# Mapping Atmospheric Exposure of the Intertidal Zone with Sentinel-1 CSAR in Northern Norway
## 2 Materials and Methods
### Methods
#### 2.2.3. In Situ Data Collection
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Time**} & **Water Level (Pred.)** & **Water Level (Obs.)** & **Approx. Tidal** \\  & **(Ref. MSL)** & **(Ref. MSL)** & **Reference Level** \\ \hline
20:20–20:30 (6 June) & \([-111\) cm; \(-110\) cm] & \([-98\) cm; \(-96\) cm] & Minimum on 6 June \\
20:45–20:54 & \([-108\) cm; \(-106\) cm] & \([-95\) cm; \(-93\) cm] & MLWS \\
21:38–21:53 & \([-87\) cm; \(-76\) cm] & \([-74\) cm; \(-62\) cm] & MLW \\
22:12–22:19 & \([-65\) cm; \(-59\) cm] & \([-51\) cm; \(-45\) cm] & MLWN \\
23:09–23:27 & \([-18\) cm; \(-3\) cm] & \([-4\) cm; \(+11\) cm] & MSL \\
23:59–00:08 & \([+24\) cm; \(+31\) cm] & \([+39\) cm; \(+45\) cm] & MHWN \\
00:32–00:52 (7 June) & \([+48\) cm; \(+61\) cm] & \([+60\) cm; \(+75\) cm] & MHW \\
01:08–01:25 & \([+70\) cm; \(+78\) cm] & \([+85\) cm; \(+92\) cm] & MHWS \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Times and predicted and hindcast modeled tidal levels of GPS tracks in Avlosbukta (Hillesy) during fieldwork on 6–7 June 2020. Approximative tidal reference levels extracted from Kartverket [12]. Figure 9: Photos taken at Langnes showing the variability of intertidal zone type and challenges in accessing and precisely defining the water line: (**a**) coastal meadow, (**b**) stone beach, (**c**) stone dam, (**d**) stone beach covered by kelp and floating kelp, and (**e**) mudflat with some kelp. * _Lille Grinday_  
The ITZ at Lille Grindey was also visited on two occasions: 19 July 2019 and 25 July 2020. This is a nature reserve on Kvaloy, close to Tromso and part of the RAMSAR convention [47] site Balsfjord Wetland System [48], with extensive mud- and sandflats with coastal meadows. Low tide on 19 July 2019 was at 9:59 a.m., with a predicted water level of \(-113\) cm [12]; the site was visited only during low tide, from ca. 9:30 to 11:20 a.m., with an estimated tidal water level range between \(-113\) cm and \(-88\) cm, which is between MLWS (\(-111\) cm) and MLW (\(-84\) cm) [12]. During the field visit, ground photos with GPS coordinates were taken to identify different ecosystems. A GPS track was recorded walking along the water starting about 30 min before the lowest tide until about 1.5 h after the lowest tide. The track is shown as a dotted red line in Figure 11. On 25 July 2020, the mudflat area to the west of Lille Grindey was revisited, and the water line was mapped using a handheld GPS at seven stages during a half tidal cycle from low tide to high tide: the lowest water level on this day (LWL), MLWS, MLW, MLWN, MSL, MHWN, and MHW. These stages correspond to approximately 2%, 5%, 15%, 25%, 50%, 75%, and 85% atmospheric exposure. At each stage, the water line was tracked for about 20 min, from 10 min before to 10 min after the calculated time, to limit the change in tidal stage during the track. This tidal area was characterized by ca. 300 m wide mudflats with and without seaweed, permanent pools, rocky outcrops with salt meadows, large boulders, sandy beaches, and a rocky coastline. Figure 11 shows the collected GPS tracks superimposed on an aerial photo mosaic from [36]. The aerial photo also demonstrates the difficulty in defining the water line using aerial images alone. Shallow water would probably have been falsely classified as ITZ by most analysts. Figure 10: Aerial image mosaic taken with a DJI Mavic Air quadcopter over Avlosbukta at Hillesoy (Tromso Municipality, Norway) with GPS tracks of the water line at the approximative tidal reference level from 6 June. The orange line shows the GPS track taken at 3:10 p.m. on 14 July 2020 at a tidal level of \(-56\) cm between MLW and MLWN. The background is an aerial mosaic from 2017 from norgebilder.no (© Norge Digitalt, Kartverket, Blom Norway AS).