## 4 Results
On the other hand, points characterized by high cumulative displacements show absolute offset values that are well above the GPS positioning error  
Figure 5: Cumulative displacements retrieved by PS-MTI (red) compared with GPS validation surveys (blue): CR4; CR6 CR8 and CR11 are part of the track zone; CR13 and CR53 are part of the transit zone; and CR23, CR25, CR28, CR49, CR57 and CR58 characterize the depletion zone. Note that, due to the large displacements recorded, the diagram of CR53 is out of scale respect to the others. (CR11, CR23, CR25, CR28, CR49, CR53 and CR57). The highest absolute values are obtained for CR53, CR28, and CR11. Indeed, these points show strong non-linear trends, with acceleration periods that are not detected by the PS-MTI time series. The large absolute offset of some points is not necessarily reflected in a large relative offset, since the latter, being calculated as a ratio, depends only on the magnitude of the movements. This is clearly the case of CR53, which has the largest cumulative displacement of all the CRs and is characterized by a large absolute offset and a relatively low relative offset value. In terms of differential displacements, the differences between PS-MTI and GPS-LOS results are presented in Figure 7, which shows that PS-MTI cannot adequately record the acceleration events occurred during spring 2014 and autumn 2014. An example for this is given by the differential displacement boxplots for CR11, CR13, CR23, CR28 and CR53. The comparison between differential displacements obtained by PS-MTI and GPS-LOS using Equations (6) and (7) resulted in the boxplots of Figure 8. It shows that the median of the offset values between differential displacements range between +9% and \(-\)211%. Globally, it appears that the PSI-MT underestimates differential displacements by a magnitude of approximately 16% (which is the median of the median values). Specifically, CR58 shows a very different behavior since its relative offset median value corresponds to \(-\)211%. The absolute offsets distribution shows that all records have median values very close to  
Figure 6: Evaluation of PS-MTI and GPS-LOS cumulative displacement time-series: (**a**) relative offset; and (**b**) absolute offset. The red shaded area represents the median GPS error computed for each monitoring point. Boxplots represent the 1 st and 3 rd quartiles (limiting the bottom and top of the boxes) and the median (2 nd quartile) is depicted by the black horizontal line. Whiskers represent the difference between the 1 st quartile and 1.5 times the Inter Quartile Range (IQR), bottom whisker, and the 3 rd quartile minus 1.5 times the IQR, top whisker. 0 mm and always below the GPS error (red shaded area). This means that, in the large majority of the time intervals, the PS-MTI provides results that are in very close agreement with GPS-LOS. However, outlier values are obtained in correspondence to sudden acceleration events during which the PS-MTI differential displacement is substantially lower than the GPS-LOS displacement. This is also the reason why a few erroneous PS-MTI measurements (probably due to phase cycle slips during acceleration events) can severely affect the cumulative displacement assessment. Figure 7: Differential displacements retrieved by PS-MTI (red) compared with GPS validation surveys (blue). The blue shaded areas represent the positioning standard deviation calculated for every GPS acquisition. Note that, due to the large displacements recorded, the diagram of CR53 is out of scale respect to the others.