# Groundwater and Urban Planning Perspective
## 2 Exploring the Connection between Urban Development and Urban Subsurface Hydrology
### Land Instability in Bucharest City
It considers specific elements of the studied area, such as: subsidence occurrence; land-cover changes; hydrogeological, geological, and hydrological characteristics; the history of the study area (considering the location and the extension of old quarries for building materials); the last 100 years of topographical changes to the Colentina River chained lakes; and climatic aspects. Figure 3: Colentina River corridor study area. Map generated in Esri® ArcMapTM 10.3. Base map source: OpenStreetMap. Source: elaborated by the authors. Short Overview of the Processing and Analysis of Satellite Data for Identification and an Outline of the Main Subsidence Triggering Factors  
To generate the PSI maps for the 2004-2010 and 2014-2018 time intervals, two different data sets and methodologies were used. Based on the data coming from the radar European mission ENVISAT ASAR, a land displacement map was generated for the June 2004-July 2010 time interval using the ENVI SARScape software (ENVI SARScape version 5.5, L3 Harris, purchased from ESRI Romania (Bucharest)-authorised distributor of Harris Geospatial products (ENVI and SARScape) for Romania) for data processing [33] and by applying the PSI technique. ENVISAT ASAR data are available in the archive of the European Space Agency (ESA) along with data from other historical missions of ESA [34]. For this study, some of the SAR ENVISAT ASAR data used for processing was purchased through the ESA C1P 6050 proposal, using the Romanian Space Agency as coordinator. The main PSI processing steps for the ENVISAT ASAR data in ENVI SARScape include [33]:  
* ENVISAT ASAR images were imported to ENVI SARScape;
* Application of the Orbit File: precise orbit files were downloaded from ESA archive while the SAR images were downloaded;
* Connection Graph Generation: the SAR pair combination (Master and Slaves) are defined together with the connection network, used for the generation of the multiple differential interferograms. The Master image that was used was from 24 February 2007;
* Area of Interest Definition: Bucharest city;
* Interferometric process, including the coregistration, interferogram generation, and the flattening and Amplitude dispersion index. Here, the digital elevation model (DEM) is specified. The STRM-3 v4 DEM was used for this processing;
* Inversion: First Step--estimation of the first model inversion to derive the residual height and the displacement velocity;
* Inversion: Second Step--estimation of the atmospheric phase components, considering the first linear model product coming from the previous step;
* Geocoding--the PS products are geocoded, and the displacements can be displayed. During satellite data processing, the baseline was considered between the master and the slave scenes to have values lower than 300 m. The coherence threshold established for the ENVISAT ASAR data series is 0.8. For 2014-2018 surface displacements in Bucharest, data from the SAR Sentinel-1 sensor have been used. The data that were used, available on the \"Copernicus Open Access Hub\" website [35], cover the period between October 2014-April 2018 and consider acquisitions from both a descending (109D) and an ascending orbit (131A). The PSI maps for both ascending and descending orbits were produced by the Norwegian Geological Survey within the Norwegian Ground Motion Service [36], by applying the classic PSI technique. The PSI processing of the Sentinel-1 data was carried out as an activity within the INXCES-INnovations for eXtreme Climatic EventS project [37]. Based on the ascending and descending PSI maps, where the land displacements are measured along the Line of Sight (LoS), a PSI map of vertical displacements was computed considering the methodology described in [38]. Several areas showing subsidence on the PSI map have been outlined: even the general trend of the city of Bucharest for the period 2014-2018 showed stability. Most of these areas are located along the corridors of each the two rivers crossing the city.