Jökull - 01.01.2012, Side 105

Post-Little Ice Age volume loss of the Kotárjökull glacier,
Figure 7. A schematic aerial view explaining routine 2. See text for clarification. – Yfirlitsmynd fyrir aðferð
2. Ef fjarlægð frá Howellsnöf í hvern stakan sprungukoll (d) og hornið (α) á milli þeirra eru þekkt, fæst lengd
mótlægrar hliðar (a) samkvæmt tan α=a/d og hversu stórt hornbilið er í metrum í þekktri fjarlægð. Þann-
ig finnum við stærð myndeiningarinnar í sömu fjarlægð. Yfirborðslækkun jökulsins (∆h) er hlutfall af lengd
mótlægu skammhliðarinnar. Punktalínur gefa til kynna hvernig aðlaga má hliðar þríhyrninga samkvæmt því í
hvaða punkt er mælt.
(∆θ) is a certain ratio of the opposite side, we can
calculate the change in metres at a distance d from the
site of photography:
∆h = ∆θ (d tanα)/n
where n is the number of pixels of the corresponding
angle α. This is possible since horizontal and vertical
scales of the pixels are the same. We can extend the
triangle’s hypotenuse, depending on the point we aim
at (illustrated by the dotted line in Figure 7).
The LIA glacier and volume calculations
We assume unchanged glacier geometry in the accu-
mulation area, as seen from the photos in Figures 3a-
b. Calculated elevation changes were used to raise
the contour lines of the LiDAR DEM to an 1891
level. The reconstructed cross-valley profiles of the
two branches, below the equilibrium line, are convex,
with a maximum height of 20 m at the glacier cen-
ter line. This assumption is based on the photographs
in Figures 4a-b and 5a-b. Our estimate for the un-
certainty limits of the reconstructed 1891 surface is
±2 m. A gradual thickness changes along the longi-
tudinal profile of Kotárjökull is assumed (Figure 1),
and interpolated between data thickness points using
a least-square fit to a log-linear equation. The volume
loss of Kotárjökull was calculated by subtracting the
glacier surface of 2011 from the reconstructed surface
of 1891.
Elevation changes on Öræfajökull’s plateau
The 1904 map of Öræfajökull of the Danish Gen-
eral Staff was georeferenced with the LiDAR DEM,
and the elevation of selected trigonometrical points
from the older map compared with the DEM, to re-
solve possible glacier surface changes. Nine geode-
tic points on mountain peaks or nunataks and eleven
on the glacier surface were selected for this purpose,
spanning an altitudinal range of 1700–2100 m.
JÖKULL No. 62, 2012 103