# Airborne GNSS Reflectometry for Water Body Detection
## 6 Data Analysis
### Water Body Surface Detection  
The proposed radar technique is first applied for detecting water body surfaces in landforms. We represent the specular points of reflection corresponding to the detected water body surfaces by our radar technique for the three satellites in study on maps provided by the French National Institute of Geographic and Forest Information (IGN) maps using QGIS software. IGN maps maintain geographical information for France managing geodesic and leveling networks, aerial photographs and geographical databases and maps. IGN maps provide up-to-date map schemes that clearly show the actual locations of the water body surfaces at the day of the experimentation. The aim is to assess the performance of the proposed method for water body surface detection. Figure 15 shows the detected water body surfaces using our proposed method for the area between Guines and Ardres represented by a blue coloring of the specular points of reflection.  
Figure 14: Detection of water body surfaces in landforms using the proposed automatic radar technique. (**a**) Lake (50.882083\({}^{\circ}\)N, 1.976546\({}^{\circ}\)E); (**b**) Swamp (50.876854\({}^{\circ}\)N, 1.948390\({}^{\circ}\)E); (**c**) River (50.887720\({}^{\circ}\)N, 1.878526\({}^{\circ}\)E); (**d**) Streams (50.888385\({}^{\circ}\)N, 1.868127\({}^{\circ}\)E); (**e**) Wetland (50.883070\({}^{\circ}\)N, 1.900203\({}^{\circ}\)E); (**f**) Pool (50.886888\({}^{\circ}\)N, 1.884547\({}^{\circ}\)E).  
The proposed method detects water body surfaces whenever the reflectivity measurements are beyond a specified threshold. From the analysis of the GNSS measurements obtained, the reflectivity threshold used in this study is 0.21 for the three satellites. We compare the number of water body surfaces provided by IGN maps along the satellite footprints traces with the percentage of detections of these water bodies using our proposed approach. Table 2 details the results of the manual inspection applied between the IGN images and our radar technique for water body surface detection.  
We show in Table 2 that our radar technique detects 96% of in-land water body surfaces (i.e., 45 out of 47 surfaces) along the satellites footprint traces as compared to the details provided by IGN maps. We detect 100% of large-size contained water body surfaces (i.e., lakes/large swamps) and of large waterways (such as rivers and canals). However, we miss one small-size contained water body surface and one stream along the trajectory. We cannot clearly observe the reason using maps such as IGN or Google Earth as it is not evident whether the miss is due to a detection inaccuracy or due to the water bodies being masked by vegetation (such as trees or groves).  
Although IGN maps provide updated map imageries, we can observe from Figure 15 that these maps do not provide sufficient information about the water body characteristics (e.g., type, shape, and approximate size) nor the vegetation possibly covering it. Thus, we propose to use satellite images provided by Google Earth software for water body edge localization.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Water Body** & \
\begin{tabular}{c} **Number of Surfaces** \\ **Using IGN Maps** \\ \end{tabular} &
\begin{tabular}{c} **Percentage of Detection** \\ **Using Our Radar Technique** \\ \end{tabular} \\ \hline Lakes/Large Swamps & 20 & 100\% \\ \hline Ponds/Swamps/Wetlands & 17 & 94\% \\ \hline Rivers/Canals & 4 & 100\% \\ \hline Streams/Brooks & 6 & 83\% \\ \hline
**Total** & **47** & **96\%** \\ \hline \hline \end{tabular}
\end{table}
Table 2: Performance assessment of the proposed radar technique for water body surface detection. Results of the manual inspection applied between the IGN images and our radar technique per water body type and in total (bold) for the study area.  
Figure 15: The detected water body surfaces using our proposed radar technique for the area between Guines and Ardres. The specular points of reflection, represented by a blue coloring, are superimposed on IGN maps.