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Glacier Inventories of Iceland: Evaluation and Use of Sources of Data

Published online by Cambridge University Press:  20 January 2017

R.S. Williams Jr.
R.S. Williams Jr.*
Affiliation:
U.S. Geological Survey, Reston, Virginia 22092, U.S.A.
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Abstract

All sources of cartographic, aerial photographic, satellite image, and related data, from the 18th century to the present, for the eight geographic groups of Iceland’s glaciers, were evaluated for use in preparing a preliminary inventory of Iceland’s glaciers, based on information requirements of the Temporary Technical Secretariat for World Glacier Inventory. On the basis of an evaluation of all sources of historic and modern data for the Langjökull Group, the 1:50 000 scale U.S. Army Map Service Series C762 maps of Iceland were determined to be the best maps from which to derive information for a preliminary inventory, as long as the limitations of these maps are considered and accommodated. The fluctuations of Langjökull’s principal outlet glaciers on maps and Landsat images were found to be consistent with field observations at the International Hydrological Decade monitoring stations. Accumulation area ratios were calculated from late summer snow lines on 1973 Landsat images of Vatnajökull (0.70), Langjökull (0.78), and Mýrdalsjökull (0.35), Measurements of the area of the now stagnant glacier on Ok showed a rapid reduction in area (68 per cent) between 1910 and 1960, but a decline in rate of wastage since 1960 (73 per cent between 1910 and 1978). From 1910 and 1945 topographic maps, the volume of the glacier on Ok was found to be reduced by 0.62 km3.

Information

Type
Research Article
Information
Annals of Glaciology , Volume 8 , 1986, pp. 184 - 191
Copyright
Copyright © International Glaciological Society 1986

Introduction

A great variety of maps, both historic and modern, aerial photographs, satellite images, and related information are available of Iceland and these have been used by many scientists for glaciological studies, including the preparation of glacier inventories. It is the objective of this paper to evaluate the most important of these sources of data and to assess the usefulness of the 20th century cartographic and image data for the preparation of a modified Level 4 inventory of Iceland’s glaciers, according to the “Guidelines for Preliminary Inventories” set forth by the Temporary Technical Secretariat for the World Glacier Inventory (TTS/WGI) (Reference SeherlerSeherler 1983). Because of the difficulty in preparing comprehensive glacier inventories of many regions, according to the initial guidelines set forth by Reference Müller, Caflisch and MüllerMuller and others (1977), Reference SeherlerSeherler (1983) developed a simplified Level 1, 2, 3, or 4 inventory scheme that is based on Landsat images with or without the supplementary use of maps and aerial photographs.

The latter part of the paper will apply the glaciological information that can be gleaned from various types of modern cartographic, image, and other data to one of Iceland’s eight glacier groups, the Langjökull Group (Fig.1.). In this way the glaciological value of these diverse data for the preparation of a glacier inventory can be directly evaluated for a complex group of nine glaciers in the Langjökull Group, first studied in the mid-l700’s (Reference VidalinVidalin 1754, Reference ÓlafssonÓlafsson 1975 (1772)) and first mapped in 1792 (Pálsson 1795).

Fig. 1

Index map to the 8 groups of glaciers included in an inventory of Iceland’s glaciers. The drainage divides between the 12 sectors of the Hydrological Network of Iceland are shown with a dashed line (Iceland National Energy Authority, 1972). The 34 numbers associated with many of the glaciers are International Hydrological Decade (IHD) index numbers assigned by Rist (Reference Rist1967 and Reference Rist1977) to margins of glaciers monitored annually.

Distribution of Glaciers

Most of the glaciers in Iceland, both in terms of area and volume, occur as ice caps (Fig.1.). The ice caps range in area from a few square kilometers (for example, 4.0 km2 for the glacier on Ok) to several thousand square kilometers (8300 km2 for Vatnajökull).

It was found convenient, from a glacier-inventory viewpoint, to group Iceland’s glaciers according to eight geographic clusters (Fig.1.): Vestfirdir, Snaefellsjökull, Langjökull, Myrdalsjökull, Hofsjökull, Tröllaskagi (Nordurlandsjöklar), Vatnajökull, and Austfirdir. Rist (written communication, 1985) has also grouped Iceland’s glaciers into eight clusters, although his grouping is slightly different from Fig.1. The number of glaciers in each group ranges from one in the case of Snaefellsjökull, a solitary ice cap, to about 115 Alpine-type cirque and valley glaciers (according to Reference BjörnssonBjörnsson 1980b) in the Tröllaskagi Group. The names of the glaciers in the glacier groups are from the 1:100 000 scale Atlas sheets of Iceland and the published works of Thorvaldur Thoroddsen, Jón Eythórsson, Sigurdur Thorarinsson (especially Reference ThorarinssonThorarinsson 1943), and volumes of the journal Jökull.

