# Comprehensive Annual Ice Sheet Velocity Mapping Using Landsat-8, Sentinel-1, and RADARSAT-2 Data
## 2 Data and Methods
### Landsat-8  
Landsat-8 is an American Earth observation satellite launched on 11 February 2013 to continue the Landsat program started in 1972. The Operational Land Imager (OLI) on-board Landsat-8 acquiresimages in the visible and infrared parts of the spectrum with a revisit time of 16 days between 82.7\({}^{\circ}\) North and South latitude. Images are provided as geocoded products by USGS, either in UTM or Polar Stereographic coordinates. Optical imaging depends on the solar illumination, which results in large number of acquisitions during austral and boreal summers and few to none in winter.  
Here, we use the persistence of surface features to track ice displacements between two images over the ice sheets as shown in [20; 21]. In Antarctica, we systematically search for suitable pairs of images with revisit periods between 16 and 80 days with cloud cover less than 80%, and for 368-day pairs with cloud cover below 20%. In Greenland, we search for pairs between 16 and 64 days repeat cycle with cloud cover below 80%. We do not process images with more than 80% of cloud cover (or 20% cloud cover for 368-day pairs) to decrease the computational load and because small additional spatial coverage would be expected. Due to surface melt in large areas of Greenland in summer, pairs with longer time separation, such as 368-day, do not correlate well. To minimize stereoscopic effects [10], we only consider image pairs that have the same observation geometry, i.e., same path and row in the global notation system of Landsat data. The search is done automatically using the Python utility landsat-util ([[https://github.com/developmentseed/landsat-util](https://github.com/developmentseed/landsat-util)]([https://github.com/developmentseed/landsat-util](https://github.com/developmentseed/landsat-util))). Landsat-util is a open source command line utility to search, download Landsat imagery. We also use it to download the selected pairs from Google Storage\(\copyright\) or Amazon AWS\(\copyright\), which mirror the Landsat-8 archive from USGS, but provide faster download rates. For the shortest repeats (\(<\)32 days), we focus our processing on the paths and rows located along the coastline of the ice sheets, where ice is moving the fastest. For repeat passes between 48 and 80 days, we apply no spatial restriction and process all available pairs in Antarctica and Greenland. Finally, for the revisit time of 368 days, we focus our processing only on the Antarctic ice sheet interior where ice motion is slow.  
We use only the panchromatic channel (Band 8), which provides the highest resolution (15 \(\times\) 15 m) and covers a wavelength range between 0.5 and 0.68 \(\upmu\)m. A 3 \(\times\) 3 Sobel filter is applied in the x- and y-directions in order to enhance small scale features and attenuate albedo changes, e.g., different solar illumination between the scenes [23]. To assist offset tracking, we employ the a-priori displacements of ice from prior velocity mappings [2; 3]. This step makes it possible to reduce the search area for pixel offsets at each location to windows about 4 \(\times\) 4 pixels in size, hence considerably reduces the computational load. The offset (or pixel offset) maps are generated using the ampcor routine from ROI_PAC [24] using 32 \(\times\) 32 sub-images. For some glaciers, the correlation fails because the search window is too small compared to the range of motion between the reference image and the slave image. In these areas, we perform a second run using a larger search window.