## Appendix B SfM Mapping with UAS Platforms and Considerations for Snow-Covered Arctic Locations  
A UAS SfM survey is carried out by flying over the interested area and acquiring a set of overlapping pictures with a camera. Mission planning is a critical phase for successful DEM generation. It is optimal to have at least a degree of forward overlap (along the flight line) of 80% and side overlap of 60% between the images acquired through the flight route [35] (Figure 11). Given that the UAS is a moving platform, the camera's time interval must be set to guarantee the image overlap. The image footprint represents the ground level area covered by the image and is an important factor to consider in order to plan the mission for optimal UAS SfM survey (Figure 11). The height and width of the photographic footprint can be calculated from the camera Field of View (FOV) and orientation, as seen in Figure 11, by using Equation (11):  
\[ \begin{split}& FOV_{width\ (height)}=2\times tan^{-1}\Big{(}\frac{x_{s}( y_{s})}{2\times f}\Big{)}\\ & Footprint\ Width=H_{f}\times\big{[}\tan(\alpha+0.5\times FOV _{width})-\tan(\alpha-0.5\times FOV_{width})\big{]}\\ & Footprint\ Height=H_{f}\times\big{[}\tan\Big{(}\beta+0.5\times FOV _{height}\Big{)}-\tan\Big{(}\beta-0.5\times FOV_{height}\Big{)}\big{]}\\ & Photographic\ Footprint\ from\ an\ UAS=Height\times Width \end{split}  \tag{12}\]The resolution and accuracy of a DEM generated through SfM methodology relies on the ability of the image to detect the smallest feature possible, in other words on the spatial resolution of the imaging system used. If a centimeter-precision DEM is desired, the spatial resolution should be of the same order. A good way to measure the spatial resolution is the ground sample distance (GSD), which is the distance between pixel centers measured on the ground; the smaller the GSD, the better the spatial resolution achieved, which leads to an increase in the number of visible details. Given a defined camera setting for image acquisition, the GSD is directly related to the UAV flight height and can be calculated from Equation (A2) [51]. GSD is the ground sample distance (cm pixel\({}^{-1}\)), Pixsize is the sensor pixel size (\(\mu\)m pixel\({}^{-1}\)), \(H_{f}\) is the flying altitude (m), and f is the camera focal length (mm); \(x_{s}\) is the width of the camera sensor (mm), \(y_{s}\) is the height of the camera sensor (mm), and \(\alpha\) and \(\beta\) are the gimbal angles from the x and y axis, respectively (degrees). \[GSD=\frac{Pix_{Size}\times H_{f}}{f}\] (A2)  
Overall, all mapping parameters need to be considered when performing a UAS photogrammetric survey; the ground area to be covered by the mapping, the GSD (resolution of the mapping), the platform flying speed, the time interval between each frame acquisition, the camera's internal and external parameters, and the flying altitude have to be calculated based on each scenario and desired outcomes. These photogrammetry elements of the survey have to meet the technical capabilities of the UAS itself. Different types of UASs and camera set-ups allow for a range of customization capabilities for mapping snow depth at the required scale. For example, in the case of multi-rotor UAVs, the maximum flying time is heavily dependent on the payload the vehicle is carrying, the type of batteries used, and the multi-rotor specifications. While detailing the technicalities of flying UAVs goes beyond the scope of this appendix, it can still be concluded that targeted mission planning is critical for these kinds of surveys. For UAS survey missions on snow it is important to recall the highly reflective nature of the snow surface. Therefore, special attention must be given to avoid overexposure of the images on clear sky days. This snow property is, however, taken as an advantage and allows a high shutter speed, which is extremely important to prevent blurred images caused by UAS movement and vibrations.