Historical Review of Glacier Inventories

Reference ThorarinssonThorarinsson (1960) provides an excellent review of the published historical record on observations of Iceland’s glaciers before the beginning of the 19th century, especially with reference to the pioneering work of Sveinn Pálsson, who prepared the first detailed maps of some of the glaciers of Iceland (Pálsson 1795, Reference Eythórsson and EythórssonEythórsson 1945, Reference SigurdssonSigurdsson 1978). The primary basis for compiling an inventory of glaciers of any area, however, is the availability of adequate maps and, until the beginning of the 20th century, when 1:50 000 scale and 1:100 000 scale map series were initiated by Danish surveyors and cartographers, the lack of a good cartographic base to plot field observations of glaciers was a serious handicap.

Thorvaldur Thoroddsen was the first scientist to study all of Iceland’s glaciers systematically (Reference ThoroddsenThoroddsen 1892, Reference Thoroddsen1906, Reference Thoroddsen1911). Thoroddsen had a significant advantage over Sveinn Pálsson in that the 1:480 000 scale color map of Iceland by Reference GunnlaugssonGunnlaugsson (1844) was available. Although the Reference HellandNorwegian, Amund Helland (1883), used this map to calculate areas of eight of Iceland’s glaciers, Thoroddsen (Reference Thoroddsen1892, Reference Thoroddsen1906) provided information on areas, as well as information of glaciological value (for example, highest and lowest elevation above sea level, height of the snow line, and number of outlet glaciers) about Iceland’s glaciers, Thoroddsen gave the total area of glaciers in Iceland as 13 530 km2 (Reference ThoroddsenThoroddsen 1906), but this area can only be used in a semi-quantitative sense because of geodetic deficiencies in his base map (Reference ThoroddsenThoroddsen 1901), which is a modification of the 1844 Gunnlaugsson map.

In 1930, Jón Eythórsson began a systematic program of annua] measurements of the fluctuation of termini of selected glaciers in Iceland (Reference EythórssonEythórsson, 1963). Since Eythórsson’s death in 1968, the annual measurements have been supervised by Sigurjón Rist (Reference Rist1967b, Reference Rist1977, Reference Rist1983). During the International Hydrological Decade (IHD) (1965–1974), glacier termini, measured on an annual basis, were assigned the numbers shown on Figure 1. At the present time, Rist and his colleagues monitor 34 termini. Eythórsson’s and Rist’s work provides a continuous record of fluctuation of selected glacier termini for more than 50 years, in some cases, and provides an important bridge between the discontinuous record from maps, aerial photographs, and satellite images. Also during the 1930s, Eythórsson (Reference Eythórsson1931, Reference Eythórsson1935), Reference EirikssonEiriksson (1932), and Reference BárdarsonBárdarson (1934) prepared glacier-inventory information on a few glaciers in Iceland.

Reference ThorarinssonThorarinsson (1943) used 1:50 000 scale and 1:100 000 scale Danish Geodetic Institute maps in compiling a comprehensive inventory of Iceland’s glaciers and also published new areas of the 13 largest ice caps in Iceland, from later revisions to the 1:100 000 scale Danish maps (Reference Th[orarinsson]Thorarinsson 1958).

Reference BjörnssonBjörnsson (1974) discussed the information needs for a glacier inventory of Iceland on the basis of various sources of data and briefly reviewed previous inventories of Iceland. Reference BjörnssonBjörnsson (1980a) measured the area of ice caps on Landsat images and used aerial photographs, acquired by the U.S. Air Force in 1960, to measure the areas of Drangajökull, Snaefellsjökull, and 115 glaciers in the Tröllaskagi Group. He added areas of other glaciers (˜20 km2) for a total of 11 260 km2. Reference WilliamsWilliams (1983) summarized information about the changing areas of the 13 largest ice caps in Iceland (the same 13 as discussed by Reference Th[orarinsson]Thorarinsson (1958)).

Table I gives a summary of the total area of Iceland covered by glaciers, from various sources. Although all of these are based on the best available maps at the time, or from vertical aerial photographs and satellite images, it is clear that each source of data has deficiencies that limit its usefulness in preparing a comprehensive and accurate inventory of Iceland’s glaciers. Because of serious deficiencies in maps of glaciers of Iceland prior to 1903, data from such maps can only be used in a qualitative sense.

