In order to conduct disaster-prevention inspections without overlooking falling-rock sources, in this paper, we aim to establish a disaster-prevention inspection method using microtopography highlight maps and verify its effectiveness. First, we create the map using the data obtained from a high-density aerial laser. In our study, measurements were carried out using a measurement helicopter loaded with a laser measuring machine capable of irradiating 80,000 to 400,000 points per second. For creating a microtopographical representation, grid data, contour maps, inclination-amount diagrams that calculate the amount of inclination for each grid datum and change the lightness accordingly to express the topography, and wavelet-analysis diagrams that emphasize the change in the unevenness through wavelet analysis, are generally used. However, it is difficult to extract sources of falling rocks, because it is impossible to express the topographic change point between the contour lines in a contour map. It is also difficult to distinguish between ridges and valleys in the inclination-amount diagram because there is no information indicating the height difference. Furthermore, it is difficult to distinguish the microtopography in the wavelet-analysis diagram, because there is no information indicating the height difference or inclination. Therefore, in this study, we created a microtopography highlight map by overlaying 50 cm of grid data, the inclination-amount map, contour diagram, and wavelet-analysis diagram created from the measured laser data. A field survey verified that, by using this map, it was possible to detect a steep cliff of height 2 m or more, which is a possible source of falling rocks. In our study, we were able to extract sources of falling rocks from a microtopography highlight map.
The necessity of tackling falling-rock projects is increasing, especially, because of the recent heavy rains and earthquakes in Japan. In September 2017, a man riding a bike in the Hualien County in East Taiwan was hit by a falling stone and died. To avoid such painful accidents, improvements in countermeasure projects for falling rocks are necessary.
In Japan's rock-fall countermeasure projects, visual tools (drawings) such as basic forest maps and aerial photographs are used to investigate slopes and grasp the locations of falling rocks. However, on slopes covered by trees, it is difficult to prepare a plan view by actual measurement. As a background of investigation using these drawings, this is because these can be made relatively easily. However, in the current countermeasure projects, it is difficult to grasp accurately the positions of the falling-rock origins, from the surveys using these drawings. This leads to a decrease in the investigation efficiency, which is caused by poor positioning accuracy during the survey and because of overlooking objects. In addition, there are problems related to the safety of investigators at the time of investigation, because survey routes must be selected using these drawings and reliable research routes cannot be obtained from them. To solve these problems, it is necessary to improve the accuracy of the drawings used for the survey.
