# 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  
As stated earlier, five field sites were visited on seven occasions. Ground photos were taken for type description, and GPS tracks of the water line were collected using the GPS of mobile smartphones or a handheld GPS. On one occasion, an aerial mosaic was collected using a DJI Mavic Air drone. * _Langnes_  
Langnes, close to Tromse Airport on Tromsey (Island of Tromse), was visited on 8 May 2020. This region is quite variable and includes mudflats with and without vegetation (kelp and algae), stone beach with and without kelp and algae, rocks with and without vegetation (kelp and algae), and a manmade stone dam. An adjacent small grassland area (coastal meadows) can be flooded at high spring tides. The field visit lasted half a tidal cycle from high to low tides from 3:00 p.m. to 9:00 p.m. on 8 May 2020, an above-average spring-tide day, during which the position of the water line at different tidal stages was collected nearly hourly by mobile-phone GPS while walking along the water. The water line tracks were thereafter associated with the correspondent reference tidal levels (Figure 8). Slippery rocks and stones because of kelp and algae were very challenging to walk on. It was also clear that floating kelp in low waters challenged the definition of the water line. Some water line positions could, therefore, vary by up to about 10 m inaccuracy. The same was valid for mudflats where small sand hills from invertebrates extended above shallow water. Photos taken during the fieldwork (Figure 9) illustrate some of these issues. The choice of where the \"exact\" water line was under these conditions was, therefore, also subject to the interpretation of the observer and could be associated with an error of up to 10 m.
* _Hillessy_  
Avlosbukta at Hillesoy was visited on two occasions, 6 June and 14 July 2020. On 6 June 2020, the site was observed over a half tidal cycle from 8:20 p.m. to 2:20 a.m. (7 June) by taking GPS tracks of the water line around the times of the predicted tidal reference levels of the lowest tide (LTL), MLWS, MLW, MLWN, MSL, MHWN, MHW, and MHWS from [12], as summarized in Table 4. On 14 July, the site was revisited taking one GPS track at 3:10 p.m. during low tide. At the same time, aerial photos were collected using a small drone, a DJI Mavic Air quadcopter, which were then stitched together into an aerial photo mosaic using Pix4 Dmapper [46]. Two overlapping drone flights were done to cover the area. The first flight lasted from 2:53 p.m. to 3:04 p.m. and the second flight lasted from  
Figure 8: (**a–c**) Backscatter threshold contour lines (white) limiting the land mask in the fifth, 50 th, and 95 th percentile images, respectively, with the respective GPS tracks at tidal stages collected at Langnes (contains modified Copernicus Sentinel-1 data 2018–2019). (**d**) resulting atmospheric exposure map. 3:32 p.m. to 4:44 p.m. Low tide, at \(-61\) cm between MLWN and MLW, was at 2:40 p.m. on 14 July 2020. During the two flights, the tidal height varied from \(-59\) cm to \(-37\) cm around MLWN (\(-46\) cm). Figure 10 shows the aerial mosaic over Avlosbukta. Even on this very high-resolution aerial mosaic, it is difficult to see the water line, especially in the mudflat area in the norther part. Flying at higher altitude and using a better imaging sensor (e.g., thermal infrared) could improve the water line detectability.