## 2 Materials and Methods  
### Study Areas  
This study encompasses two different areas to explore the use of VHR remote sensing data to examine glacier behavior. A part of the Otz valley in the Austrian Alps, in which substantial glaciological and environmental field data collected over the last six decades is available, as well as high-resolution remotely sensed data from airborne platforms, and the Khumbu Himal in the Himalayas of Nepal, for which field data is sparse and glacier behavior not well documented.  
The study site in the Alps offers a wide array of available data, which is used to investigate the quality of our results. The high Himalaya provides more interesting challenges and unique opportunities to test and apply new area-wide remote sensing tools, such as the Pleiades twins, in order to improve the knowledge and understanding of the mountain cryosphere of the Himalaya.  
The study area in the European Alps encompasses the Hochjochferner in the upper part of the Rofen valley, which is a side valley of the upper Otz valley (see Figure 1). Hochjochferner is a relatively low-angled, predominantly clean-ice glacier that since the Little Ice Age has disintegrated into four separate glaciers, one of which was investigated in this study. In recent years, temperatures in the area have been above the long-term climatological mean [24], and Hochjochferner has been experiencing significant losses in area and mass during this time [25, 26]. Hochjochferner is investigated to compare the DEM calculated on the basis of Pleiades data (PLE-DEM) to a high-accuracy DEM based on airborne light detection and ranging (LiDAR) data (ALS-DEM), which is available for the area (see Section 3.1).  
In the Khumbu Himal of Nepal, a variety of glacier sizes, types, dynamics, topography, and debris cover is present (Figure 2). The wide altitude range and the variability in debris cover make the Himalayan glaciers particularly sensitive to climate forcing [27, 28]. Most importantly, field-based measurements of mass balance in the Himalaya are sparse, and Himalayan glaciers are conspicuously absent from global mass balance records [29]. About 60% of the glaciated area investigated in this study  
Figure 1: The study area in the Austrian Alps. One is the main part of Hochjochferner, which is investigated in this study. UTM 32 N.  
is debris covered, and, in the absence of in situ measurements, remotely sensed geodetic mass balances based on DEM differencing offers the best means of measuring current change in these glaciers. The role of the different processes contributing to the mass change of debris-covered Himalayan glaciers is still under investigation [30; 31; 32].  
In particular, avalanched snow is thought to contribute significantly to glacier accumulation, and rapid melting at exposed ice faces within the debris-covered zone have been shown to significantly influence the total glacier ablation [31], but both of these processes remain poorly quantified at the glacier scale. Here, we investigate the capabilities of the DEM calculated from Pleiades imagery to address those questions by observing the sub-annual and inter-annual surface changes of five glaciers: the Ngozumpa, Khumbu, Nuptse, Lhotese Nup, and Lhotes glaciers. They all flow southward, and are heavily debris-covered in the lower parts. While Nuptse, Lhotese Nup, and Lhotese are of similar dimensions, with similar lengths and terminate roughly at the same elevation, the Ngozumpa glacier, which originates on the summit regions of Cho Oyu with elevations of above 8000 m, and the Khumbu glacier are significantly larger and more complex, dendritic glaciers.