## 3 Materials and Methods
### Data Processing
#### 3.4.1 Snow Imagery Pre-Processing  
Manual image pre-selection was done to remove blurred images. These images are often the result of wind-driven instabilities causing abrupt maneuvers or sudden changes in UAS altitude. The aim of the pre-processing phase was to explore the potential of image manipulation on the SDEM generation process and to enhance the photogrammetric reconstruction performance over typically featureless and homogenous snow surfaces. Pre-processing approaches for SfM have proven successful for improved 3D model reconstructions in other disciplines [31]. Another aim was to assess and correct the effects of systematic errors arising from DEM generation with radial and/or wide angle acquired imagery, in particular in this application where the subtraction of two UAS generated DEMs could introduce undesired effects [33]. The pre-processing solution was tested and implemented in two steps, (1) image content enhancement and (2) radial lens distortion correction. 1. **Content enhancement. ** Image contrast, defined as the difference in luminance (light) intensity between an image's object and background [36], is central to feature tracking algorithms. This is because these algorithms extract features from the pixel radiometric intensity levels in an image area and compare them with adjacent ones [37]. An image histogram is a graphical representation of the intensity distribution in a digital image and plots the number of pixels per intensity value. A narrow histogram means that most pixels have similar intensities. For images where snow is the dominant element (around 95% of these Arctic winter datasets), the histogram is usually thin and bell-shaped because of the relatively narrow range of intensities. The aim was to set the histogram as centered as possible with an initial proper exposure correction. This allows the application of a high local contrast enhancement to the mid-tones of the image, which increases the detail in the snow textures while conserving the global contrast in the image, thus safeguardinglarge-scale shadows and highlighting details. In other words, the process increases the contrast without reducing the dynamic range (defined as the range of light intensities from the darkest shadows to the brightest highlights). This results in a wider image histogram compared to the non-processed one and plays a similar role as traditional histogram stretching and equalization methods for enhancing features in image data [38]. Centering and spreading the histogram along the whole dynamic range means that more nuances in intensity values will be available for pattern recognition, improving the performance of the SfM framework. Traditional edge sharpening is also applied to enhance small-scale sastrugi and snow ripples. However, both contrast enhancement and sharpening can introduce undesired digital noise [39], so the effect of this noise on the reconstructed surfaces was carefully evaluated. The process was applied in batch mode for each area (characterized by the same camera settings and luminance). 2. **Radial lens distortion correction. ** The geometry of the camera lens accounts for some degree of distortion in the images [36; 40]. Radial distortion particularly affects the geometry of the image and is accentuated as the focal length is reduced, thereafter having a direct impact in SfM reconstructions and generating non-linear deformations in the 3D models if the FOV angle is wide [40]. Recent studies show that systematic errors in topographic models derived from UAS-borne SfM surveys might arise in the photogrammetric reconstruction due to a combination of the near-parallel imaging collection pathways taken in traditional UAS mapping surveys, and an inaccurate correction of radial lens distortion [33]. A solution to this issue was investigated by testing two different approaches; the use of the freely available Agisoft Lens and the addition'striped' oblique imagery over a small snow-covered surface generated from an imagery sub-set.