Jökull - 01.06.2000, Page 33

Helgi Björnsson et al.
west. A total sounding line length of 900 km was used
in the compilation of the maps.
Glacier surface mapping
The ice surface elevation was measured by precisi-
on barometric altimetry and recorded automatically at
50 m intervals along the sounding lines. Corrections
were made for the effects of temperature, and variati-
ons in atmospheric pressure were logged by a control
barometer at a base station within the caldera.
T RP
Loran-C
G PS
t
D
Ice
T: R adar transm itter
R : R adar receiver
P: B arom etr ic p ressure
t : radar pulse travel tim e
D: Ice th ickness
B edrock
Figure 2. Schematic illustration of the survey
technique of ice thickness, position and elevation.–
Skissa af tækjum sem mæla ísþykkt, staðsetningu og
hæð jökulsins.
Elevations were calculated from the standard
barometric altimeter equation:
 

  
 !#"%$'& (1)
in which   and   are the absolute temperature and
the atmospheric pressure, respectively, at elevation  
(elevation of base camp), = 0.0065 K/m is the tem-
perature lapse rate for an international standard at-
mosphere (i.e. ()  * + ), and p is the obser-
ved pressure at the elevation  . For height reference
a 40 km long optically levelled profile was surveyed,
traversing the ice cap from SSW to NNE and tied to
benchmarks of the Icelandic Geodetic Survey on both
sides (Fig. 3). The elevation accuracy along this ref-
erence line was close to 1 m. Other survey lines were
tied to 15 reference points on the optically levelled
line and neighbouring mountain peaks. In addition,
the elevation was measured at about 230 gravity mea-
surement points, spread over the ice cap. The absolute
accuracy in elevation is considered to be , 5 m on
the sounding lines, whereas the relative elevations are
accurate to , 3 m.
S lé ttjöku ll
B otn jökull
Ö ldufe lls jöku ll
Entu jöku ll
Só
lhe
im
a jö
ku
ll
Kötlu jö ku ll
K ötluko llar
G oðabunga
H áabunga
A ustm anns -
bunga
E nta
Sandfe lls jöku ll
B-II
B-I
Figure 3. Data source map. The position of the sound-
ing lines and the optically levelled profile ( - ) across
the ice cap, the location of radio echo soundings (+)
by Mackintosh et al. (2000), base camps (B-I and
B-II). Names of the main outlet glaciers. – Gagna-
safnskort og nöfn skriðjökla. Lega mælilína og sniðs
sem landmælt var yfir jökulinn, mælipunktar við ís-
sjármælingar á Sólheimajökli, lega bækistöðva.
Radio echo sounding measurements
The radio echo sounder consists of a mono-pulse
system (Sverrisson et al., 1980). Pulses of 0.2 . s
duration are transmitted into a 30 m long resistively-
loaded dipole antenna at the repetition rate of 1 kHz.
The reflected signal is picked up by an identical an-
tenna and fed into a receiver which has a 1-5 MHz
bandwidth. The transmitter and the receiver are placed
on sledges at the centre of the antennae and towed on
a line along the glacier surface by a tracked vehicle.
The intensity modulation (Z-scope display) of the
received signal is recorded photographically with a
35 mm camera. The speed of the scanning beam is
32 JÖKULL No. 49