The new Swiss Glacier Inventory SGI2010: relevance of using high-resolution source data in areas dominated by very small glaciers

Observed fluctuations of mountain glaciers are known to be among the best indicators of climate change (e.g., IPCC, 2013). Continuous glacier monitoring is thus crucial and must include the repeated creation of glacier inventories (Haeberli, 2004) in order to understand observed glacier changes and project their future evolution (e.g., Hoelzle et al., 2007; Abermann et al., 2009; Huss, 2012). Furthermore, updated inventories are essential for assessing climate change impacts on future runoff in glacierized catchments (e.g., Huss, 2011; Sorg et al., 2012). For the European Alps, mapped glacier outlines are stored in regional or national inventories. They are available for different acquisition dates and result from different methods (automatic or semi-automatic approaches, manual digitization) and data sources (satellite data, aerial photography, laser scanning, topographic maps) (e.g., Paul et al., 2002; Lambrecht and Kuhn, 2007; Knoll and Kerschner, 2009; Abermann et al., 2010; Carturan et al., 2013). Based on a set of Landsat Thematic Mapper (TM) scenes (30 m resolution) acquired in autumn 2003 and a semi-automated approach, Paul et al. (2011) were the first to map the extents of glaciers in the entire European Alps spatio-temporally consistently. For the Swiss Alps, the latest glacier outlines prior to the inventory presented here originate from that study. The rapid mass loss and shrinkage of Alpine glaciers observed during the past years (e.g., Zemp et al., 2008, 2012, 2013) requires more frequent updates of glacier inventories (every 5-10 years; Paul et al., 2007). The 2003 outlines are thus already outdated for the Swiss Alps. Moreover, the spatial resolution of the source data used for creation of the 2003 inventory might be too poor for the assessment of specific questions on smaller scales. Using mediumresolution (e.g., Landsat, ASTER) satellite imagery has indeed become the state-of-the-art for creating new glacier inventories for remote areas (cf. Raup et al., 2007; Bolch et al., 2010; Andreassen and Winsvold, 2012; Frey et al., 2012; Falaschi et al., 2013; Nuth et al., 2013). If medium-resolution source data are used, however, the accuracy of such methods is rather limited for investigating changes in very small glaciers (Paul et al., 2013), here defined according to Huss (2010) as being smaller than 0.5 km. This is problematic because very small glaciers are numerous in most glacierized areas around the world and can even play a role for the accuracy of global glacier change assessments (Bahr and Radić, 2012). Hence, trying to understand the peculiarities and the response of glaciers in the Swiss Alps to climate change asks for as accurate as possible glacier outlines. Using high-resolution airor spaceborne imagery helps to overcome the potential shortcomings of medium-resolution source data for the delineation of glacier boundaries (cf. DeBeer and Sharp, 2009). Therefore, the 50/25 cm resolution aerial orthoimagery available for the entire Swiss Alps since the late 1990s from the Swiss Federal Office of Topography (swisstopo) is highly valuable source data for glacier mapping. In this article we present the new Swiss Glacier Inventory (SGI2010) derived from manual delineation of glacier outlines based on aerial orthoimagery acquired between 2008 and 2011. The source data used for its creation and the chosen methodological approaches are described in detail. Further, some selected aspects of glacier change assessment are shown. The accuracy of our inventory is evaluated by comparing glacier outlines for clean, snowand/or debris-covered glacier boundaries from multiple digitization by different experts. Glacier outlines for the eastern Swiss Abstract