ESTIMATION OF OVERTURNING RATIO OF TOMBSTONES BY IMAGE ANALYSIS OF AERIAL PHOTOGRAPHS

摘要

As a method to estimate the seismic intensity in a strong earthquake, the overturning ratio of tombstones in a cemetery has been used widely in Japan. The purpose of this study is to estimate the overturning ratio of tombstones due to an earthquake based on image analysis of aerial photographs. It is expected to be an alternative of field survey to grasp seismic intensity widely and densely. In image processing, the edge, which is the outline of an object, is extracted because displaced tombstones are considered to be detected through the distribution of the edge direction. Firstly, the experiments with a model tombstone were conducted under various conditions such as the different displaced-state of tombstone, the position of spotlight, and the resolution of images. Removing noises by a threshold of edge intensity is found to be necessary to extract only notable edges. The examination based on the ratio of vertical and horizontal edge directions shows that not only the state of tombstone but also shadow affects this ratio largely. Hence the direction of sunlight and the shape of shadow should be considered properly. An estimation formula for the overturning ratio of tombstones will be proposed in the near future..