## 4 Materials and Methods
### Image Processing Tools  
Considering the extensive and heterogeneous image datasets available, different tools were used for the image pre-processing and then for the DIC analyses. The pre-processing phase is required to prepare the pre-post image pairs from each single dataset for processing using DIC software. The following software tools were used:  
* ENVI(r) v. 5.4 (Environment for Visualizing Images) [58]: ENVI(r) was used to visualize the available dataset and perform oversampling of both aerial orthophotos to 0.8 m/pixel geometric resolution (by using the nearest-neighbor interpolation method);
* ESRI ArcGIS [59]: This software performs a number of surface operations and generates eight shaded reliefs before performing the DIC analysis on the pre-post DTM pair; and,
* SARPROZ(r) (SAR PROcessing tool by periZ) [60]: This tool was used to perform time-averaged filtering on the entire CSK SAR amplitude dataset (in both ascending and descending geometries). The DIC analyses were undertaken using two open source software programs: COSI-Corr and GOM Correlate. * COSI-Corr (Co-registration of Optically Sensed Images and Correlation) [61] is a sub-pixel image correlation algorithm (developed by the authors of [62; 63] that is available as an open-source plug-in for the ENVI(r) software package. According to References [64; 65; 33; 62; 63], to allow for displacement measurements, an initial parameter setting has to be chosen as follows: (i) a window size, which is the size in pixels of the patches that will be correlated in the x and y directions; (ii) a step, which determines the step in the x and y directions in pixels between two sliding windows; and (iii), the type of correlator engine to be chosen, between frequency (Fourier based) and statistical typology. Further detailed descriptions of the algorithms and characteristics of this software are available in References [66; 62]. * GOM Correlate [67] is a DIC evaluation software program used for materials research and component testing. GOM Correlate software is based on a parametric concept that forms the underlying foundation for every single function [68; 69]. This parametric approach ensures that all process steps are traceable, thereby guaranteeing process reliability for measuring results. In addition, in GOM Correlate, parameters must be initialized. While establishing \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Dataset** & **Platform** & **Sensor** & **Image** & **Geometrical Resolution (m/pixel)** & **Pre-Failure Image** & **Post-Failure Image** \\ \hline COSMO-SkyMed (CSR) ASC & Satellite & SAR & SAR absolute amplitude & 3 & 3 December 2013 & 18 December 2013 \\ \hline COSMO-SkyMed (CSR) DESC & Satellite & SAR & SAR absolute amplitude & 3 & 31 March 2013 & 20 December 2013 \\ \hline COSMO-SkyMed (CSR) ASC & Satellite & SAR & SAR temporal average amplitude & 3 & 19 May 2011-3 & 18 December 2013-14 \\ \hline COSMO-SkyMed (CSR) DESC & Satellite & SAR & SAR temporal average amplitude & 3 & December 2013 & May 2015 \\ \hline LANOSTA & Satellite & Multi-spectral & Panchromatic & 15 & 26 October 2013 & 15 February 2014 \\ OLI-THS & Airborne & LIDAR & Shued relief & 1 & July 2013 & December 2013 \\ \hline Orthophoto & Airborne & Optical & High resolution & \textless{}1 & July 2013 & December 2013 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Detailed scheme of the technical specifications of the different analyzed datasets. the surface component, the software finds square-shaped facets in the collected scenes. Basically, the square facets, set in GOM Correlate, are equivalent to the subset, set in COSI-Corr analyses. GOM Correlate software identifies these facets by the stochastic pattern quality structure. The distance between the individual square shapes has to then be properly set. The point distance describes the distance between the center points of the adjacent square facets. This setting influences the measurement point density within the surface component. The measurement point density increases as the point distance decreases.