Jökull - 01.12.1967, Blaðsíða 36

II. THE RECESSION OF HAGAFELLS-
JÖKULL EYSTRI AND LANGJÖKULL
Introduction
I have made some atlempts at collecting all
available data on the recession oí Hagaí'ells-
jökull Eystri and Langjökull as a whole in
order to measure and calculate the glacial re-
cession at dií'ferent times. The most important
sources of information on the regime of the
glaciers are old maps. The oldest map showing
the southern margin of Langjökull with some
degree of accuracy is a map at the scale of
1:480,000 drawn aíter Björn Gunnlaugsson’s
measurements dating from the years 1834—35.
This map should be fairly reliable since Björn
Gunnlaugsson travelled in this area. Unfortun-
ately there are, however, some obvious errors on
it, although his writings give additional points
in some cases (Gunnlaugsson 1835 and 1836).
According to these, the Jarlhettur Valley was
filled with ice nearly to the same degree as
during the maximum extent of the glacier (Fig.
3). The map shows the snout of Hagafellsjökull
Eystri reaching as far to the south as the south
end of Hagafell, but in its maximum stage it
reachecl about one km farther south. The glacier
lobe has therefore probably advanced somewhat
in more recent times.
The next mapping of this area was carried out
by the Cambridge Icelancl Expedition in the year
1934 (Wright 1935). This map covers the south-
ern margin of Langjökull from Thórisdalur in
the west to Jarlhettur in the east. The contour
intervals on it are only 100 m except on a
detailed map of Hagavatn ancl surroundings on
which 25 m contour lines are drawn. This map
is reliable in all main points and shows clearly
the position of the ice front ancl the marginal
lakes.
In the years 1937—39 all this area was sur-
veyecl and mapped by the cartographers of the
Geodetic Institute, Copenhagen, as a part of a
geodetic survey of Iceland. In the preparing of
this map aerial photographs were used. The
next map of the area was made by the U. S.
Army Map Service. It was drawn by photo-
grammetric methods, based on aerial photo-
graphs from the year 1945 and a triangulation
by the Geodetic Institute. Finally a part of this
area was surveyed and mapped with 5 m con-
tour intervals at the scale of 1:20,000 by the Na-
tional Energy Authority. Tliis map is drawn
by photogrammetric methods, based on aerial
photographs from the year 1960. In addition
to these maps a student group from the Dur-
ham Exploration Society mapped Hagavatn and
its surroundings in the year 1950 (Green 1952).
The snout of Hagafellsjökull Eystri was also
mapped by English students in the years 1956,
1960 and 1964. I have obtained a map, which
shows the position of the ice front in these
years, but otherwise I have not been able to
approach any details of their work. But in
accuracy their map does not seern to be as good
as the map of 1960.
Since the year 1934 Jón Eythórsson a. o. have
measured the retreat of the glacier snouts of
Hagafellsjökull Eystri and Hagafellsjökull Ytri.
To begin with these measurements were sparse,
but since 1960 they have been carriecl out al-
most every year.
The map on Fig. 1 has been compiled from
these sources, which can be counted reliable.
The maximum extent of Hagafellsjökull Eystri
has been drawn by using aerial photographs,
the interpretation of which has been checked
by field observations. I have estimated the posi-
tion of the ice front in the years 1902 ancl 1929
with reference to the “jökulhlaups” of these
years and by using photographs and historical
accounts.
The profile AB on Fig. 1 is drawn in vertical
scale on Fig. 5, where the longitudinal profiles
of the glacier lobe at different times are shown
together with the changing level of Hagavatn.
The maps have also been usecl as base for the
profiles, except that the maximum thickness of
the glacier is estimated in the following way:
The maximum height of the recent glacial fea-
tures was measured on most of the peaks in the
Brekknafjöll Range and also in Hagafell and
the results of these measurements were project-
ed on the profile AB. Aneroid barometer was
used for these measurements, but the air pres-
sure was unstable in some cases and some errors
may have been caused by that.
I have used these maps and measurements
as a base for calculations of the shrinkage ol
the glaciers in this century, both in length,
area and volume. The following three areas
were selected for that purpose (Fig. 7):
270 JÖKULL 17. ÁR