## 2 Materials and Methods
### Validation and Interpretation of SAR Shorelines and Change Rates
#### 2.5.3 Start Bay  
For this study site, we are interested in gaining a better understanding of how much of the tidal range is captured by SAR-SLs and if this dataset and metrics of change can capture the beach rotation observed at Start Bay. To address these questions, we started by visualizing the satellite-derived SLs individually and at different scales. This allowed  
Figure 8: Subaerial and inter-tidal beach profile for Salinas beach from in situ surveys (profile No. 6). The horizontal distance is referenced to the baseline in Figure 7 (HAT is the high astronomical tide; MHW is the mean high water; MSL is the mean sea level; MLW is the mean low water; and LAT is the low astronomical tide). the overall characteristics of the lines to be assessed, including their completeness and the presence of any spurious lines, loops, or gaps. The lines were visualized over publicly available high-resolution aerial and satellite optical imagery, allowing for an assessment of where they accurately captured the shoreline and where there were issues. We used Quantum Geographical Information System software (QGIS; version 3.28.0-Firenze) for visualization and analysis. For this study, we used the high-resolution 30 cm imagery layer provided by ArcGIS online services via World Imagery (MapServer), available at [[https://server.arcgisonline.com/arcgis/rest/services/World_Imagery/MapServer/3](https://server.arcgisonline.com/arcgis/rest/services/World_Imagery/MapServer/3)]([https://server.arcgisonline.com/arcgis/rest/services/World_Imagery/MapServer/3](https://server.arcgisonline.com/arcgis/rest/services/World_Imagery/MapServer/3)) (accessed on 6 December 2023). World Imagery provides satellite and aerial imagery at one-meter or better resolution in many parts of the world and lower-resolution satellite imagery worldwide. The map includes 15 m TerraColor imagery at small and mid-scales (\(\sim\)1:591 M down to \(\sim\)1:288 k) for the world. The map features Maxar imagery at 0.3 m resolution for selected metropolitan areas around the world, 0.5 m resolution across the United States and parts of Western Europe, and 1 m resolution imagery across the rest of the world. In addition to commercial sources, the World Imagery map features high-resolution aerial photography provided by the GIS User Community. This imagery ranges from 0.3 m to 0.03 m in resolution (down to \(\sim\)1:280 in selected communities). For more information on this map, including the terms of use, visit ArcGIS online (available at [[http://goto.arcgisonline.com/maps/World_Imagery](http://goto.arcgisonline.com/maps/World_Imagery)]([http://goto.arcgisonline.com/maps/World_Imagery](http://goto.arcgisonline.com/maps/World_Imagery)), accessed on 6 December 2023). To address the question regarding how much of the tidal range is captured by the SAR dataset at Start Bay, we used astronomical tide model POLTIPS3 tidal prediction software (POLTIPS-3; Standard version 3.9.02/21 Standard) [43], which allows us to compute the astronomical tide around 700 UK ports using harmonic components provided by the National Oceanographic Centre. The tide elevations (daily high and low tides) are computed at Start Point relative to the chart datum and are converted to the ordnance datum using Vertical Offshore Reference Frames software (VORF; version 2.11) [44]. The datum used for the British mainland (and some of the closer offshore islands) is called Ordnance Datum (Newlyn), as it is based on the average value of the mean sea level at Newlyn for the six-year period from 1915 to 1921. Due to the subsequent rise in mean sea level, this datum is now about 0.2 m below mean sea level at Newlyn. The tide level at the time when the satellite data were collected is obtained through linear interpolation of the daily high and low tides using the date and time (in coordinated universal time).