Jökull - 01.01.2012, Page 88

F. Pálsson et al.
Figure 5. The surface of Langjökull constructed from SPOT5 HRG stereo image pairs (acquired on August
12th, 14th, 17th and 19th 2004). The red triangles are static or kinematic GPS surveyed control points, the
blue lines kinematic GPS profiles. The yellow triangles are sites on the glacier surface surveyed in October
2004 with DGPS. The numbers are the residue between the DEM and GPS surveyed elevation. The LIAmax
outline is shown in red. – Yfirborð Langjökuls reiknað út frá þrívíddar-myndpörum sem tekin voru með SPOT5
gervitunglinu dagana 12., 14., 17. og 19. ágúst 2004. Lega jökuljaðars við lok litlu ísaldar (um 1890) er sýnd
með rauðri línu.
The older maps (1937 and 1945) were somewhat
distorted laterally, due to errors in the old trigonomet-
ric network for Iceland, used as the original reference
(surveyed by the Danish Geodetic Institute in 1904–
1937). To overcome this, the maps were scanned
and the contour lines digitized. During the digitiz-
ing process, the maps were shifted locally using the
2004 DEM as a reference, and the rugged landforms
at the glacier margin and nunataks as controls. The
upper parts of both maps were reconstructed, based
on the 1997 map. We assumed only small changes
in the landforms (shape of contour lines) and that the
elevation difference to decay linearly over the accu-
mulation zone. The elevation difference at the high-
est peaks was estimated from model run, applying the
coupled ice-flow mass balance model given in Gud-
mundsson et al. (2009a). This yielded in only a few
metres elevation difference at the highest ridges. The
86 JÖKULL No. 62, 2012