Jökull - 01.01.2012, Side 137

Öræfajökull central volcano, SE-Iceland
Figure 3. Scatter plot of the ice thickness
from the 2009 RES survey data against
sinθ, where θ is the surface slope in flow
direction. The red line shows a linear re-
lation used to estimate ice thickness in ar-
eas where surface slope, θ, is between 8◦
and 20◦ and no RES data are available
(Figure 2). – Dreifirit sem sýnir ísþykkt
mælda með íssjá á Öræfajökli árið 2009
á móti sinθ, þar sem θ er yfirborðshalli í
flæðistefnu jökulsins. Rauða línan sýnir
metið línulegt samband sem notað var til
að áætla ísþykkt á svæðum þar sem yfir-
borðshalli var á bilinu 8–20◦ (2. mynd).
each x-coordinate of the image. We finally apply
2-dimensional migration along the surveyed profiles
(e.g. Claerbout, 1985) to compensate for the width of
the radar beam illuminating the subglacial bedrock,
which may be up 200 m wide for the range of the
observed ice thickness. The accuracy in derived ice-
thickness is estimated ±15 m (Björnsson et al., 2000).
The accuracy in derived bedrock elevation can be con-
sidered the same for the 2009 observations but some-
what higher at ±17 m for the 1991–1993 observa-
tions, due to additional uncertainty in the surface ele-
vation.
The RES point measurements, mostly carried out
in 2005 and 2006, are primarily from the lower part
of glacier, where the rough and crevassed glacier sur-
face makes continuous profiling very difficult. They
were made with the system used in 1991–1993, but
with a modification on how data was recorded. For
a single observation the antennae are generally align-
ing parallel on the glacier surface 20–60 m apart. We
assume that the difference in travel time between the
direct wave and the wave reflected from the bedrock
reveals the ice thickness midway between the anten-
nae. x,y,zs coordinates are taken at that point. Bedrock
elevation is calculated as surface elevation (zs) minus
ice thickness. Migration of this dataset is not possible
due to its discrete nature and thus the uncertainty of
the bedrock elevation is on the order of ±20 m. The
data discontinuity also makes misinterpretation of the
bedrock reflection more likely than for the continuous
RES measurements.
Surface DEM
The surface DEM used in this study was obtained with
an airborne laser scanning commonly referred to as
LiDAR (an acronym for Light Detection And Rang-
ing). The measurements were carried out by Top-
Scan (http://www.topscan.de) in 2010, at elevations
>∼1300 m a.s.l. and 2011, at elevations <∼1300 m
a.s.l. The DEM has a pixel size of 5 m x 5 m and ver-
tical accuracy <0.5m (Jóhannesson et al., in press). It
covers the whole of Öræfajökull ice cap in addition
to most of the ice free part of the Öræfajökull vol-
cano and adjoined mountains (Figure 5). The survey
of Öræfajökull is part of an ongoing project led by the
Icelandic Meteorological Office and Institute of Earth
Sciences, University of Iceland, which aims to com-
plete topographic mapping of nearly all glaciated ar-
eas in Iceland with LiDAR (Jóhannesson et al., 2011;
Jóhannesson et al., in press).
Construction of bedrock DEM
To construct the bedrock DEM of Öræfajökull we first
outlined the glacier margin and nunataks. This was
done by viewing the LiDAR DEM as both a shaded re-
lief image and as elevation contours with 10 m interval
(sometimes smaller if needed) in Surfer©10 (Golden
Software, Inc.). This reveals changes in both sur-
face slope and texture, making manual outlining of
JÖKULL No. 62, 2012 135