Jökull - 01.01.2011, Page 24

T. Jóhannesson et al.
tive bias may be due to compressive ice deformation
over the four days that elapsed between the aerial sur-
vey and the GPS-measurements, which would tend to
raise the ice surface slightly during this time of the
year when summer melting is mostly over. However,
as the bias is of similar magnitude outside the ice mar-
gin, it is most likely that the negative bias is the result
of a small bias in the LiDAR measurements, which is
well within the expected error of the measurements.
The Loftmyndir DTM is based on digital process-
ing of aerial photographs from a flight altitude of
3000 m. The 10x10 m DTM was reprocessed for the
purpose of this study using new GPS-measurements
of distinguishable features from ice-free areas around
the ice margin to improve the vertical accuracy. The
surface elevation according to the 1999 and 2008
DTMs along eleven profiles across the ice cap from
west to east shows that the DTMs are consistent in
ice-free areas (Figure 4). The consistent variations
of the 1999 and 2008 DTMs within the ice margin
indicate that the digital stereoscopic processing of
the aerial photographs from 1999 successfully iden-
tified the ice- and snow-covered surface of the glacier.
The surface of Snæfellsjökull in the fall is compar-
atively suitable for stereoscopic processing of aerial
photographs due to numerous nunataks and ice-free
ridges, dust blown onto the glacier from the barren
lands outside the ice margin and dense crevasse pat-
terns that provide visual features that aid the stereo-
scopic processing.
A
lti
tu
de
(
m
a
.s
.l.
)
60
0
80
0
10
00
12
00
14
00
1 km
1
2
3
4
5
6
7
8
9
10
11
Figure 4. Measured elevation in 1999 (green) and 2008 (red) on eleven profiles crossing the ice cap from west to
east (see Figure 6 for the map location of the profiles). The part of the profiles that is within the 2002 ice margin
is drawn with a thicker curve on the 1999 profile. The profiles are progressively shifted to the east as indicated
by the horizontal location of the numerical labels to reduce overplotting. – Landhæð eftir ellefu sniðum frá
vestri til austurs yfir jökulinn (lega sniðanna er sýnd á mynd 6). Grænir ferlar sýna mælingar Loftmynda ehf.
frá 1999 en rauðir ferlar LiDAR mælingar TopScan haustið 2008. Sá hluti sniðanna frá 1999 sem er innan
jökuljaðarsins frá 2002 er dreginn með breiðum ferli. Sniðunum er hliðrað jafnt og þétt til austurs eftir því sem
númer þeirra hækkar til þess að þau skiljist betur að á myndinni.
24 JÖKULL No. 61, 2011