Technical Note. Mapping seagrass beds using IKONOS satellite image and side scan sonar measurements: a Japanese case study

Abstract

In this study, we mapped seagrass beds in Japanese coastal areas by using a newly developed combination method of satellite and side‐scan sonar images. Traditionally, sea‐truth data used for satellite‐image analysis are collected through direct observations requiring scuba divers or by aquatic video‐camera observation. The method proposed here used side‐scan sonar measurements for collecting accurate sea‐truth data and succeeded in efficiently obtaining precise information about seagrass distribution. Side‐scan sonar images also enabled us to assess the reliability of satellite‐image results from an area perspective and not only from a traditional point data one. The IKONOS image was analysed in two different ways. First, mapping was realized without any reference to bottom depth. Second, mapping was processed with consideration for depth gradients. Results were compared using error matrices and showed that the method proposed here is suitable to map several specific areas efficiently.

Acknowledgements

We thank Mr Koichi Morita, Mr Masataka Kurosawa, and Mr Kouichi Shoudoshima from the International Coastal Research Center, the Ocean Research Institute, the University of Tokyo, who helped us in the field and gave us advice. We are also grateful to Mr Takurou Nakagawa and Mr Song‐zhe Bai from SEA Corporation who were of great help in sonar equipment measurements. We are also grateful to Mr Yoshikazu Ozeki from Japan Space Imaging Corporation for his cooperation to this analysis.