# Evaluation of Satellite Imagery for Monitoring Pacific Walruses at a Large Coastal Haulout
## 4 Discussion
### Optical Imagery  
Clouds were the most prevalent obstacle for detecting walruses with optical imagery at the Point Lay haulout during the haulout season. Although aggregations could sometimes be delineated through thin clouds using a near-infrared band, opaque cloud cover was common and often persistent. Even the PlanetScope constellation with near-daily revisitations was precluded from collecting clear view imagery for periods lasting upwards of ten days during each year. Cloud cover varied across the study years, but it is reasonable to expect that on-average roughly two-thirds of optical images at Point Lay could be contaminated by clouds, resulting in data gaps of variable duration (Table 1 and Figure 2). The Sentinel-2 and PlanetScope time series, taken together, had gaps lasting >10 days in both 2019 and 2020, and 2 such gaps in 2018. Periods of persistent clouds were also reflected in a 2005-2020 record of weather station data from the nearby Point Lay airport [33]. Annually during 25 July to 23 October, at times suitable for optical remote sensing (10:00-16:00 local), the fraction of hourly sky-condition records classified as being 'clear' or with 'few clouds' averaged 35% (sd 13%, n 14 years), consistent with the roughly two-thirds fraction of cloud-contaminated satellite images that we had obtained for 2018-2020. Nighttime also prevents use of optical imagery for detecting walruses, a limitation that rapidly establishes during autumn at high latitudes. For example, at Point Lay's latitude (~69.7\({}^{\circ}\)N) in mid-October, the maximum sun elevation is ~12\({}^{\circ}\), and by late-October it drops to <6\({}^{\circ}\)[34]. No Sentinel-2 or PlanetScope images of Point Lay were available in the archives after ~23 October, presumably due to insufficient solar illumination. Although walruses appeared to have vacated the haulout by mid-October during the years of our study (Figure 2), haulouts on the northern Russian coast of Chukotka are commonly used throughout November, including the largest known haulout at Cape Sertlse Kamen' [35], so objectives to monitor haulouts range-wide with optical sensors would face late-autumn limitations. Given previously published accounts of detecting walruses with ~1-m resolution satellite imagery [9; 10; 11; 12], it was not surprising that the DigitalGlobe image of our study area rendered very close observer agreement on both the location and area occupied by the large walrus aggregation present (Figure 4, [31]). Monitoring a walrus haulout with commercial high-resolution imagery, such as that collected by the DigitalGlobe satellites (currently Geoeye-1 and WorldView-1, 2, and 3), requires prearranged scheduling to ensure data collections. DigitalGlobe will also soon deploy \"WorldView Legion\", a constellation of six satellites (first launch planned for 2021) that will be capable of imaging most places in the world multiple times a day, and Planet Labs is now operating the SkySat constellation of 21 satellites capable of imaging targeted sites with 0.5 m resolution imagery collected up to 7 times daily [20]. In comparison, Planet Labs operationally collects and archives imagery worldwide every day with its PlanetScope constellation, and Sentinel-2 does the same every 2-3 days at Arctic latitudes, so prearranged acquisitions are not necessary and the broad swath width of these imagery types better enables detection of walrus aggregations that may sometimes move several kilometers within a haulout area. We found that both PlanetScope and Sentinel-2 imagery yielded reasonable agreement among observers on the location and area occupied by walruses (Figure 4). Although PlanetScope images have higher resolution (4 m) than Sentinel-2 (10 m), the latter's superior radiometric quality seemed to help compensate its coarser spatial resolution.