# Evaluation of Satellite Imagery for Monitoring Pacific Walruses at a Large Coastal Haulout
## 3 Results  
We examined a total of 562 satellite images collected over our study area during 25 July-15 November 2018, 2019, and 2020 [31], among which 207 images had clear views (all 80 SAR images and 127 of the 482 optical images). Of the 207 clear view images, we obtained full-resolution digital data for 195 (DigitalGlobe = 3, LandSat-8 = 11, PlanetScope = 54, Sentinel-1 EW = 18, Sentinel-1 IW = 46, Sentinel-2 = 47, TerraSAR-X staring spotlight = 12, and TerraSAR-X stripmap = 4). Examples of each imagery type when walnves were present are shown in Figure 1. Most of the clear view optical images were obtained by PlanetScope and Sentinel-2. Al though both of these missions collected imagery every 1-2 days over the study area, the median clear view revisit rate for each mission was 3 days (Table 1), and with considerable variability (PlanetScope IQR, 4.7 day and Sentinel-2 IQR, 5.0 day) resulting in an irregular time series for both imagery types. Few clear view images were collected by the Landsat-8 mission due to the long 16-day revisit time and variable cloud conditions. During the 2020 haulout season PlanetScope browse images documented only 12 clear view days. Although we had not prearranged for image collections by DigitalGlobe, three clear view images were available in the archives: one each from GeoEye-1, WorldView-1, and WorldView-2. Regardless of the mission, no optical images were available after October 23, presumably because solar illumination failed to meet data collection or processing thresholds. Figure 1: Examples of the imagery types evaluated in this study for detecting Pacific walrus haulout aggregations on the beach near Point Lay Alaska. Each panel is labeled with the name of the imagery  
type that is used in subsequent figure legends. Each panel has an inset locator map on the right that includes a latitude/longitude gratictic to enable precise geographic location of the walrus aggregations. The left column includes optical imagery types displayed with a true-color RGB enhancement (red, green, and blue bands): (**A**) Landsat-8 (30 m resolution, 2019-08-17 22:31Z), (**B**) Sentinel-2 (10 m, 2018-09-07 23:05Z), (**C**) PlanetScope (4 m, 2018-09-07 22:08Z, ( PlanetLabs), (**D**) WorldView-2 (1.5 m, 2019-08-11 22:57Z, ( DigitalGlobe), and (**E**) unoccupied aerial system (UAS) imagery (2 cm, 2018-09-07 20:49Z, included for comparison with satellite imagery). The right column displays SAR images of normalized backscatter (sigma nought): (**F**) Sentinel-1 EW with VV polarization (40 m, 2020-09-15 17:39Z), (**G**) Sentinel-1 IW with HV polarization (10 m, 2018-09-08 17:41Z), (**H**) TerraSAR-X stripmap dual polarization (3.5 m, 2020-09-17 04:19Z, ( DLR 2020), and (**I**) TerraSAR-X staring spotlight with VV polarization (1.4 m, 2020-09-18 04:02Z, ( DLR 2020). SAR images \"F\", \"G\", and \"I\" display sigma nought across a pseudo-color ramp ranging from dark purple to light yellow, while image \"H\" is displayed as RGB pseudo-color with red = HH polarization, green = null, and blue = HV polarization. Archived Sentinel-1 SAR images of the study area were available during 2018, 2019 and 2020 in both the IW (n = 46) and the EW (n = 18) imaging modes [31]. Limited daylight in November did not impede the collection of Sentinel-1 imagery. Sentinel-1 IW images were available for Point Lay every 12 days in 2018 (collected by Sentinel-1A only), and in 2019 and 2020 (with collections by both Sentinel-1A and Sentinel-1B) with a revisit rate of roughly 10 days. Sentinel-1 EW images were only available in 2019 and 2020, with mostly a 12-day revisit interval, but occasionally a 4 or 8 day. For TerraSAR-X, we obtained data from two imaging modes, staring spotlight (n = 12) and dual-polarized stripmap (n = 4). Pooling the walrus detections across all image types conveyed information about the seasonal timing and within-season dynamics of occupancy at the Point Lay haulout (Figure 2).