Jökull - 01.01.2012, Page 104

S. Guðmundsson et al.
Table 1. Dimensions of the crevasse areas, and surface elevation change measurements in pixel units. – Tafla
1. Grunnupplýsingar um sprungukollana (c1–c9) sem notaðar voru til þess að reikna yfirborðslækkun. Meðal-
hæðarbreyting (í myndeiningum) út frá 7 mælingum á hverjum sprungukolli.
measurements (pixels)
area latitude longitude altit. (m) d (m) bearing◦ I1 l2 l3 center r1 r2 r3 ∆θ
c1 63.9683 16.7031 1166 1405 60.2 14.36 14.92 13.40 13.2 13.25 12.83 10.02 13.14
c2 63.9716 16.6939 1319 1977 56.7 5.73 5.73 6.33 5.29 4.62 4.73 3.85 5.18
c3 63.9722 16.6865 1398 2328 59.9 5.76 5.19 4.05 5.76 5.99 5.85 5.99 5.51
c4 63.9713 16.6778 1429 2673 66.2 2.86 2.36 2.29 2.86 2.86 2.86 2.92 2.72
c5 63.9779 16.6703 1677 3335 56.60 2.36 2.29 2.36 1.81 1.81 1.81 1.81 2.04
c6 63.9680 16.6872 1283 2113 70.80 4.62 4.01 3.49 5.29 5.19 6.41 8.05 5.29
c7 63.9697 16.6790 1367 2723 70.00 1.72 1.72 1.72 1.72 1.72 2.29 1.72 1.80
c8 63.9682 16.6757 1403 2666 73.90 2.92 3.63 3.49 2.92 2.92 3.49 2.06 3.06
c9 63.9650 16.7077 1083 1058 72.50 13.78 13.88 11.70 15.5 15.59 16.60 16.07 14.73
Figure 6. A sketch of the crevassed accumulation area of Kotárjökull. The edge of each crevasse area in 1891
(c1-c9) shown as a dotted line. Surface lowering of each area derived from 7 points, as illustrated for crevasse
area c1. Collimation points are indicated in Roman numerals (I-IV). – Sprungukollar (c1-c9) á safnsvæði
Kotárjökuls voru notaðir í útreikninga á hæðarbreytingu milli 1891– 2011. Á sprungukolli c1 er sýnt hvernig
gerðar eru 7 mælingar á þykktarbreytingu milli áranna 1891 (punktalína) og 2011 (heil lína), til þess að fá
fram meðalbreytingu. Hamrarnir suðvestanmegin í Rótarfjallshnúk voru notaðir sem viðmið til þess að áætla
þykktarbreytingar í aðferð 1. Ljósmyndirnar tvær voru stilltar af með fjórum punktum merktum I-IV.
m (Figure 6). The southern face is HII = 120 m and
nmII = 52 pixels, hence θuII = 2.3 m/pixel and the
distance from Howellsnöf DII = 3.320 m. To test the
quality of θu as a satisfactorily accurate value, we cal-
culated the size of θuII at distance DI ;
(θuII ×DI )/ DII = θuII/DI = 2.5 m/pixel
and then obtained an average value for θuI of 2.25
m/pixel (θu) at distance DI , which was used in all
calculations. Glacier surface changes were also calcu-
lated for crevasse area r1 (Figures 1 and 4a-b), using
routine 1.
Routine 2
The pixel unit size, which is used as a reference for
surface elevation changes, was determined from the
photographs by calculating a lateral scaling distance
(a = d tan α) perpendicular to the line of sight from
Howellsnöf to each distinct crevasse area. The dis-
tance from the site of photography is d, and α the
angle between two crevasse areas, as seen from How-
ellsnöf (Figure 7). The length of the opposite side
is used to find the size of the pixel unit at distance
d. Since the measured surface lowering in pixel units
102 JÖKULL No. 62, 2012