## Results
### Comparison with Fremantle
Figure 12: _Cont._  
In the case of Fremantle with the introduced gaps of Aden, the breakpoint technique with assumptions of the break-points suggests an increase of the second segment versus the first of 44.9 mm, an increase of the third segment versus the second of 34.9 mm, and decrease of the fourth segment versus the third of 102.9 mm. Over the time window 1916 to 2013, with all the data considered and no gaps, the sea level trend in Fremantle was +1.65 mm/yr, as seen in Figure 12a. By simply introducing the gaps, the sea level trend increased to +2.27 mm/yr, Figure 12a. The simple presence of gaps makes the estimation non-accurate. By applying the breakpoint alignment technique, detailed in Figure 12b, the aligned data suggest a trend of +2.45 mm/yr (Figure 12c), that is, an 8% difference. Figure 13 presents the analysis of the Fremantle MSL records, with the same gaps as Karachi. The sea level oscillations appeared much stronger in Fremantle than Karachi. In Karachi, the first three segments all had negative trends, from \(-\)2.67 mm/yr (the longest segment) to \(-\)4.62 mm/yr (the intermediate segment). The first, shortest segment, less than a decade, had a linear trend of \(-\)3.47 mm/yr. The last segment, less than two decades long, had a linear trend of +2.93 mm/yr. In Fremantle, with the same gaps as Karachi (Figure 13b), the first segment had a huge negative trend of \(-\)23.49 mm/yr. The other segments had a linear trend variable between \(-\)1.85 and +3.86 mm/yr. The breakpoint technique with three gaps worked poorly, suggesting a decrease in the second segment compared to the first of 46.5 mm, a decrease in the third segment compared to the second of 5.7 mm, and a decrease in the fourth segment compared to the third of 117.5 mm. Over the time window 1916 to 2017, with all the data considered, the sea level trend in Fremantle was +1.65 mm/yr, as seen Figure 13a. By simply introducing the gaps, the sea level trend increased to +2.11 mm/yr. The simple presence of gaps makes the estimation inaccurate. By applying the breakpoint alignment technique with three gaps, as seen in Figure 13b, the aligned data suggested a trend of +0.67 mm/yr, which is a significant difference. Figure 12: MSL of Fremantle with the same gaps as Aden. (**a**) The effect of the gaps. (**b**) Breakpoints alignment. (**c**) Aligned series with gaps. The breakpoint alignment technique only works in specific conditions, when the trends for the different segments suffer from fewer oscillations. If this is not the case, the technique does not help. As there are usually interannual, decadal, and multidecadal oscillations up to 60 years, which can  
Figure 13: MSL of Fremantle with the same gaps in Karachi. (**a**) The effect of the gaps. (**b**) Breakpoints alignments. (**c**) Aligned series with gaps. be very strong in the sea level signals, trends computed with only 10 or 20 years of data may be particularly misleading. The best advice when there are segments is not to couple together the segments, that, at the most, can be analyzed only independently [1; 5; 6]. PSMSL, NOAA, and the other holders of tide gauge data, should simply quality flag all the tide gauges that are segmented.