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Mishra V.D.,Snow and Avalanche Study Establishment SASE | Sharma J.K.,Rayat Institute of Engineering and Information Technology | Khanna R.,Thapar University
Annals of Glaciology | Year: 2010

The topographic effects of differential terrain illumination in optical satellite imagery of rugged mountainous regions have serious consequences for qualitative and quantitative analysis for various snow applications. Therefore, effective removal or minimization of topographic effects is necessary in satellite image data of mountainous regions. Different methods for topographic corrections, including C-correction, Minnaert corrections (including slope) and slope-matching method, are analysed in the context of snow reflectance. Combination of dark-object subtraction models DOS1 and DOS3 is used for image-based atmospheric corrections while considering the effect of Rayleigh scattering on the transmissivity in different spectral bands of AWiFS and MODIS image data. The performance of different models is evaluated using (1) visual analysis, (2) change in snow reflectance on sunny and shady slopes after the corrections, (3) validation with in situ observations and (4) graphical analysis. The results show that the slope-matching technique could eliminate most of the shadowing effects in Himalayan rugged terrain and correctly estimate snow reflectance from AWiFS and MODIS imagery. The validation of results with in situ observations for both types of imagery suggests that all other methods significantly underestimate reflectance values after the corrections.

Sharma J.K.,Rayat Institute of Engineering and Information Technology | Mishra V.D.,Snow and Avalanche Study Establishment SASE | Khanna R.,Thapar University
Journal of the Indian Society of Remote Sensing | Year: 2013

The present paper discusses the impact of topography on accuracy for land cover classification and "from-to class change using improved spectral change vector analysis suggested by Chen et al. (2003). Two AWiFS sensor images of different dates are used. Double Window Flexible Pace Search (DFPS) is used to estimate threshold of change magnitude for change/no change classes. The topographic corrections show accuracy of 90% (Kappa coefficient 0.7811) for change/no change area as compared to 82% (Kappa coefficient 0.6512) in uncorrected satellite data. Direction cosines of change vector for determining change direction in n-dimensional spectral space is used for image classification with a minimum distance categorizing technique. The results of change detection are compared (i) Improved CVA with conventional two bands CVA and (ii) Improved CVA before and after topographic corrections. The improved CVA with topographic correction consideration using slope match show maximum accuracy of 90% (Kappa coefficient 0.83) as compared to conventional CVA which show maximum accuracy of 82% (Kappa coefficient 0.6624). The overall accuracy of "from- to class using improved CVA increases from 86% (Kappa coefficient 0.7817) to 90% (Kappa coefficient 0.83) after topographic corrections. The improved CVA with proper topographic corrections is found to be effective for change detection analysis in the rugged Western Himalayan terrain. © 2012 Indian Society of Remote Sensing.

Gusain H.S.,Snow and Avalanche Study Establishment SASE | Mishra V.D.,Snow and Avalanche Study Establishment SASE | Arora M.K.,Indian Institute of Technology Roorkee
Remote Sensing Letters | Year: 2014

The aim of this letter is to estimate incoming and net shortwave radiation fluxes of large snow-covered area of western Himalaya and to evaluate the results with in situ observations. Radiation fluxes are estimated at spatial levels using remote sensing data from Moderate Resolution Imaging Spectroradiometer (MODIS) sensor and Shuttle Radar Topographic Mission (SRTM) digital elevation model (DEM), supplemented with sparse field data obtained from automatic weather stations (AWSs). Snow cover albedo has been estimated from MODIS data using narrowband to broadband conversion method for clear sky days. Geospatial maps of air temperature (Ta) and relative humidity (RH) have been generated for the study area using AWS recorded Ta/RH and DEM. Parameterization techniques have been used for estimating incoming and net shortwave radiation fluxes, which have been validated from in situ AWS observations. The root mean square error (RMSE) in estimation of incoming shortwave radiation flux and net shortwave radiation flux has been found to be 75 and 84.9 W m-2, respectively. Further, the higher radiation fluxes have been observed on south aspect slopes than those observed on north aspect slopes. © 2013 Taylor & Francis.

Datt P.,Snow and Avalanche Study Establishment SASE | Kapil J.C.,Snow and Avalanche Study Establishment SASE | Kumar A.,National Institute of Technology Kurukshetra
Cold Regions Science and Technology | Year: 2015

The damage process within snow, a porous sintered multiphase material, results in the emission of weak acoustic signatures. It is also responsible for the failure of weak snow layers on mountain slopes leading to snow slab avalanches release. Monitoring these acoustic emissions and their characterization is useful to understand the complex damage behaviour of snow and also for identifying the relevant parameters associated with in situ snowpack stability assessment. In this paper, we present the outcomes of an acoustic emission (AE) study for the damage analysis of snow subjected to uni-axial compression under controlled laboratory conditions. The distribution of various AE characteristics of snow such as peak amplitudes, hit duration, AE energy and emitted acoustic frequency spectrum corresponding to different displacement rates was analysed. The amplitude, hit duration and AE energy substantially increased with increasing displacement rate. Furthermore, the b-value for different displacement rates was estimated using least square fittings as well as maximum likelihood estimates. The temporal variation of the b-value was also estimated for particular time windows of the time series of AE data for all three displacement rates. The b-values were correlated with the external mechanical loading of the snow and were observed to vary from 3.6 to 2.3 for the displacement rates from 1. mm/min to 10. mm/min. Furthermore, the AE hit duration was found to be an important indicator of the damage behaviour. The AE energy of snow increased for decreasing grain size. Analysing the waveform of the AE signatures revealed that the emitted frequency spectrum was between 30. kHz and 70. kHz. The outcome of this study may be useful to identify the relevant AE parameters associated with evolving damage behaviour of the snowpack and for AE applications towards in-situ monitoring of snowpack stability and subsequent avalanche release. © 2015 Elsevier B.V.

Kapil J.C.,Snow and Avalanche Study Establishment SASE | Kumar A.,Ranchi College | Negi P.S.,Snow and Avalanche Study Establishment SASE
Annals of Glaciology | Year: 2010

Under melt-freeze conditions crusts may evolve within a snowpack, which may favour avalanche initiation by forming a hard bed surface for weakly bonded faceted grains. We used a parallelprobe saturation profiler (PPSP) to record the distribution of water contents within the snowpack. Diurnal effects of melt-freeze action on the growth of crusts were monitored with the help of the PPSP device. Saturation profiles were collected from a partially wet snow cover. Snow stratigraphy was conducted manually in the morning, after overnight freezing, to identify the location and the granular compositions of the crusts that had evolved. A one-to-one correspondence between the saturation spikes collected using the PPSP and the actual positions of the crusts was established. The PPSP was also used to monitor three-dimensional variations in the maximum percolation depths within a south-facing snowpack. The operation of the PPSP is faster than existing dielectric measurement techniques, so it was applied to study the spatial variability of maximum percolation depths on the slopes of different aspects.

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