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To determine the behavior of rainwater after rainfall, it is important to obtain local ground-slope information and topographic-feature information, e.g., ridges and valleys. To obtain such information, a method that uses overground openness (OO) and underground openness (UO) has been proposed previously [1]. In this method, OO and UO are calculated using a digital elevation model (DEM). In addition, a method has developed to generate a red relief image map [2] by combining the ridge–valley degree (RVD) calculated as (OO-UO)/2 and the gradient of the ground to visualize general topographical characteristics. However, these methods do not fully utilize the features of currently available highresolution DEM data, and, to the best of our knowledge, automatic extraction of ridge valley lines has not been considered to date. To address the former problem, a new method [3] that uses CG techniques was proposed to enable an openness calculation that reflects the DEM resolution. Thus, in this paper, we propose a method to extract ridge and valley lines automatically using OO and UO values obtained from a high-resolution DEM [3].
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