Normalized Difference Water Index - NDWI - is intended for creating an image that visually separates land from open water, and is especially useful for monitoring flood extents, water levels in lakes, and other open water bodies, such as rivers. NDWI is known to have comparatively high values in built up areas, and as such, is not well suited for mapping the extents of urban flooding, as separating wet concrete and asphalt from flooded areas is prone to errors.