Jökull - 01.01.2012, Side 102

S. Guðmundsson et al.
DATA
The photographs of Frederick W.W. Howell were
taken from two locations on Sandfell mountain (Fig-
ures 1 and 2): a) one shot towards east to Rótar-
fjall from Howellssteinn (N63.9497◦, W16.7587◦,
elevation 573 m a.s.l.) and b) two shots from
Howellsnöf (N63.9624◦, W16.7286◦, elevation 1020
m), 1)northeast towards Rótarfjallshnúkur (1833 m)
across the upper slopes of the Kotárjökull, and 2)east
towards Rótarfjall. Howellssteinn and Howellsnöf are
not geographical place names, but used as landmarks
by the authors. The photographs confirm, that the
highest lateral moraines, trimlines and glacial errat-
ics in the narrow gorges of Kotárgil and Berjagil are
from the 1890 LIA maximum (Figure 2) . In Novem-
ber 2011 the two locations were revisited, the pho-
tos reframed, and the acquired duplicates (Figures 3a-
b, 4a-b and 5a-b) used to calculate the recession of
the glacier between 1891 and 2011. Howell’s pho-
tographs are available in the Fiske Icelandic Collec-
tion, at the Cornell University Library website.
High-resolution aerial images of Loftmyndir ehf©
(2003) were used to outline the LIA maximum glacier.
In situ and oblique aerial photographs of 2006 and
2010, helped derive the glacial extent.
A recent DEM, produced from airborne LiDAR
measurements in August 2010 and September 2011
(data from the Icelandic Meteorological Office and
the Institute of Earth Sciences, University of Iceland,
2011; Jóhannesson et al., 2012); provides accurate
position and elevation (Table 1). The DEM has a
horizontal resolution of 5x5 m and vertical accuracy
within 0.5 m. It provides precise elevation of the lat-
eral moraines and trimlines.
METHODS
The extent of Kotárjökull at the LIA maximum, was
based on photographic and geomorphological evi-
dence, and the LiDAR DEM providing basic topo-
graphical data. The glacier margin in the ablation
area, is delineated from the highest lateral moraines,
glacial erratics, and trimlines. Data on elevation
changes above the equilibrium line are restricted to
the old photographs. The idea of obtaining quanti-
tative estimates of glacier changes from the photo-
graphic duplicates, originates from methods used in
astrometry. The movements of distant objects are
measured over time from separate images, taken hours
to decades apart.
Repeat photography
The three photographic pairs of Kotárjökull were col-
limated in GIS ArcMap (Figures 3a-b, 4a-b, 5a-b). A
3D-image, a duplication of Figure 3b, was produced
from the DEM in ArcScene, to improve the accuracy
of our measurements. The southeastern flank of Rót-
arfjallshnúkur has apparently undergone some land-
form changes since 1891, perhaps a landslide. The
photos in Figures 3a-b were collimated, using the
northern (I) and southern (II) peaks of Rótarfjalls-
hnúkur for reference, the top of Sandfell (III), and a
crevasse area (IV) on the horizon to the west of Rót-
arfjallshnúkur (Figure 6). The photographs in Figures
4a-b were referenced with four points, and in Figures
5a-b, with 10 points. The ease of collimating the du-
plicate photos, indicates minimal errors related to the
older camera’s focal length.
Nine crevasse areas in the accumulation area (Fig-
ures 3a-b), were used for surface elevation calcula-
tions. The lowering was measured in pixel units and
converted to metres. A total of 7 measurements evenly
distributed over each crevasse bulge, from center to-
wards left (l1−3) and right (r1−3), were used to obtain
a mean pixel value for the surface lowering (Figure 6
and Table 1). Two independent routines to calculate
the glacier surface changes in metres were used.
Routine 1
The vertical glacier surface change (∆h) at any dis-
tance (d) from the site of photography was estimated
by scaling in metres the pixel unit size (θu= H/nm)
based on a known vertical height of a mountain cliff
(H spanning nm pixels) at a known distance (Do), in
this case Rótarfjallshnúkur (Figure 6). If the measured
surface lowering of a crevasse area in pixel units is nc,
the corresponding glacier surface change in metres is:
∆h = θu × (d/Do) × nc
The northern face of Rótarfjallshnúkur is 60 m high
(HI ) and nmI = 30 pixels, hence θuI = 2.0 m/pixel;
the distance to the face from Howellsnöf isDI= 3.650
100 JÖKULL No. 62, 2012