Jökull - 01.12.1981, Blaðsíða 13

Recent Mapping of Gljúfurárjökull and Gljúfurárdalur
C. J. CASELDINE and R. A. CULLINGFORD
Department of Geography, University of Exeter,
Amory Building, Rennes Drive, Exeter, EX4 4RJ.
INTRODUCTION
Gljúfurárjökull is a small valley-head
glacier lying in the uppermost section of a
hanging valley above the major glacial
trough of Skíðadalur in the Tröllaskagi
peninsula of Northern Iceland (Lat. 65°43’N,
Long. 18°39’VV). Observations have been
made of the glacier by several workers over the
past forty years (Eythórsson 1956, 1963; Rist
1977) and in recent years it has been visited by
a number of parties from the United Kingdom
as part of the North Iceland Glacier Inven-
tory. Although rates of retreat have been
worked out by Eythórsson (1963) and several
minor surveys have been carried out by the
British groups it was not until 1977 that an
accurate survey was made of the extent of the
giacier by the British Schools Exploring
Society (B.S.E.S.). This survey provided the
basis for the work of the 1979 Exeter Univer-
sity North Iceland Expedition and the prin-
cipal results obtained in 1979 are presented
below.
SURVEY METHODS
The 1979 survey programme was greatly
facilitated by the availability of the B.S.E.S.
map, which was based on a rigorously exe-
cuted surveying programme using a combi-
nation of 3rd order triangulation, plane table
and tacheometric methods. The existence of a
network of identifiable ground stations esta-
blished by the 1977 party and earlier ex-
peditions, and surveyed accurately by the
1977 party, enabled triangulation work to be
kept to a minimum. The 1979 survey was
conducted almost entirely using a Wild RDS
self-reducing tacheometer in conjunction with
a 4m l'ristaff. The precision of the altitudinal
aspects of the survey was checked during the
fieldwork when it became clear that the altitudes
of particular stations as determined indepen-
dently from two or more other stations showed
only very small differences (Caseldine 1981).
The datum for the altitudinal determinations
was that used by the B.S.E.S. survey in 1977.
This datum is not related precisely to mean
sea level, its altitude having been determined
with reference to photogrammetrically-
surveyed contours on the AMS Series C762
Sheet 5824 III 1:50.000 topographic map. The
planimetric accuracy could be checked only
during the plotting of the survey data, when it
was apparent that positions of stations deter-
mined by tacheometry from two other stations
corresponded very closely, often within the
practical limits of draughtmanship.
Taking the maximal observed discrepancies
as a guide to accuracy, it is believed that
altitudes (relative to the local datum) are
accurate to within 0.5 m, and locations to
within 5 m. Table 1 lists stations established
and/or used during the 1979 survey.
i) Glacier Survey — The margins of the snout
of the glacier were surveyed from Ll, G and
V *. The glacier margin was surveyedd at inter-
vals of about 15 — 20 m although difficulties
were encountered where the ice margin was
obscured by snow. In such cases the marginal
position was estimated from the surface con-
figuration of the snowbeds and as these melted
the actual positions were checked. Because of
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