# Monitoring and Forecasting Green Tide in the Yellow Sea Using Satellite Imagery
## 4 Results  
### Semi-Automated Green Tide Extraction  
#### 4.1.1 Characteristics of Different RS Data  
To compare the extraction results of different satellite data sources, we matched the contemporaneous satellite images of each data source and selected the same area for comparison to ensure that the objective conditions, such as cloud volume, weather conditions, imaging effect, and green tide coverage, of the satellite images, were as similar as possible. Due to the different revisit periods of the satellites, it was difficult to match the data exactly to the same day, so we tried to select a set of data with relatively close dates, and the results of the comparison are shown in Table 3. Even though the selected data for GF-1 and HJ-1 did not match the RS image on the same day as the other data, both of them are the main sources of data for the Yellow Sea Green Tide traceability, so we still added them to the comparison. The region selection schematic diagram and extraction effect of each dataset are shown in Figure 8.  
In terms of the ability of the RS image to display the green tide information, the high-resolution RS images (Figure 8b,d,f,h) presented the green tide information more completely, regardless of whether the green tide was large or sporadically distributed, whereas the low-resolution RS images (Figure 8j) cannot present the sporadic green tide information. In terms of the effect of the extraction method, the green tide information presented by each data source was extracted successfully, although the extraction results were, as expected, significantly better for the high-resolution RS images (Figure 8c,e,g,i) than for the low-resolution images (Figure 8k), especially when the early green tide bloom was sporadically distributed [36]. To sum up, thanks to the advantage of the presentation of sporadically distributed green tides, the high-resolution RS images had absolute advantages over low-resolution images for the traceability of early green tide sources.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Satellite** & \begin{tabular}{c} **Time of** \\ **Guardian Film** \\ \end{tabular} & **Resolution (m)** & \
\begin{tabular}{c} **Number of** \\ **Grids** \\ \end{tabular} &
\begin{tabular}{c} **Corresponding** \\ **Area (km\({}^{2}\))** \\ \end{tabular} \\ \hline GF-1 & 20 June 2021 & 16 & 562 \(\times\) 562 & 80.86 \\ Landsat & 23 June 2021 & 30 & 300 \(\times\) 300 & 81.00 \\ HJ-1 & 22 June 2021 & 30 & 300 \(\times\) 300 & 81.00 \\ HY-1C & 23 June 2021 & 50 & 180 \(\times\) 180 & 81.00 \\ MODIS & 23 June 2021 & 250 & 36 \(\times\) 36 & 81.00 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Selection regions of the same area size from different data sources.  
Figure 7: Spectral information of one _U. prolifera_ pixel from GF-1 image (6 April 2021).  
Figure 8: Schematic diagram of the location of the satellite extraction area. (**a**) is the selected approximate area; (**b,d,f,h,j**) are false-color images of GF-1, Landsat, HJ-1, HY-1C, and MODIS, respectively (band combination: GF-1: band 3-2-1; Landsat: band 4-5-3; HJ-1 and HY-1C: 3-4-2; MODIS:1-2-1); and (**c,e,g,i,k**) are the images’ respective green tide extraction effects (the red point represents FP, the white point represents FN, and the green point represents TP).