Jökull - 01.01.2012, Page 86

F. Pálsson et al.
Figure 3. The surface of Langjökull based on April 1997 DGPS survey data (25 m contour lines). Survey lines
of DGPS and RES profiles are shown in red. – Kort af yfirborðshæð Langjökuls eftir GPS hæðarmælingum á
íssjármælisniðum frá 1997. Rauðar línur sýna legu mælisniðanna.
Three DEMs were constructed from four 5x5 m
pixel resolution SPOT5 HRG images acquired from
12, 14, 17 and 19 August 2004, using the PCI Ge-
omatica software (Toutin, 2006); the images of the
dates 12/14, 14/17 and 17/19 as stereo pairs. None of
the DEMs covered the whole glacier, but by combin-
ing them one complete DEM could be compiled (Fig-
ure 5). A collection of ground control points (GCPs)
such as crossroads, a road crossing a river (i.e. items
recognized on the SPOT5 images) were surveyed by
static or kinematic GPS (red triangles in Figure 5).
Profiles along tracks and roads were surveyed by kine-
matic GPS. Some of the GCPs were used as input to
Geomatica to calibrate the SPOT5 satellite model, to-
gether with carefully chosen tie-points recognized on
the both images of each stereo pair used to extract
the DEMs. The profiles, GCPs and 22 GPS surveyed
sites on the glacier surface were then used to evaluate
the resulting complete 2004 DEM (the final product),
yielding an accuracy of ∼1 m in elevation.
Three published maps exist for Langjökull, all
with 20 m contour lines: from 1937 (Geodetic Insti-
tute, Copenhagen), 1945 (AMS series C762, mapped
by the Army Map Service, Corps of engineers, U.S.
Army, Washington D.C.) and 1986 (DMA series C761
produced by the Defense Mapping Agency Hydro-
graphic/Topographic Center (DMAHTC), Washing-
ton DC). The 1986 and 1945 maps are derived by stan-
dard aerial photographic methods and the 1937 map
from trigonometric geodetic survey and oblique pho-
tographs. All the maps include only estimated con-
tour lines (indicating shape or form, not actual con-
84 JÖKULL No. 62, 2012