ABSTRACT. Estimates of glacier mass balance using geodetic methods can differ significantly from estimates using direct glaciological field-based measurements. To determine if such differences are real or methodological, there is a need to improve uncertainty estimates in both methods. In this paper, we focus on the uncertainty of geodetic methods and describe a geostatistical technique that takes into account the spatial correlation of the elevation differences when calculating spatially averaged elevation changes. We apply this method to the western Svartisen ice cap, Norway, using elevation differences from the surrounding bedrock derived from stereophotogrammetry. We show that the uncertainty is not only dependent on the standard error of the individual elevation differences but is also dependent on the size of the averaging area and the scale of the spatial correlation. To assess if the geostatistical analysis made over bedrock is applicable to glacier surfaces, we use concurrent photogrammetrical and laser scanning data from bedrock and a range of glacier surfaces to evaluate the dependency of the geostatistical analysis on the surface type. The estimated geodetic mass balance, and its uncertainty, is -2.6 AE 0.9 m w.e. for the period 1968-85, and -2.0 AE 2.2 m w.e. for 1985-2002.
Abstract.We investigate the velocity field of the Larsen C ice shelf, Antarctic Peninsula, over the periods 2002-2006 and 2006-2009 based on repeat optical satellite data. The velocity field of the entire ice shelf is measured using repeat low resolution MODIS data (250 m spatial resolution). The measurements are validated for two ice shelf sections against repeat medium resolution Landsat 7 ETM+ pan data (15 m spatial resolution). Horizontal surface velocities are obtained through image matching using both orientation correlation operated in the frequency domain and normalized crosscorrelation operated in the spatial domain, and the two methods compared. The uncertainty in the displacement measurements turns out to be about one fourth of the pixel size for the MODIS derived data, and about one pixel for the Landsat derived data. The difference between MODIS and Landsat based speeds is −15.4 m a −1 and 13.0 m a −1 , respectively, for the first period for the two different validation sections on the ice shelf, and −26.7 m a −1 and 27.9 m a −1 for the second period for the same sections. This leads us to conclude that repeat MODIS images are well suited to measure ice shelf velocity fields and monitor their changes over time. Orientation correlation seems better suited for this purpose because it produces fewer mismatches, is able to match images with regular noise and data voids, and is faster. Since it can match images with regular data voids it is possible to match Landsat 7 ETM+ images even after the 2003 failure of the Scan Line Corrector (SLC off) that leaves significant image stripes with no data. Image matching based on the original 12-bit radiometric resolution MODIS data produced slightly better results than using the 8-bit version of the same images. Streamline interpolation from the obtained surface velocity field on Larsen C indicates ice travel times of up to 450 to 550 years between the inland boundary and the ice shelf edge. In a Correspondence to: T. Haug (torborg.haug@geo.uio.no) second step of the study we test our method successfully on 10 other ice shelves around Antarctica demonstrating that the approach presented could in fact be used for large scale monitoring of ice shelf dynamics.
The geodetic mass balance of the western Svartisen ice cap in northern Norway is determined, in this work, from photogrammetry on vertical aerial photographs taken in 1968, 1985 and 2002. The existing 1968 digital terrain model (DTM) was generated using analogue photogrammetry, and the 1985 and 2002 DTMs are newly generated using digital photogrammetry. The geodetic mass balance for 1968–85 is –2.6±0.8mw.e., and for 1985–2002 it is –2.0±1.6mw.e. The area of western Svartisen decreased from 190 km2 in 1968, to 187 km2 in 1985 and to 184 km2 in 2002. The outlet glacier Flatisen in the southeast retreated 1700 m over the two periods. The geodetic mass balance is also determined for Engabreen drainage basin, as –2.1±0.9mw.e. for the first period, and –0.3±2.4mw.e. for the second. The results for Engabreen are compared to traditional mass balances, and the large deviations cannot be explained from uncertainties determined for the geodetic method. The assessed errors contributing to the uncertainty in the geodetic mass balance are elevation errors, uncertainties from the applied melt correction, and the use of Sorge’s law, assuming constant snow thickness and density.
Abstract. We investigate the velocity field of the Larsen C ice shelf, Antarctic Peninsula, over the periods 2002–2006 and 2006–2009 based on repeat optical satellite data. The velocity field of the entire ice shelf is measured using repeat low resolution MODIS data (250 m spatial resolution). The measurements are validated for two ice shelf sections against repeat medium resolution Landsat 7 ETM+ pan data (15 m spatial resolution). Horizontal surface velocities are obtained through image matching in both frequency and spatial domain, and the two methods compared. The uncertainty in the displacement measurements turns out to be less than 70 m for the MODIS derived data, and less than 15 m for the Landsat derived ones. The difference between MODIS and Landsat based speeds is −15.4 m a−1 and 13.0 m a−1, respectively, for the first period for the two different validation sections on the ice shelf, and −26.7 m a−1 and 27.9 m a−1 for the second period for the same sections. This leads us to conclude that repeat MODIS images are well suited to measure ice shelf velocity fields and monitor their changes over time. The frequency domain image correlation method seems better suited for this purpose because it is faster, produces fewer mismatches, and is able to match images with regular noise and data voids. The latter makes it possible to match Landsat 7 ETM+ images even after the 2003 failure of the Scan Line Corrector (SLC off) that leaves significant image sections with no data. Image matching based on the original 12-bit radiometric resolution MODIS data produced slightly better results than using the 8-bit version of the same images. Streamline interpolation from the obtained surface velocity field on Larsen C indicates ice travel times of up to 450 to 550 a between the inland boundary and the ice shelf edge. In a second step of the study we test our method successfully on 10 other ice shelves around Antarctica demonstrating that the approach presented could in fact be used for large scale monitoring of ice shelf dynamics.
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