We present the Global Water and Flood Mapping System (GWFMS), an experimental web-based portal supported by NASA, designed to facilitate access to high-resolution surface water and related products derived from commercial satellite data. The system architecture is modular, enabling integration of additional commercial satellite data sources; its current configuration is optimized for PlanetScope optical imagery, which offers near-daily global coverage at high spatial resolution. This temporal frequency enables the detection and monitoring of ephemeral or small-scale flood events that are typically missed by lower-resolution or less frequently updated datasets. Products generated by GWFMS are disseminated through an on-demand online service, which can be activated for example, during emergency scenarios, such as developing flood events, to support rapid response. The current system configuration utilizes the PlanetScope Ortho Analytic 4B Surface Reflectance (SR) product from Planet Labs. Surface water is detected using the Normalized Difference Water Index (NDWI), in conjunction with a multi-Otsu thresholding algorithm to delineate water bodies. Validation of the water detection output against the Global Surface Water dataset demonstrates strong overall accuracy exceeding 90%, with a false positive rate of approximately 3.5%. Despite its effectiveness, the system adopts a conservative detection approach in areas characterized by complex terrain, cloud shadow contamination, or high turbidity, which may reduce sensitivity in these regions. Nevertheless, GWFMS significantly enhances the capacity to monitor surface water dynamics globally, offering a valuable complement to existing global water datasets-particularly for disaster response agencies and other stakeholders requiring timely, high-resolution hydrological information.
Policelli et al. (Tue,) studied this question.