Table I

Area of Iceland Covered By Glaciers (from published and unpublished sources)

Evaluation of Sources of Data for Glacier Inventories

Maps

Table II summarizes maps of Iceland, published in the 20th century, that provide qualitative (pre-1905) and quantitative (post-1904) information needed to compile an inventory of Iceland’s glaciers. Both the 1:100 000 scale Atlas sheets (Atlasblöd) and the 1:50 000 scale Quarter sheets (Fjórdungsblöd) contain glacier-inventory information but have several deficiencies. According to Reference NørlundNørlund (1944), the Danish General Staff started preparations for the mapping of Iceland in 1900, with the first 1:50 000 scale map of a glacier (south-eastern Vatnajökull) surveyed in 1903. By 1920, the Danish General Staff had worked clockwise around the coast to the western part of the Tröllaskagi peninsula. In 1930, the Danish Geodetic Survey started up the mapping again, reaching the south-east coast in 1936. The central part of Iceland was mapped by a combination of field surveys and oblique aerial photo-grammetry from aerial photographs acquired in 1937 and 1938 (Reference NorlundNørlund 1938). Figure 2 shows the course of the 37-year mapping program by the Danes. The 36-year span of the surveys of the glaciers of Iceland limits the use of these maps for glacier-inventory work for two reasons: (1) country-wide, time-restrictive comparisons of glacier area are impossible because glaciers were mapped at various times during the period, which was at a time of rapid recession of Iceland’s glaciers, because of the climatic warming (Reference ThoroddsenThoroddsen 1916–17, Reference EythórssonEythórsson 1949, Reference Sigbjarnarson and EinarssonSigbjarnarson 1969, Central Intelligence Agency 1974, Reference BergthorssonBergthorsson 1985), and (2) Iceland’s largest ice caps were mapped piecemeal (for example, Vatnajökull from 1903 to 1939, Myrdalsjökull from 1904 to 1938). Another deficiency is in the plane-table survey method. Topographic contours and other features, such as lakes, are sketched in by the surveyor between stadia-rod positions. Geodetic monuments and summit elevations, however, are generally surveyed accurately. The determination of glacier margins from field observations or analysis of oblique aerial photographs (1937 and 1938) is necessarily subjective and depends on the ex-perience of the surveyor, rodman and cartographer.

Table II

Maps and Map Series Used to Compile Inventories of Iceland’s Glaciers

Fig. 2

Index map to the 87 1:100 000 scale Atlas Sheets (Atlasbiöd) (light solid lines) and 118 1:50 000 scale Quarter Sheets (Fjórdungsblöd) (dashed lines) of Iceland, mapped between 1902 and 1939 by the Danish General Staff (1900–1920), the Danish Geodetic Institute (1930–1944), and the Iceland Geodetic Survey (1944 to present) and the years in which field surveys were carried out (heavy solid lines). Modified from Reference NørlundNørlund (1944).

Those in the AMS Series C762 are the most accurate large scale maps available of the glaciers of Iceland, but several deficiencies exist for glacier-inventory work: (I) variety of compilation methods, (2) lack of ground control, and (3) time of year when surveys were made. The AMS Series C762 maps of Iceland’s glaciers were compiled by photogrammetric (multiplex) methods, but each map has been compiled by one or more of the following methods: photo-stereo, photo-planimetric, Danish Atlas or Quarter sheets. The Danish maps were used where no aerial photographs were available (cloud cover) or as a base for planimetrie revisions from aerial photographs, where no stereo coverage was available. Ground control for the maps was based on bench marks shown on the Danish maps and identified on the photographs. Some of the aerial photographs were acquired in late September or October, when new snow cover masked the glaciers and surrounding terrain.

The AMS Series C762 maps have considerably more topographic and geomorphic information than the Danish maps. This is important in properly positioning the 1945–1946 and more recent aerial photographs to obtain additional glaciological information and to make measurements of marginal fluctuations. The AMS Series C762 maps also show the transient snow line on the glaciers. The greatest value of aerial photographs is in their precise historical record of a glacier on a specific date.

Aerial Photographs

The most valuable sets of vertical aerial photographs of Iceland’s glaciers are the two flown by the United States military in support of the two 1:50 000 scale topographic map series of Iceland. The 1945 and 1946 aerial surveys by the U.S. Army Air Force provide nearly complete coverage of Iceland, except where cloudy weather prevented aerial photography. These photographs are the basis for the 1:50 000 scale AMS Series C762 maps of Iceland. The 1956 (no glaciers surveyed) and 1959–61 aerial photographic surveys by the U.S. Air Force also provide good coverage of the glaciers of Iceland, except for parts of Vatnajökull (persistent cloud cover). These photographs were to be the basis for a new 1:50 000 scale set of maps of Iceland (AMS Series C76I), but only 11 maps of southwestern Iceland, in cooperation with the Iceland Geodetic Survey, have been completed. All other sets of aerial photographs, German (early 1930s (Reference IwanIwan 1935) and 1942 (Reference BragasonBragason 1985)), Danish (1937 and 1938 (Reference NorlundNørlund 1938, Reference Nørlund1944)), Icelandic (1954 to the present (Iceland National Research Council 1976; Reference BragasonThorvaldur Bragason, written communication, 1985)), U.S. Air Force Cambridge Research Laboratories (1968). U.S. Navy (1973), and National Aeronautics and Space Administration (1968 and 1973)), provide only limited coverage of Iceland’s glaciers.

