On the Automated Mapping of Snow Cover on Glaciers and Calculation of Snow Line Altitudes from Multi-Temporal Landsat Data

Philipp Rastner,Gabriele Schwaizer,Claudia Notarnicola,Rudolf Sailer,Lindsey Nicholson,Frank Paul,Rainer Prinz

doi:10.3390/rs11121410

Abstract

Mapping snow cover (SC) on glaciers at the end of the ablation period provides a possibility to rapidly obtain a proxy for their equilibrium line altitude (ELA) which in turn is a metric for the mass balance. Satellite determination of glacier snow cover, derived over large regions, can reveal its spatial variability and temporal trends. Accordingly, snow mapping on glaciers has been widely applied using several satellite sensors. However, as glacier ice originates from compressed snow, both have very similar spectral properties and standard methods to map snow struggle to distinguish snow on glaciers. Hence, most studies applied manual delineation of snow extent on glaciers. Here we present an automated tool, named ‘ASMAG’ (automated snow mapping on glaciers), to map SC on glaciers and derive the related snow line altitude (SLA) for individual glaciers using multi-temporal Landsat satellite imagery and a digital elevation model (DEM). The method has been developed using the example of the Ötztal Alps, where an evaluation of the method is possible using field-based observations of the annual equilibrium line altitude (ELA) and the accumulation area ratio (AAR) measured for three glaciers for more than 30 years. The tool automatically selects a threshold to map snow on glaciers and robustly calculates the SLA based on the frequency distribution of elevation bins with more than 50% SC. The accuracy of the SC mapping was about 90% and the SLA was determined successfully in 80% of all cases with a mean uncertainty of ±19 m. When cloud-free scenes close to the date of the highest snowline are available, a good to very good agreement of SC ratios (SCR)/SLA with field data of AAR/ELA are obtained, otherwise values are systematically higher/lower as useful images were often acquired too early in the summer season. However, glacier specific differences are still well captured. Snow mapping on glaciers is impeded by clouds and their shadows or when fresh snow is covering the glaciers, so that more frequent image acquisitions (as now provided by Sentinel-2) would improve results.

Highlights

Glacier mass balance is a key measure to determine the contribution of glaciers to regional hydrology [1,2] and global sea level [3,4,5]
As direct measurements reveal a high correlation of glacier mass balance with (a) the snow cover on a glacier and (b) the elevation of the snow line altitude (SLA), mapping snow cover (SC) on glaciers from satellite images offers a proxy for glacier mass balance (e.g., [12,13,14]), whereby the remotely sensed snow cover ratio (SCR) is taken as a proxy for the accumulation area ratio (AAR) [12,13,15] and the elevation of the snow line at the end of the ablation period [12,16] as a proxy for the equilibrium line altitude (ELA)
The comparison of the automatically- and manually-derived SLAs for the four test glaciers reveals a good agreement with a coefficient of determination (r2) > 0.8 for all test glaciers (Figure 5)

Summary

Introduction

Glacier mass balance is a key measure to determine the contribution of glaciers to regional hydrology [1,2] and global sea level [3,4,5]. As the number of glaciers with direct measurements of mass balance (using the glaciological method) is limited (e.g., [6]) and their representativeness for the larger mountain ranges often unknown [7], it is appealing to obtain glacier mass changes from remote sensing data. As direct measurements reveal a high correlation of glacier mass balance with (a) the snow cover on a glacier and (b) the elevation of the snow line altitude (SLA), mapping snow cover (SC) on glaciers from satellite images offers a proxy for glacier mass balance (e.g., [12,13,14]), whereby the remotely sensed snow cover ratio (SCR) is taken as a proxy for the accumulation area ratio (AAR) [12,13,15] and the elevation of the snow line at the end of the ablation period [12,16] as a proxy for the equilibrium line altitude (ELA). Mapping SC extent has been applied to reconstruct missing mass balance measurements [18] on the basis of manually selected satellite scenes coinciding with the end of the ablation season, revealing a high correlation between satellite-derived snowlines and ELAs measured in the field

Results

Discussion

Conclusion