Fundamental Study on Estimation Method of Vegetated Area of Ulva spp. by Using NDVI

Field observation and Aerial photography have been widely used for grasping seaweed growth. In recent years, the utilization of remote sensing is expected to grasp it. It can be observed in a wide area at once, but it is not used to grasp the situation of vegetation growth in the coastal zone. In our previous studies, it is suggested that considering the influence of water depth, it is possible to grasp the vegetated area by Landsat data. In this study, we investigated to measure the vegetated area of Ulva spp. in Yastsu tidal flat by UAV (Unmanned Aerial Vehicle) and satellite data.

Yatsu tidal flat is located in the deepest part of Tokyo Bay. We measured the vegetated area of Ulva spp. in Yatsu tidal flat based on the orthoimage which took data by UAV. On the other hand, using the data obtained by Landsat 7/8, the normalized vegetation index, NDVI, was calculated in Yatsu tidal flat. After that, we examined the growing condition of Ulva spp. by comparing the area of its confirmed by orthoimage and the NDVI value by Landsat 7/8.

The vegetated areas of Ulva spp. by UAV on 27 Oct. 2016, 9 May 2017 and 16 Nov. 2017 were 80%, 45%, and 0% respectively. In Nov. 2017, the area of Ulva spp. has become 0% because of the death of its due to the influence of red tide occurred in Tokyo Bay in July 2017. Next, we considered the relationship between NDVI and water depth. The tide level was low in Oct. 2016, all of the NDVI showed a positive value. Conversely, the tide level was high in Nov. 2016, the NDVI showed a negative value in more than 90% of the vegetated area. Furthermore, in Apr. 2017, positive values of the NDVI were shown at many places in the vegetated area, and it was very different from the vegetated area confirmed by UAV.

Therefore, the NDVI value was corrected for each observation day so as to match the value of the vegetated area at the field. Then, the relationship between the corrected value and the water depth of Yatsu tidal flat was examined. As a result, it was clearly confirmed that the NDVI correction value increases with increasing of water depth. From the result, it was shown that it is possible to grasp the vegetated area by Landsat 7/8 based on the modified NDVI.