# Potential and Limitations of Open Satellite Data for Flood Mapping
## 4 Results
### Semi-Automatic Mapping of the Flooded Area
#### 4.2.2 Multispectral Data Analysis  
In this section, we present the results of the multispectral analyses relying on single data methods (e.g., supervised classification) and on methods based on two-image comparisons. **(a) Supervised classification. ** Figure 7 shows the supervised classification based on MODIS and Landsat-8 multispectral images. We have defined four primary land-use typologies: Vegetation, urban area and bare soils, water body/wetland (that mostly correspond to the flooded areas) and cloud cover. The supervised classification showed good results for both the 2 March 2015 MODIS-Terra image (Figure 7A) and the 3 March 2015 Landsat-8 (Figure 7B) collected over the Ebro River valley, for which the scenes were almost cloud-free and the river was at the maximum discharge. The Landsat-8-based classification showed errors in the areas where the vegetation emerged from the water. The details were provided by the finest resolution. In the case of the Po basin flood, the supervised classification of the 26 November 2016 MODIS-aqua image (12-24 h after the flood peak) showed an underestimation of the flooded areas in the sectors inundated by the Bormida (Figure 7C) and by the Oitana and Chisola streams (Figure 7D). Classification errors due to partial cloud cover and shadow classified as water can be seen in the area around Alessandria (Figure 7C), but they do not significantly affect flood detection. **(b) Post-Pre flood image comparison. ** The comparison of co-flood or post-flood images with pre-flood images allowed mapping the flooded areas using different band-ratio variations. Figure 7: Supervised classification based on **(A)** MODIS image acquired on 2 March 2015 and **(B)** Landsat-8 Images acquired on 3 March 2015 over Ebro valley; MODIS-Aqua images acquired on 26 November 2016 over Alessandria **(C)** and Turin **(D)** areas. (I) The MNDWI\({}_{\text{var}}\) shows different performances depending upon the time of the satellite pass and local area setting. The threshold used to detect flooded areas ranged from 0.1 to 0.2. There was a decrease in the efficacy of this index with time elapsed from the flood event. In the Zaragoza case study (Figure 8A), the flooded area was almost entirely detected by the co-flood Landsat-8 image acquired on 3 March 2015. Some false positive variation at SW is related to cloud cover. The MNDWI\({}_{\text{var}}\) computed with the 2 March 2015 MODIS image provided the same results. In the case of the November 2016 Po basin flood, only post-flood data are available at high resolution. At south of Turin, the flooded areas were satisfactorily detected with the 26 November 100 m spatial resolution Proba-V image (Figure 8B), whereas by using Sentinel-2 imagery acquired (Figure 8C) five days after the water was withdrawn, it remains possible to detect a pattern of positive MNDWI\({}_{\text{var}}\) inside the inundated areas, but with more false positive areas. Over the Alessandria area, the MNDWI based on 100 m spatial resolution Proba-V (Figure 8E) and 500 m spatial resolution Proba-V (Figure 8E) data acquired approximately 8-12 h after the flood peak detected most of the areas inundated by Tanaro, despite cloud cover and shadows. Sentinel-2 images (Figure 8F) acquired approximately 10 days after the flood peak only show residual patterns. Many pixels located out of flooded areas are classified as inundated, most likely due to a general increase of soil moisture. We also applied a cloud mask to the images of 26 November of MODIS and Proba-V; fortuitously, the cloud cover and shadow did not affect the detection of the flooded area. In the case of the Arahal flood (Figure 8F), the MNDWI\({}_{\text{var}}\) based on the Sentinel-2 image acquired 10 days after the flood event allowed the detection of only some residual parts of the original flood. The NDMI\({}_{\text{var}}\) based on Landsat images showed a similar result. Figure 8: Maps of inundated areas based on MNDWI\({}_{\text{var}}\). (**A**) Ebro flood: Landsat-8 5 March 2015.