## 3 Materials and Methods
### UAS Surveys  
The UAS surveys were performed in agreement with local regulations for each case. A brief overview of the regulations of flying UAS in the Arctic can be found in Appendix A. The initial aim was to acquire millimeter-scale resolution imagery so datasets can then be downscaled to the maximum practical point that yielded desirable spatial resolution from a snow variability context. The SDEM generation was divided into four chronological steps (Figure 3): (1) allocation of GCPs on the survey site, (2) UAS-borne image acquisition, (3) GNSS survey of the GCPs, and (4) manual snow depth probing. 1. **Allocation of GCPs. ** GCPs are required for georeferencing the reconstructed 3D model [22; 23]. The GCPs were distributed across the surveyed area as widely as possible [32]. All GCPs had a distinctive mark on their surface to precisely pinpoint the GCP on the imagery. They consisted of hand-made targets (circular plates with a centered red-cross) and snow-free features (e.g., overlying boulders, rocks, and ground patches) during winter. During summer, GCPs consisted of easily identifiable marked rocks. Features on snow-free areas, termed co-ground control points (GCPs), are selected in support of co-georeferencing the SDEMs with the TDEMs [26]. The number of GCPs and CGCPs for each area are listed in Table 1 and positions can be seen in Figure 1. GCPs were carefully deployed to keep the majority of the snow undisturbed. This limited the addition of unnatural features in the snow that could unfairly assist the SfM reconstruction [26]. 2. **UAS-borne image acquisition. ** With the aim of achieving millimeter-scale ground sample distance (GSD), the UAS flying parameters (altitude and speed) and the camera internal parameters (focal length and exposure) were set according to the local surveying characteristics (area extent and environment conditions shown in Table 2) using standard photogrammetric formulas (Appendix B). Millimeter-scale GSD is needed to ensure the downscaled (resampled) image resolution is sufficient to produce DEMs of the variable snow surfaces at the optimal scale. The guidelines and formulas for calculating typical UAS mapping parameters are found in Appendix B. The image acquisition frequency was set to 1 image per second. Once the cameras were configured for each surveyed area, the UAS was flown with a typical systematic mapping pattern with the camera directed orthogonal to the surface (Figure 2a and Appendix B). Additionally, as suggested by [33], a set of slightly oblique imagery is taken for each area by tilting the camera gimbals of ~20 degrees from vertical. With this setting, two additional flight transects were conducted for each area with the tilted camera pointing towards the center [33]. The ratio oblique/orthogonal imagery resulted to be around 0.2. Oblique imagery yields an increased ground footprint (Appendix B). Overall, this resulted in a high mean image overlap for the smaller areas mapped with the minimum set-up of Svall (>9 for SDEM), Svall2 (9) and Green1 (>9). Areas mapped with the advanced set-up resulted in a medium image overlap of Svall (8 for TDEM), Green2 (6), Green3 (8) and Green4 (7). The Nikon D3200 utilized the RAW-file format, i.e., non-compressed data from the camera sensor, which allowed a wider range of post-processing possibilities. The GoPro(r) Hero 3 compressed the images to JPEG-file format. 3. **GNSS survey of the GCPs. ** For the positioning of the GCPs and of a few additional snow depth samples (3 for area Sval2 and 1 for area Green4, taken post-survey), a static GNSS survey was carried out with a JAVAD Legacy receiver and a JAVAD RegAnt antenna (Figure 2b). The geometric center of the antenna was aligned with the center of the GCP marker (red cross midpoint). In cases of irregular topography, we took repeat measurements of the distance from the snow surface to the antenna measuring point and used the average for determining the antenna height. The static GNSS acquisition times ranged from 30 to 90 min with a 1 Hz frequency.