# Mapping Atmospheric Exposure of the Intertidal Zone with Sentinel-1 CSAR in Northern Norway
## 4 Discussion
On the basis of these comparisons, we are confident that our overall approach is applicable to any region in the world if the time series period includes a comparable number (i.e., >100) of S1 observations. Over Europe, S1A and S1B both operate constantly in all ascending and descending paths, i.e., four samples per 12 day S1 cycle, acquiring about 120 observation per year. Hence, this should be sufficient for producing an atmospheric exposure map anywhere in Europe on a yearly basis. In regions of the Earth where S1A or S1B operate in only one flight direction, the data acquisition period must be extended accordingly, depending on the overlap of adjacent paths. As the overlap decreases with decreasing latitude, the method needs longer times series at lower latitudes. This means also that natural changes in the ITZ can be monitored only on a time scale of at least twice the time series period. Natural changes in the intertidal zone occur generally on longer time scales than anthropogenic changes such as construction. Anthropogenic changes could, however, lead to natural changes in the intertidal zone, for example, due to a change in coastal currents. As constructions often induce double-bounce scatterers, i.e., corner reflections, they should be detectable on a much shorter timescale. Reducing the integration time will primarily affect the speckle noise reduction and extraction of the lowest (second) and highest (98 th) percentiles, i.e., tides higher than MHWS and lower than MLWS, respectively. The corresponding atmospheric exposure classes of >95% and <5% in our study reflect mainly single pixels or only a few pixels and are, therefore, often negligible. These two classes represent extreme tidal events that occur occasionally but not periodically and will, therefore, rarely be captured. Defining the highest and lowest water lines with the fifth and 95 th percentiles as proposed by [27] could, therefore, be sufficient for most studies and could filter out most of false detections due to boat traffic. One issue affecting the accuracy is the detection of the water line in the percentile images. After a first attempt with automatic thresholding, our experience was that the accurate position of the water lines was improved by refining the thresholds according to GPS tracks from field data. At this stage, due to lack of funding and Covid-19 travel restrictions, we only have a limited amount of field data of the Tromso Municipality. Itneeds to be further investigated if such thresholds are universal or regional and maybe ITZ-type specific. Because of the ITZ's dynamic nature, a strict accuracy assessment of the product is challenging and needs extensive observations of the position of the water line at different tidal reference levels. Exact timing of the observations during the field observation is crucial but difficult, as the position of the water line in tidal flats can change with a speed of some cm\(\cdot\)s\({}^{-1}\). As times of observations are based on tidal model predictions that are only given in 10 min intervals, the positioning error due to timing alone can be on the order of 10 m. Additionally, the water line in mudflats is often not a clear line but a diffuse area of small features above water. These features can be both inorganic and organic: small sand elevations, single stones, rocks, or vegetation that is either floating at the surface (e.g., kelp) or extending above the water surface as in salt marshes. Vast areas of salt marshes, however, are not common in this study region [51]. Such small features can also change quite rapidly due to currents, wind, and benthic activities. The definition and the interpretation of the water line in such areas are, therefore, highly dependent on the observer and the exact time, and they can also be associated with a 10 m inaccuracy. Collecting such field data is a time-consuming effort, and a more rigorous field data collection than presented here could improve the accuracy assessment.