# Mapping Atmospheric Exposure of the Intertidal Zone with Sentinel-1 CSAR in Northern Norway
## 5 Conclusions  
This paper presents a new approach to map atmospheric exposure in the ITZ purely on the basis of a statistical analysis of S1 radar imagery without the need for tidal models or tidal gauge observations. The method was developed and applied in Northern Norway with near-daily satellite coverage. Specific tidal reference levels of mean low and high water at near, mid, and spring tides are directly linked to specific percentages P of atmospheric exposure. These can be extracted as water lines by thresholding VV and VH polarized SAR images that represent the \"100-P\"th percentiles of S1 backscatter time series. The tidal reference levels MLWS, MLWN, MSL, MHWN, and MHWS correspond to the percentages of atmospheric exposure of 5%, 25%, 50%, 75%, and 95% and inversely to the water lines extracted from the 95 th, 75 th, 50 th, 25 th, and fifth backscatter percentiles. These constitute the class limits of the final atmospheric exposure map product. Areas of potential extreme tides below MLWS and above MHWS with atmospheric exposures <5% and >95% are also classified but with much lower accuracies. Currently, S1 is the only radar satellite constellation providing such temporally consistent dense time series that allow such a statistical approach to sample the whole tidal cycle. Other more sporadically acquiring satellites, specifically, optical satellites, need to have their observations associated with tidal height in situ data. As S1 is freely available and acquires data consistently worldwide with at least one acquisition every 12 days, the method should be transferable to any region as long as the number of acquisitions allows a statistically significant analysis. As the method is based on extracting specific backscatterpercentiles, we recommend time series of at least 100 observations. Our results were based on 240 acquisitions collected over two summer periods, June-November of 2018 and 2019, resulting in accuracies when mapping low water lines at neap, mid, and spring tide of 93%, 84%, and 64%, respectively. Higher elevated tidal reference levels, i.e., MSL and mean high water levels, were always correctly mapped as part of the ITZ area. The method has been applied to map the ITZ atmospheric exposure along the whole Norwegian coast and combined with S2-based ITZ type classifications [37] in order to improve the overall accuracy of mapping the ITZ area [49]. These three ITZ map products (atmospheric exposure, type [37], and area) will provide the most consistent ITZ dataset currently available for Norway. The products can be used in the identification and delineation of important ITZ areas and the official land border by extraction of the MSL water line. The bathymetry in the ITZ can be derived from the atmospheric exposure and the locally known tidal ranges. In addition. High-biodiversity areas can be associated with large areas of ITZs. In southern Norway, where the tidal range is relatively low and ship traffic appears to be more often misclassified as ITZ, the product might be less accurate than our accuracy assessment in Tromse Municipality suggests. Conceptualization, methodology, formal analysis, writing--original draft preparation, and project administration, J.H. ; validation, investigation, visualization, and funding acquisition, J.H. and C.D. ; writing--review and editing, C.D. All authors read and agreed to the published version of the manuscript. This research was funded by the Norwegian Environment Agency as part of the project \"Satellite-Based National Intertidal-Zone Mapping of Continental Norway with Sentinel-1&2\" under agreement No. 19087497. Not applicable. Not applicable. Not applicable.