Questo layer mostra la variazione dell'indice delle alghe galleggianti (FAI) sulle superfici d'acqua tra due periodi annuali. Il FAI viene calcolato dalle immagini di riflettanza al suolo Sentinel-2 (COPERNICUS/S2_SR_HARMONIZED, Copernicus/ESA, circa 20 m) usando la banda del vicino infrarosso B8A rispetto a una linea di base rosso-SWIR. La palette divergente si legge cosi: rosso per una diffusione o intensificazione delle alghe galleggianti e della vegetazione acquatica, blu per una riduzione, bianco per nessuna variazione significativa.
The layer compares the Floating Algae Index (FAI) between an initial annual period and a final annual period to reveal where floating algae and aquatic vegetation on water surfaces have spread or receded. FAI is a baseline-subtraction index sensitive to the reflectance peak that floating vegetation produces in the near-infrared.
| Code | Provider | Resolution | Year |
|---|---|---|---|
| WRD_S2XXX_99 | ESA / Copernicus | 20 m | 2017-present (rolling) |
For each period a cloud-free mean composite is built from the Sentinel-2 collection COPERNICUS/S2_SR_HARMONIZED, with clouds and shadows removed via the Scene Classification Layer (SCL) and only water pixels retained (SCL class 6). FAI is computed per scene as FAI = (B8A - [B4 + (B11 - B4) * 0.211]) / 10000, where B8A is the narrow near-infrared band (865 nm, 20 m), B4 is red (665 nm) and B11 is SWIR1 (1610 nm); the division by 10000 rescales the Sentinel-2 surface-reflectance digital numbers to physical reflectance, and the factor 0.211 is the wavelength-based interpolation coefficient (Hu, 2009). The change layer is the pixel-wise difference between the final and initial period composites, rendered with a diverging blue-white-red palette clipped to the range -0.1 to +0.1.