Jökull


Jökull - 01.01.2016, Síða 33

Jökull - 01.01.2016, Síða 33
First documented surge of Kverkjökull, central Iceland The 2011 ALS measurements were carried out on 10th September 2011. They have a mean point den- sity of 0.3 per m2 and were aggregated onto a 5 x 5 m regular grid based on the mean point density being slightly lower than that for the 2007 ALS data (Ice- landic Meteorological Office and Institute of Earth Sciences, University of Iceland, 2013: DEMs of Ice- landic glaciers; Jóhannesson et al., 2013). The abso- lute vertical accuracy of the 2011 DEM is estimated to be better than 0.5 m but the relative accuracy may be expected to be similar to the 2007 DEM. The 2011 DEM (Figure 2B) was corrected ver- tically for the geoid height using the NKG96 geoid model for Iceland which is practically identical to the more recent 2011 geoid model (National Land Survey of Iceland, 2016). In contrast, the absolute vertical po- sitioning of the 2007 DEM was derived on the basis of the calculated altitude above sea-level of the base dGPS station (see above). In order to eliminate pos- sible vertical biases, the 2011 DEM was shifted ver- tically based on elevation differences in 130 random points off-glacier, snowfields and ice-cored moraines (yellow triangles in Figure 2A). We interpolated a sur- face through our points of difference and found no spatial pattern, suggesting that both ALS datasets are spatially coherent and indeed uniform in tilt and pitch and yaw (i.e. IMU) corrections. The 2007 and 2011 DEMs were each hillshaded to produce a 3D visual impression of the glacier surface with unprecedented spatial resolution when compared to other analyses of glacier surges. Each DEM was analysed for its representation of ice surface texture, specifically by (i) simply using a local (between adja- cent grid cells) slope, and (ii) calculating local rough- ness as the elevation range between adjacent cells in a detrended elevation grid. Elevation in each DEM was detrended by subtracting the mean elevation within a 4 x 4 cell (i.e. 20 m) moving window from the local (per grid cell) elevation. The local relief of crevasses and seracs was quantified by calculating the difference in elevation between adjacent detrended grid cells, i.e. over a horizontal distance of 10 m. After the vertical shifting of the 2011 DEM, the DEMs were subtracted on a cell-by-cell basis to give a new grid: a DEM of difference (DoD) where posi- tive values represent an increase in elevation between August 2007 and August 2011, and negative values represent a decrease in elevation between those dates (Figure 2B). Satellite images We checked Loftmyndir ehf. (Loftmyndir.is) for aerial photographs and they had no imagery of Kverkjökull for the time period 2007 to 2011. We therefore ob- tained WorldView imagery (Table 2) that were resam- pled to a common 0.5 m grid cell size for efficiency of data management purposes. Our georeferencing of the WorldView images could only be to ∼2 m via co- registration to the ALS data and orthorectification was not possible because the images have a very oblique view and belong to a slightly different strip and to a different time of day, which together cause long and deep shadows. The images are from single strips and are not stereo-pairs so they could not be used pho- togrammetrically. Nonetheless, the WorldView im- ages could be used for manual measurement of plan- form and surface texture changes via on-screen digi- tising, both at an unprecedented spatial resolution for glacier surge analyses. Our manual feature tracking focussed on linked points, such as the ends of neigh- bouring major crevasses, and linear features such as longitudinal foliation and major dirt band edges, be- cause we were rather uncertain of finding individual points is multiple images. Bed topography and ice thickness Bed topography and ice thickness are not usually known for surging glaciers outside of Iceland but Murray et al. (2012) have reported measurements of ice thickness in their surge analysis. In Iceland, many subglacial bed profiles have been interpolated to es- timate subglacial bed topography (Björnsson, 1988; Björnsson et al., 2003; Aðalgeirsdóttir et al., 2005). However, bed elevation has not been measured along the main trunk of Kverkjökull. The only bed eleva- tion measurements that exist in the Kverkfjöll area are; (i) with uncertainty of ±50 m elevation and from a single traverse made in 1988 across the southern rim of the northern caldera that identified ice thickness of ∼200 m (Finnur Pálsson pers. comm.; Björnsson and JÖKULL No. 66, 2016 33
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