Extraction of windthrown trees gaps using data photographed before and after typhoon by UAV

The risk of windthrown trees due to typhoons has increased because of climate change, making it imperative to be able to rapidly assess the extent of damages. Typhoon No. 21, which landed on the Japanese archipelago in September 4, 2018, was used for a survey of woodlands in the Natural and Cultural Gardens located in the Expo’70 Commemorative Park (Suita, Osaka Prefecture). By comparing a Digital Surface Model (DSM) obtained by processing data sets on two dates, before the typhoon on July 11, 2018, and after the typhoon on September 26, 2018, using Structure from Motion, I was able to extract the extent of the windthrown trees gaps caused by the typhoon. The correlation between the gap-patch area and breast height cross-sectional area was overall higher than correlation between the gap-patch area and number of trees from within the gap patch. I was able to extract gaps formed by fallen trees for a difference of ≥ 6 m between the two DSM periods. The total gap-patch area generated by the typhoon was 6,926 m2, which was equivalent to approximately 64％ of the gap area artificially caused within the same area over 17 years between 2000 and 2017. Taking regular data sets with UAV will be effective for forest monitoring.