## 3 Results
### Start Bay
At these collection times, the median tide elevations seen by the satellites are 0.5 m and 0.6 m for the ASC and DESC tracks, respectively, which are higher than the median value for the site of 0.3 m. The minimum astronomical tide elevation at the site is \(-\)2.6 m, which is lower than the minimum elevation at the time of satellite data collection, which is \(-\)1.0 m for the ASC tracks and \(-\)1.1 m for the DESC tracks. The range of tide elevations, calculated here as the difference between the maximum and the minimum elevation, is 5.6 m, while the range of elevations captured by the satellite is 3.8 m. These results indicate that satellite observation at this study site only captures about 68% of the astronomical tide elevation range of the daily high and low values and is 0.2 m to 0.3 m biased towards high elevation. Table 7 shows the collection dates of the DTMs used for this analysis and the shorelines closest in time derived from the ASC and DESC satellite tracks. Out of the six total SLs (three ASC and three DESC), only two of the DESC SLs were collected on the same day of the DTM data collection on the 12 th September 2018 and the 19 th September of 2020, both at the time when the tide was rising. None of the ASC SLs were collected on the same day of DTM collection but within \(\pm\)2 days of DTM data collection and all of them during ASC tides, with the SLs being collected on the 17 th September 2020 two minutes before high tide, which was at 17:59 UTC. The only SL collected during descending tide was on the 8 th of March 2016 and close to the daily high tide, which was at 05:20 UTC. The astronomical tide elevation at the time of these SL collection operations varied from 1.1 m to 2.7 m.  
Figure 18 in Appendix A shows the location of the ASC and DESC SLs over the DTMs used for the elevation analysis for Start Bay, and Table 8 shows the main stats of the terrain elevation values along the SLs. The seaward limit of the DTMs is constrained by the lower sea water level at the time of data collection, and most of the SLs are located within the DTM spatial domain, except for some segments of the SLs for the year 2020 and most appreciably for the DESC SLs in the year 2020. The mean number of points with no nan-values (e.g., line is mapped within DTM spatial domain) for all the SLs of the two types in the year 2020 is of the order of 8000 points, while for the years 2016 and 2018, it is of the order of 10,000 points. The minimum, maximum, mean, and median terrain elevations  
Figure 17: Astronomical tide elevation at Start Bay study site for the years 2015 to 2021 from the daily high and low tides, and subsets of when ASC and DESC images were taken. Elevations are obtained using POLTIPS software at the Start Point tide gauge station and refer to Ordnance Datum Newlyn. BGS ©UKRI. for all six lines are \(-2.4\pm 0.2\) m and \(37.9\pm 8.8\) m, \(3.8\pm 0.9\) m and \(3.0\pm 0.5\) m, respectively. The points where we observe the largest elevations are located in places where there is a high cliff with a steep slope and a narrow inter-tidal area. Both the mean and the median elevation values are higher than the astronomical tide levels shown in Table 8. Surprisingly, the mean and median terrain elevation are at the minimum on the 17 September 2020, which is the date with the largest astronomical tide level. The lagoon has been mapped as part of the open coast by the only SL captured during descending tide on 08 March 2016, and there is no evidence of breaching in the DTM closest to this date. Figure 18 shows the changes observed at the Blackpool Sands embayment in Start Bay between 2016 and 2018 and the closest SAR-SLs to the observed DTMs together with the metrics of change obtained using all lines for the study period between 2015 and 2021. Blackpool sands is a small embayment located at the north end of Start Bay that shows how the beach rotates: sediment eroded from one end is deposited at the other end of the embayment, with a nodal point of no elevation change close to the center of the embayment.