Abstract
Mapping of glacier extents from automated classification of optical satellite images has become a major application of the freely available images from Landsat. A widely applied method is based on segmented ratio images from a red and shortwave infrared band. With the now available data from Sentinel-2 (S2) and Landsat 8 (L8) there is high potential to further extend the existing time series (starting with Landsat 4/5 in 1982) and to considerably improve over previous capabilities, thanks to increased spatial resolution and dynamic range, a wider swath width and more frequent coverage. Here, we test and compare a variety of previously used methods to map glacier extents from S2 and L8, and investigate the mapping of snow facies with S2 using top of atmosphere reflectance. Our results confirm that the band ratio method works well with S2 and L8. The 15 m panchromatic band of L8 can be used instead of the red band, resulting in glacier extents similar to S2 (0.7% larger for 155 glaciers). On the other hand, extents derived from the 30 m bands are 4%–5% larger, indicating a more generous interpretation of mixed pixels. Mapping of snow cover with S2 provided accurate results, but the required topographic correction would benefit from a better orthorectification with a more precise DEM than currently used.
Highlights
Glacier mapping is a key application of optical satellite data and has been widely applied, in particular after the Landsat image archives were opened to the public and offering orthorectified products [1]
The panchromatic band with 15 m resolution listed in Table 2 along with the equivalent bands of the Sentinel-2A Multi Spectral Instrument (MSI) and for reference the is only available on ETM+ and Operational Land Imager (OLI), and ASTER does not have a band in the blue part of the spectrum
Assigning th1 to the threshold for the respective band ratio and th2 to the blue band threshold, we find that the values chosen for th1 have some impact on the mapped extent for snow and ice in direct sunlight (Figure 3), whereas th2 causes little or no change
Summary
Glacier mapping is a key application of optical satellite data and has been widely applied, in particular after the Landsat image archives were opened to the public and offering orthorectified (i.e., terrain corrected) products [1]. Glacier classification is based on the strong difference in spectral reflectance of snow and glacier ice in the visible and near infrared (VNIR) compared to the shortwave infrared (SWIR). By dividing the raw digital numbers (DNs) of a VNIR band by those from the SWIR, illumination effects due to topography are minimized and glaciers stand out bright against a dark background. The ratio image is transformed to a glacier map (clean ice only) and corresponding glacier outlines [5]. Simple band ratios have proven to be fast, Remote Sens. 2016, 8, 575; doi:10.3390/rs8070575 www.mdpi.com/journal/remotesensing
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