Jökull - 01.01.2012, Page 87

Mass and volume changes of Langjökull ice cap, Iceland, ∼1890 to 2009
Figure 4. Location of the 23 mass balance sites on Langjökull and the two AWS’s (blue triangles), the red
line is an approximate central flow line down W-Hagafellsjökull outlet. b-d) Maps of the average winter-
(bw), summer- (bs) and net balance (bn) 1996–1997 to 2008–2009. – a) Lega 23 afkomumælistaða (punktar)
og tveggja sjálfvirkra veðurstöðva á Langjökli (þríhyrningar). b-c) Kort sem sýna meðal vetrar-, sumar- og
ársafkomu Langjökuls jökulárin 1996–1997 til 2008–2009.
tour lines of elevation) for elevations above ∼1100 m.
Considerable differences in the shape and elevation of
the accumulation zone were revealed when comparing
those maps to the 1997 and 2004 DEMs (a difference
of over 100 m at the north-dome summit). Compar-
ison of the maps and the 2004 DEM, however, in-
dicates the contour lines to be fairly accurate below
1100 m elevation.
In a joint effort by the U.S. Army and Iceland’s
Energy Authority, an elevation of 10 points in the up-
per part of Langjökull was surveyed in the summer of
1985 using an inertia positioning system in a gravity
survey campaign (accuracy ∼1 m). The elevation of
these points and the shape of the contours of the 1997
and 2004 DEMs, were used to control the reconstruc-
tion of the upper part of Langjökull in 1986, i.e. by
assuming only small changes in the shape of the up-
per glacier. The lower part was based on digitized
contour lines and co-registration with the 2004 DEM
to correct for lateral shifts. The accuracy of the recon-
structed 1986 DEM is estimated <5 m, vertically.
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