Download - GLIMS: Global Land Ice Measurements from Space

4.1. ALGORITHMS OF THE GLIMS - PROJECT 33dicult to delineate.4.1.3 Spectral RatioSnow and ice have high reectance in the VNIR spectrum whereas at λ > 1.5µmthey have very low reection. So image ratioing of two spectral bands with dierentalbedo for snow and ice enables segmentation of glaciers according to spectralproperties.The ratio of Landsat 5 TM band4/band5 reveals good results for glacier segmentation.For ASTER images the ratio to enhance glacier properties is band3 to band4.Values of planetary albedo for the Landsat 5 TM ratio image and the ASTER imagehave been calculated using following equations:r p = πL max,λd 2E sun,λ cosθ s(4.1)r p,ratio = L max,V NIRE sun,SW IRL max,SW IR E sun,V NIR(4.2)Equation (4.1) is the equation for planetary albedo, equation (4.2) is the equationfor the ratio images. L max,λ is the spectral radiance in (W/m 2 sr 1 µm 1 ), E sun,λis the spectral irradiance. d 2 is the Sun - Earth distance in AU and cosθ s is the solarzenith angle. Planetary albedo of the Landsat 5 TM image ration is r p,T M = 1.59.Planetary albedo of the ASTER image with low gain settings in the VNIR channelsis r p,AST ER = 1.1.The results from both satellites are quite similar, both ratios enhance the spectralproperties of snow and ice but have diculties with the debris covered parts of theglaciers. Glaciers and snow have high DN's in the ratio images, terrain has very lowDN's in the ratio images. Another diculty is caused by dark lakes which mightbe classied as glaciers in the Landsat 5 TM band 5 and in the ASTER band 4.Figure 4.3a shows the ASTER ratio of the bands 3 and 4, gure 4.3b shows theLandsat 5 TM ratio of the channels 4 and 5. Diculties with the debris coveredparts and the dierence in resolution between both satellites are evident. It alsoshows the problem with shadows in the Landsat 5 TM image e.g in the upper partof the Alpeiner Ferner.To remove isolated pixels and gaps and to smoothen the glacier mask obtained fromthe ratio image, a 3x3 median lter was applied (see (Paul, 2003)). Figure 4.2c andgure 4.2d show the ltered ratio image of the ASTER and the Landsat 5 TM ratio