Jökull - 01.01.2011, Side 25

LiDAR mapping of the Snæfellsjökull ice cap
A detailed comparison of the 1999 Loftmyndir
DTM with the LiDAR DTM in ice-free areas around
the ice margin indicates a ∼1.0 m one-sided bias
(mean 1.0 m, median 0.9 m). This comparison was
carried out by masking out snow covered areas out-
side the ice margin in 1999 based on a rectified 1999
aerial photograph (Figure 5, left). This leaves ∼70
thousand points on which the estimation of the bias
is based. After the subtraction of this bias, the differ-
ences between ice-free areas of the DTMs were un-
biased and less than 0.5 m in magnitude at more than
50% of the considered points. Several metres differ-
ences were found in ∼1% of the points and >10 m dif-
ferences in ∼0.1% of the points, as may be expected
in a landscape with gullies, ridges and other sharp fea-
tures that are not resolved in the 5x5 m and 10x10 m
DTMs. Comparison of the DTMs in several hand-
picked ice-free areas outside the ice margin, on three
nunataks within the northern part of the glacier and
along the ice free part of the profiles in Figure 4 in-
dicates a similar value of the bias on all sides of and
within the ice cap. Based on this analysis, 1.0 m was
subtracted from the Loftmyndir DTM before the cal-
culation of the thinning of the ice cap discussed in the
next section.
The 1999 orthophoto of Snæfellsjökull with the
GPS-measurement of the ice margin in 2002 is shown
in Figure 5 (left panel) together with a hillshade of the
Figure 5. Left: Orthophoto of Snæfellsjökull from 5 August 1999 (© Loftmyndir ehf.). The glacier margin
measured by GPS in the field in 2002 shown with a red curve is close to the location of the ice margin in 1999.
Right: A hillshade of the 2008 LiDAR digital elevation model. The glacier margin in 2008 as delineated from
the LiDAR measurements and the 2002 margin measured by GPS are shown with red and blue curves, respec-
tively. Note that the outline close to the ice margin is shown with a red curve in both pictures. The 2002 outline
measured by GPS is shown with a red curve on the left and a blue curve on the right. – Vinstri mynd: Upprétt
loftmynd af Snæfellsjökli frá 5. ágúst 1999 (© Loftmyndir ehf.). GPS-mæling á útlínu jökulsins frá 2002 er sýnd
með rauðri línu. Hægri mynd: Skygging á LiDAR landlíkaninu frá 2008. Útlína jökulsins frá 2008 dregin á
grundvelli LiDAR mælingarinnar er sýnd með rauðri línu og útlínan frá 2002 mað blárri línu.
JÖKULL No. 61, 2011 25