2.2. Paired Datasets of PlanetScope Imagery and Corresponding Red Tide Maps

In this study, we used PlanetScope imagery to detect the pixel-wise existence of red tides. The PlanetScope was launched in June 2016, and the satellite constellation consists of approximately 130 satellites and can image the entire land surface of the Earth every day [47]. Each PlanetScope satellite is a CubeSat 3U with dimensions of 10 × 10 × 30 cm³ and carries a telescope and a frame charge-coupled device (CCD) camera equipped with a Bayer-mask filter. Table 1 shows a constellation overview of the PlanetScope. The orbit altitude is 475 km, and the equator crossing time is between 9:30 am and 11:30 am at local solar time. Images around the Korean Peninsula were obtained at approximately 10:40 am every day. Each image pixel had four spectral bands and a spatial resolution of 3 m. An imagery product covers an area of approximately 24 × 7 km², with some variability depending on the satellite altitude.

Constellation overview of PlanetScope.

Imagery products are available as either individual basic scenes (level 1B), ortho scenes (level 3B), or ortho tile products (level 3A). Nevertheless, we used the orthorectified level 3B surface reflectance (SR) PlanetScope product. Level 3B ortho scenes consist of raw digital number (DN) and SR product suitable for analytic and visual applications. The DN value is converted into Top of Atmosphere (TOA) radiance (Wm²sr⁻¹μm⁻¹), then the SR is calculated from the TOA reflectance through an atmospheric correction [47]. We acquired PlanetScope imagery during the red tide event in 2018. Table 2 summarizes the PlanetScope imagery available during the red tide bloom event in the southern coast of Korea in 2018. Except for 28 July, a total of 36 images were collected consecutively from July 24 to August 2. Most images were cloud-free, with the exception of some images.

PlanetScope imagery available during M. polykrikoides bloom event in the southern coast of Korea, 2018.

The ground truths of M. polykrikoides red tide maps corresponding to 36 PlanetScope images were obtained from daily red tide reports provided by NIFS [5] (Figure 1). Each pixel value has a discrete binary value that represents the nonexistence and existence of a red tide, respectively. To construct paired red tide map matching with the spatial regions of PlanetScope imagery, the red tide data in polygon format, which were provided by NIFS, were converted into ground-truth maps in raster format. We calculated the areas of red tide blooms on the ground-truth maps corresponding to 36 PlanetScope images. Among the 36 images, three had a red tide area of over 10 km², and the others had areas of less than 5 km². The widest red tide area of 16.81 km² was observed on 29 July 2018.