Evaluation of medieval land transformation for the construction of hilltop castles based on topographic analysis of LiDAR DEMs

Land directly transformed by humans now occupies much of the terrestrial surface and has a significant impact on the natural environment and natural hazards. Premodern man-made landforms such as hilltop castles affect landslide disasters, particularly in western Japan. This study compared the topographical features of 39 ridge sites modified into medieval castles and 39 unmodified natural ridges in a granite area of Hiroshima Prefecture, western Japan. Topographic attributes, including slope and profile/plan curvature, were calculated from airborne LiDAR digital elevation models (DEMs) with a resolution of 1 m. As a result, the standard deviations of slopes, profile curvature, and plan curvature were larger at the castle sites than at the unmodified natural ridges. These indices reflect the modified topography of a castle, which is composed of flat surfaces and steep slopes. We also proposed a method to classify ridge topography with combining these indices. The proposed method is potentially useful for finding unknown ridge transformation by premodern human activity and also for evaluating the potential of shallow landslides in castle sites.