Following flooding disasters, satellite images provide valuable information required for generating flood inundation maps. Multispectral or optical imagery can be used for generating flood maps when the inundated areas are not covered by clouds. We propose a rapid mapping method for identifying inundated areas based on the increase in the water index value between the pre- and post-flood satellite images. Values of the Normalized Difference Water Index (NDWI) and Modified NDWI (MNDWI) will be higher in the post-flood image for flooded areas compared to the pre-flood image. Based on a threshold value, pixels corresponding to the flooded areas can be separated from non-flooded areas. Inundation maps derived from differencing MNDWI values accurately captured the flooded areas. However the output image will be influenced by the choice of the pre-flood image, hence analysts have to avoid selecting pre-flood images acquired in drought or earlier flood years. Also the inundation maps generated using this method have to be overlaid on the post-flood satellite image in order to orient personnel to landscape features. Advantages of the proposed technique are that flood impacted areas can be identified rapidly, and that the pre-existing water bodies can be excluded from the inundation maps. Using pairs of other satellite data, several maps can be generated within a single flood which would enable emergency response agencies to focus on newly flooded areas.

在洪灾之后，卫星图像会提供生成洪水淹没图所需的有价值的信息。当淹没区域没有云层覆盖时，可以使用多光谱或光学图像生成洪水地图。我们提出了一种快速映射方法，用于根据洪水前后卫星图像之间的水指数值的增加来识别淹没区域。与洪水前的图像相比，洪水后地区的洪水后图像中的标准差水指数（NDWI）和修正的NDWI（MNDWI）的值会更高。基于阈值，可以将与淹没区域相对应的像素与非淹没区域分开。从差异MNDWI得出的淹没图值准确地捕获了水灾地区。但是，输出图像会受到洪水前图像选择的影响，因此分析人员必须避免选择在干旱或更早的洪水年份中采集的洪水前图像。同样，使用此方法生成的淹没图也必须覆盖在洪水后的卫星图像上，以便使人员适应景观特征。所提出的技术的优点是可以快速识别受洪灾影响的区域，并且可以从淹没地图中排除先前存在的水体。使用成对的其他卫星数据，可以在一次洪水中生成几张地图，这将使应急响应机构能够专注于新近发生的洪水。