# ADAfinder Tool Applied to EGMS Data for the Structural Health Monitoring of Urban Settlements
## 1 Introduction  
The monitoring of the deformation of Earth's surface is possible today, also in the long term, exploiting the available huge archives of Synthetic Aperture Radar (SAR) images acquired from different sensors [1, 2, 3, 4], at costs much lower compared to the traditional ground monitoring techniques [5]. The SAR image archives start from 1992, from the sensors onboard the satellites ERS-1/2, Envisat and Radarsat, working in C-band. From 2007, TerraSAR-X and COSMO-SkyMed gave a great improvement, providing X-band data with a very high resolution. Moreover, the launch of the two satellites of the Sentinel-1 constellation, S-1A and S-1B, respectively in 2014 and 2016, even further improved the area coverage and the revisiting time (six days) of the available data acquired in band C. In recent times, many applications of SAR satellite data in the field of structural monitoring and assessments of single constructions have been performed [6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Moreover, radar-based techniques are very useful for the monitoring of the ground deformation of wide areas, as demonstrated by many literature works [16, 17, 18, 19, 20, 21], as well as national and regional services born in different countries in the last five years. Regarding the latter topic, at national scale, the first Ground Motion Service (GMS) started in Italy in 2007 [4], followed by Norway in 2018 [22], and Germany in 2019 [23]. Moreover, Denmark and the Netherlands are working on their own GMSs. In Italy, this kind of monitoring service has also been developed at regional scale, from 2016 in Tuscany [24, 25], from 2018 in Valle d'Aosta [26], and from 2019 in Veneto. A review of the evolution of wide-area deformation monitoring initiativebased on the DInSAR techniques applied to SAR and Copernicus Sentinel-1 images can be found in [PERSON] et al. [27]. At international level, the introduction of the European Ground Motion Service (EGMS) as new service of the Copernicus Land Monitoring Service was approved in 2017 [28; 29; 30]. The EGMS opens a new prospect in the field of the study of the ground deformation related to natural or anthropic-induced phenomena at regional scale. Moreover, it can also be seen as a new opportunity to study ground motion influencing structures and infrastructures. The multi-temporal interferometric processing of Sentinel-1 radar images at full resolution, using Persistent Scatterers (PS) and Distributed Scatterers (DS) radar interferometry approach, is the basis of the EGMS. For the majority of users, the great volume of products derived using SAR satellite data is difficult to manage and exploit and is onerous in terms of time. For this reason, the research is currently oriented toward developing new methodologies to exploit these data for the regional scale monitoring of natural or anthropic-induced phenomena (e.g., landslides, subsidence, volcanic activity, settlements). Many methodologies have been proposed to identify active areas starting from the products derived using SAR satellite data [31; 32; 33]. The use of these data can also be focused on the impact of the mentioned deformative phenomena on the built environment, with Structural Health Monitoring (SHM) purposes. A new strategy for the preliminary identification and ranking of possible critical constructions in terms of deformations, based on the clustering of the measuring points, is proposed in [PERSON] et al. [34]. In [PERSON] et al. [35], a new tool called \"ADAfinder\", aimed to the semi-automatic extraction of active deformation areas (ADA) starting from the deformation maps obtained through the Persistent Scatterer Interferometry (PSI) technique, is presented. ADAfinder is a very promising tool for the management and elaboration of PSs that allows to minimize the amount of information to be stored in relation to the amount of information entering the system. An application was presented over the Canary Islands (Spain) affected by landslides, earthquakes, and volcanic activity.