Proposal of convenient method of ortho mosaic image and DSM of forest floor in case of mangrove forest.

Abstract

Space information on the forest floor is limited compared to that on the canopy. Therefore, this study aimed to propose a simple method to obtain ortho mosaic images and digital surface models (DSM) of the forest floor and present the vegetation information (density of tall trees, density of seedlings, density of knee-roots) from interpretation of or tho mosaic images as an example. A mangrove forest of the Okukubi River run in Kin-cho, Okinawa prefecture was investigated. The canopy was captured as still images with un manned aerial vehicles (UAV), and the forest floor was captured as videos with a camera attached to a camera stabilizer. The images and videos were processed with Structure from Motion (SfM) to produce ortho mosaic images and DSM of the canopy and forest floor. The positions of tall trees and seedlings, knee-roots of Bruguiera gymnorhiza could be read out of the ortho mosaic images of the forest floor. The forest floor DSM had problems on abso lute height accuracy, such as the influence of the ʻdomingʼ effect. Nevertheless, they largely reflected the geography. In the canopy height model (CHM), differences of 2 m or more from measured tree heights were observed at some points, suggesting that the ʻdomingʼ ef fect of the forest floor DSM may be a cause of CHM errors. A method of producing ortho mosaic images and DSM of the forest floor proposed in this study is simple, and may pro vide a new investigation method. In mangrove forests, this method is expected to allow us to obtain the forest floor information that we never had before and information useful for mangrove forest management. However, it is necessary to accumulate examples further to ward practical use because of the insufficiency of accuracy verification.