## 3 Results  
### Visual Inspection and Spectral Analysis  
We investigated three water types: turbid, red tide, and the surrounding waters from a PlanetScope true-color composite image on 29 July 2018 at 01:42 GMT for visual inspection (Figure 4). The regions were selected based on ground-truth red tide information. Figure 4a shows the turbid waters between the islands. This region is an area where no red tides occur in the ground-truth red tide information. The turbid waters near the coast were brighter than the surrounding seawater, indicating a high SPM. _M. polykrikoides_ red tide patterns are generally brown or reddish in true color composite images. In the case of the surrounding waters in Figure 4b, no red tide bloom was found in the daily report. As shown in Figure 4c, the clear pattern of _M. polykrikoides_ in brown was distinguishable from the surrounding seawater through simple visual analysis. A red tide notice was issued from Goheung to Yeosu according to a daily red tide report on the same day. _M. polykrikoides_ blooms appeared continuously on the coast of Goheung and Yeosu (30-920 cells mL\({}^{-1}\)). In particular, a daily report indicated that high-density red tide strips appeared along the coast of Goheung (2500 cells mL\({}^{-1}\)).  
Figure 5 shows spectra of turbid, the surrounding, and red tide waters extracted from 3B SR PlanetScope image. All spectra were acquired near the coast of Goheung on 29 July 2018 at 01:42 GMT. Pixels were randomly selected from Figure 4a-c for spectra comparison of the three water types. For turbid water, in Figure 5a, a bright pixel near the coast was chosen. The greatest SR spectral values were observed in the green band of 545 nm and tended to decrease at shorter and longer wavelength bands. These properties appeared in complex waters (Case-2 water), which had high levels of SPM and CDOM. As result of spectral analysis in Figure 5b,c, spectra in the waters containing red tide represent the typical shape of _M. polykrikoides_ patches compared to the surrounding waters. This is consistent with spectra of _M. polykrikoides_, shown in Landsat images with wavelength bands similar to PlanetScope [23]. However, although the magnitude of the SR value in turbid waters was greater than that of the SR value in red tide waters, it is quite difficult to distinguish the two spectra based on spectra shape because the two spectra are very similar in Figure 5a,c.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & & **Red Tide Index in Ground Truth** \\ \cline{2-4}  & & **True (_mr_)** & **False (_mm_)** \\ \hline Red tide index in the predicted & True (MR) & (1) True positive & (3) False positive \\  & False (nMR) & (2) False negative & (4) True negative \\ \hline \hline \end{tabular}
\end{table}
Table 4: A confusion matrix for evaluation of red tide detection accuracy.  
Figure 4: 3B surface reflectance (SR) PlanetScope true-color composite image (R: 630 nm; G: 545 nm, B: 485 nm) acquired near the coast of Goheung on 29 July 2018 at 01:42 CMT. Selected regions, (**a–c**) in the left image are enlarged on the right panels. Regions (**a–c**) represent turbid, the surrounding, and red tide waters, respectively. Region (**c**) shows a high-density _M. polykrikoides_ strip in brown appearing along the coast of Goheung (2500 cells mL\({}^{-1}\)).  
Figure 5: Spectra of (**a**) turbid waters, (**b**) the surrounding water, (**c**) red tide waters, and (**d**) the average spectra of three water types in the coast of Goheung from 3B SR PlanetScope image on 29 July 2018 at 01:42 GMT. For spectra extraction of the three water types, pixels were randomly selected from the regions in Figure 3a–c. Blue dashed, black dash-dot, and red lines indicate the average over the extracted spectra in (**a–c**), respectively.