# Groundwater and Urban Planning Perspective
## 2 Exploring the Connection between Urban Development and Urban Subsurface Hydrology
### Land Instability in Bucharest City
Figure 4 illustrates the PSI map of vertical land displacements obtained using Sentinel-1 images. They are classified according to several criteria, depending on the main triggering factor, as it is shown in Table 2.  
To understand main the phenomena and processes that take place along the Coletina River corridor, several types of data sources were used. These include data describing natural environment of the area, geographical and urban conditions, and hydrogeological, hydrological, and geotechnics information. Table 3 mentions data types and information along with their sources used in the analysis. \begin{table}
\begin{tabular}{l c} \hline \hline
**Areas Where Displacements Have Been Identified, Classified by Land Use Category** & **Areas on the Map** \\ \hline Areas with small above-ground constructions (residential, office, or commercial buildings) and & \\ a small number of floors, where previous demolition took place & A4, A12, A10 \\ \hline Urban areas where land has shifted from green space to construction during the last 10–15 years & A6, A9, A13 \\ \hline Peri-urban areas in the pasture or forest category whose use has passed into the construction & A7 \\ category during the last 10–15 years & \\ \hline Green areas where works have been carried out for the practice of leisure sports (bicycle courts, & A3, A5 \\ tennis courts, or skateboarding) & \\ \hline Areas where the land use category has not changed, but there are specific factors producing & A1, A8 \\ displacements—e.g., the presence of an anthropogenic superficial layer of construction debris & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Areas with identified displacements (modified after [28]). Figure 4: PSI map of vertical land displacements in Bucharest city (obtained from Sentinel-1 images) and the identified subsidence areas (modified after [38]). Map generated in Esri® ArcMapTM 10.3. Source: elaborated by the authors. In addition to the SAR products used for radar interferometry, which use phase information, SAR images also contain amplitude information that can be used for other types of analysis. Thus, through the Interferometric Wide Swath (IW) mode of operation, in addition to the SLC (Single Look Complex) products used for interferometry, Sentinel-1 sensors provide GRD (Ground Range Detected) products. By using this type of product, it is possible to analyze the land-cover surface changes between the times of acquisition of the first image and second images. For consistency with previous land displacement analyzes, in which Sentinel-1 SLC data were acquired between October 2014 and April 2018, two Sentinel-1 GRD images were used to detect changes: the first from 25 October 2014, and the second from 25 April 2018 [38]. The data were downloaded free of charge from the Copernicus Open Access Hub [35]. Figure 5 shows the land-cover changes detection map generated for Bucharest. The areas where changes occurred in 2018 in respect to 2014 are marked in red. Areas where no changes occurred between the two moments of time are symbolized in gray tones. Water bodies have a high reflectivity, and the backscatter towards the radar is low; therefore, the lakes and rivers appear dark on the map. Ground changes are encountered mainly in areas where new residences (e.g., the western border of Bucharest city) or commercial centers are built. More details on the methodology and change detection trends in Bucharest city, considering the 2014-2018 time interval, are presented in [43].