# Mapping Atmospheric Exposure of the Intertidal Zone with Sentinel-1 CSAR in Northern Norway
## 3 Results
### Accuracy Assessment  
Ground-truth data for validation were based on GPS tracks collected by walking along the water at given tidal reference levels. The GPS accuracy was estimated to be better than 5 m. However, as stated above, the interpretation of the water line in a diffuse shallow water area and mudflat can vary on the order of tens of meters. An ITZ definition of being under water at high tide and above water at low tide is obviously ambiguous in such cases. In addition, the ITZ is a dynamic environment with incoming and outgoing water. In shallow areas and mudflats, the water line can change its position with a speed of several cm\(\cdot\)s\({}^{-1}\), fastest at mid tide. Therefore, the confusion matrix (Table 5) has to be considered with care. The different tidal reference levels, except MLW and MHW, were between the border of the defined atmospheric exposure classes and could, therefore, be attributed to the two bordering classes. The table cells in gray in Table 5 are, therefore, the results that we considered a correct classification of the tidal level lines. This corresponds to an overall  
Figure 12: Percentage of atmospheric exposure results over (**a**) the whole Tromso Municipality and (**b**) the areas of Langnes and Lille Grindoy (red rectangle in (**a**)). Errors in misclassification from ship traffic signs or ship traffic (A), radar side lobes reflection of steep slopes in east–west-directed fjords (B), and fish farms (C) are indicated. accuracy of 49%. If we consider only three classes, low-tide (below MLWN) areas, mid-tide areas, and high-tide (above MHWN) areas, i.e., if we considered the data inside the red rectangles in Table 5 as correctly classified into these classes, an overall accuracy of 74% was achieved. Figure 14: Different tidal water level lines superimposed on the ITZ atmospheric exposure product in Lille Grindoy, Troms. Figure 13: Different tidal water level lines superimposed on the (**left**) ITZ atmospheric exposure product and (**right**) aerial mosaic (© norgeibilder.no, Kartverket, Norge digitalt, Terratec AS/Blom, Norway AS) at Langes, Troms, Troms. Concerning the ITZ area accuracy, the method detected all intertidal areas above the MSL with a 99-100% accuracy, i.e., only pixels classified as water were wrongly detected. With decreasing tidal level, the water lines at MLWN, MLW, MLWS, and LWL were mapped as part of the ITZ area with accuracies of 93%, 84%, 64%, and 47%, respectively. If we disregard the lowest water lines observed during fieldwork, which we know were well below MLWS, the overall accuracy of mapping the area of the ITZ, using a stratified sampling design, was the average of the individual ITZ area accuracies for each class, i.e., 91%, which corresponds well to the accuracies reported by [27]. Figure 15: (**left**) GPS tracks of water lines at approximative reference tidal stages superimposed on the ITZ atmospheric exposure product in Avlosbukta at Hillesoy, Troms. The arrow indicates the position of the ground photo taken on 6 June 2020 of a water-saturated mudflat area, showing the difficulty in defining a clear water line in such areas.