Abstract
Abstract. Glacier facies are zones of snow on a glacier that have certain specific spectral characteristics that enable their characterization. The accuracy of their extraction will determine the end accuracy of the distributed mass balance model calibrated by this information. Therefore, coarse to medium resolution satellites are not preferable for this particular function as the data derived from such sensors will potentially blur out the minute spatial variations on the surface of a glacier. Very high resolution (VHR) sensors (such as, WorldView (WV)-1, 2, 3) are thus much more suited for this particular task. Hence, this study aims to extract the available glacier facies on the Sutri Dhaka glacier, Himalayas, using very high-resolution WorldView-2 (WV-2) imagery. Extensive pre-processing of the imagery was performed to prepare the data for this purpose. The steps incorporated for this purpose consist of 1) Data Calibration, 2) Mosaicking, 3) Pan Sharpening, 4) Generation of 3D surface, and 5) Digitization. Using image classification as the primary method of information extraction, this study tests the ever-popular pixel-based classification technique against the uprising object-based classification technique. In doing so, this study aims to determine the most accurate technique of information extraction for the WV-2 imagery in the given scenario. The presence of unique bands (Coastal (0.40–0.45 μm), Red Edge (0.705–0.745 μm), NIR-1 (0.770–0.895 μm) and NIR-2 (0.86–1.04 μm) in the multispectral range of WV-2, allows this study to perform facies classification through the development of customized spectral index ratios (SIRs) in the object-based domain. Establishment of thresholds was hence necessitated for information extraction through the developed SIRs. Three supervised classifiers, namely, a) Mahalanobis distance, b) Maximum likelihood, and c) Minimum distance to mean, were then used to perform classification, thereby allowing a comparative analysis between the classification schemes. Accuracy assessment for each classification scheme was performed using error matrices. The object-based approach achieved an overall accuracy of 90% (κ = 0.88) and the highest overall accuracy among the pixel-based classification methods is 78.57% (κ = 0.75). The results clearly portray that the object-based method delivered much higher accuracy than the pixel-based methods. The carry home message is that future studies must examine the transferability and accuracy of the customized SIRs in varying scenarios, as different scenarios will require varying threshold adjustments. Forthcoming studies can also develop sensor specific and unique indices for other sensors that are suitable for such applications.
Highlights
Glaciers consist of various zones that are primarily differentiated based on specific characteristics
Each facies extracted is unique to its threshold range. This implies that the spectral response pattern of each of the facies is most influenced by the utilization of these spectral index ratios (SIRs)
Classification, whether pixel or object based will be dependent on the precision of the spectral response pattern of the target
Summary
Glaciers consist of various zones that are primarily differentiated based on specific characteristics These characteristics are based on variables, which include, but are not limited to, moisture, texture, hardness, impurities, stratification, grain size, etc. These characteristics themselves are continuously transforming due to changing meteorological conditions (Benson, 1962). The line of transition between them is called the Equilibrium Line Altitude (ELA) These two zones are a concatenation of several small zones having discernible superficial expressions due to changing circumstances. These are referred to as glacier facies (Hall et al, 1988). It can be inferred that these superficial expressions (facies) are visual cues of the current health of the glacier (Jawak et al, 2018)
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