Assessment of Characteristics and Accuracy of Automatically Extracted Stream Networks for Mountainous Watersheds in Taiwan

• Main Authors: Yung-Chung Chuang, 莊永忠
• Other Authors: Jihn-Fa Jan
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/03225058530223058216

碩士 === 國立臺灣大學 === 森林學研究所 === 92 === Abstract Traditionally, stream networks were produced manually in hydrologic analysis. Though certain degree of accuracy can be achieved, the process requires large amount of time. In comparison, deriving stream networks from grid digital terrain model (DTM) is more efficient than the manual process. However, the accuracy of the results varies with different terrains. To evaluate the performance of automatic extraction of stream networks for mountainous watersheds in Taiwan, this study applied various algorithms to derive stream networks using grid DTM for three different watersheds. By comparing the conformity of the grid stream networks produced using manual and automatic method, the effects of grid cell sizes, preprocessing methods, algorithms, and threshold values on the characteristics and accuracies of the resulting stream networks were investigated. In order to find the best hydrologic analysis model, this study assessed the characteristics and accuracies of stream networks derived from 5m, 10m, 20m, and 40m cell size DTM using six common routing algorithms, two types of preprocessing method, and 13 levels of threshold value. The results indicate that the conformity of flow path increases as the grid cell size decreases, however the commission error and omission error increases as well; different preprocessing methods and routing algorithms result in different flow paths, however the differences of overall conformity are not significant; selection of threshold value primarily affects the flow length, and the error rate varies with different cell sizes. For the watersheds chosen in this study, when applying the D-infinity and Rho8 algorithms to the DTM with 40m grid resolution, the overall conformity was better than the other combinations if the threshold value was set to be between 200 and 250. In general, the overall error rate was high (greater than 35%). This was because the width of the streams produced from automatic analysis was much narrower than that of the manual process, hence resulting in large omission errors. Therefore, for the mountainous watersheds in Taiwan, though the automatic hydrologic analysis methods had the advantages of producing stream networks efficiently, the accuracy was not good enough. 【Key words】：Digital Terrain Model , Stream Networks, Hydrologic Modeling, Grid Resolution, Threshold Value, Mountainous Watersheds