former conclusions are verified.So,it is necessary to develop a new flow direction algorithm with vector character in order to improve the estimate accuracy of slope length derived from grid DEM.