Jökull - 01.01.2016, Side 5

The subglacial topography of Drangajökull ice cap, NW-Iceland
Figure 3. Example of migrated RES backscatter im-
ages with x-axis representing driven length along sur-
vey profile and y-axis elevation above sea level (us-
ing Cgl=1.70×108 m s−1) without (a) and with (b)
traced reflections (red line) from Leirufjarðarjökull
outlet glacier in NW-Drangajökull (Figure 1). Blue
lines indicate surface elevation (a and b) and corre-
sponding profile location (c). Roman numbers in pro-
file b and c show matching locations. – Dæmi um
íssjársnið af Leirufjarðarjökli (a og b) þar sem ásar
gefa til kynna vegalengd eftir sniði (x-ás) og hæð yfir
sjávarmáli (y-ás). Bláu línurnar sýna yfirborðshæð (a
og b) og legu sniðs (c). Rauða línan á mynd b sýnir
endurkast frá botni. Rómverskar tölur tengja saman
staði í sniði (b) við staðsetningar á korti (c).
Backscatter from the bedrock is usually recog-
nised as the strongest continuous reflections at depth.
They were traced with an automatic tracing algorithm,
programmed in Matlab (©Mathworks) that traces the
bed from a given clear point of bed reflection in the
backscatter image, using the maximum correlation
with the bed reflection at the chosen starting point.
Obtained traces were manually revised to reject de-
rived traces where the algorithm failed. This process
was repeated until all clear bed reflections had been
traced. The tracing obtains bed elevation with sub-
meter vertical precision every 5 m along the profile
where bed reflections were clear (see Figure 3 for ex-
ample). Traces at locations less than 50 m away from
abrupt turns (>45◦) in the survey profiles were how-
ever omitted since the assumption of fixed displace-
JÖKULL No. 66, 2016 5