Satellite Images

A complete set of optimum Landsat 1, 2, and 3 multispectral scanner (MSS) and Landsat 3 return beam vidicon (RBV) images of Iceland is available for glaciological studies. The optimum images have been gleaned from hundreds of Landsat images of Iceland, the best of which have been catalogued by Gudbergsson and Williams (unpublished). The advantages of Landsat images lie in the large area coverage, important in several ways for glaciological studies (Reference Krimmel and MeierKrimmel and Meier 1975). Some of the advantages for glacier inventories of Iceland are as follows: (1) entire glaciers or groups of glaciers of Iceland can be imaged in a single scene, (2) fluctuations in margins of glaciers can be measured, (3) areas of glaciers can be determined, (4) transient snow line positions can be delineated, and (5) each image provides a historical record on a precise date (Reference WilliamsWilliams, 1979, Reference Williams1983, Reference Thorarinsson, Sæmundsson and WilliamsWilliams and Thorarinsson 1974, Reference Thorarinsson, Sæmundsson and WilliamsWilliams and others 1974, Reference Williams, Bödvarsson, Rist, Sæmundsson and Thorarinsson1975, Reference Williams1979). The main disadvantages in using satellite images for glacier inventories are (1) time of acquisition not optimum to record minimum snow cover (end of ablation season), (2) too much cloud cover, and (3) difficulty in determining the actual margin of the debris-covered glacier termini.

The delineation of the late summer snow line on Landsat images of Icelandic ice caps has important glaciological value, particularly if images permit mass-balance characteristics to be inferred (Reference ØstremØstrem 1975). The accumulation area ratio (AAR) for Vatnajökull on the 22 September 1973 image was 0,7, while the AAR for Mýrdalsjökull on the same image was only 0.35. This may suggest that Vatnajökull was in equilibrium during 1972–73, but that Mýrdalsjökull had a negative mass balance during the same period, (c.f. Reference GlenGlen, 1963).

Related Data

Very few mass-balance studies have been conducted of Icelandic glaciers. Those that have been accomplished have only a short time series, such as the 1936–38 studies of Hoffellsjökull, an outlet glacier of Vatnajökull in southeast Iceland (Reference AhlmannAhlmann 1939, Reference ThorarinssonThorarinsson 1939) and the 1967–68 studies by Reference BjörnssonBjörnsson (1972) of the cirque glacier, Baegisárjökull, in the Tröllaskagi Group.

During the 1950s, seismic surveys were used to determine the thickness of parts of Vatnajökull (Reference EythórssonEythórsson 1951, Reference Eythórsson1952) and Myrdalsjökull (Reference RistRist 1967a). In 1975, Reference BjörnssonBjörnsson (1977) initiated a ground-based series of radio echo-sounding surveys of the major ice caps of Iceland. Parts of Myrdalsjökull and Vatnajökull have been completed (Reference BjörnssonBjörnsson 1978), as well as Hofsjökull (Helgi Björnsson, personal communication).

Changes in volume of Iceland’s glaciers can be calculated directly from measurement of changes in thickness, or inferred from discrepancies in measurements of precipitation and runoff (Reference SigbjarnarsonSigbjarnarson 1967, Reference Sigbjarnarson1971). Changes in thickness can be determined by radio echo-sounding, conventional ground traverses (Reference FreysteinssonFreysteinsson 1984), or aerial photo-grammetry.

Use of Data Sources for Glacier Inventories

LangjökuII Group

The LangjökuII Group is a group of nine glaciers situated in the west-central part of Iceland (Fig.1.). The following discussion of the various types of cartographic, image, and other data used to describe this group, will illuminate some of the problems associated with compiling a glacier inventory of Iceland and in merging information from different sets of data. Figure 3 shows the geographic names of 5 of the glaciers in the group that have been used on Icelandic maps and literature (for example, Reference ThoroddsenThoroddsen 1911, Reference MatthiassonMatthiasson 1980). The outlines of the five glaciers are drawn from the 28 August 1980 image. Table III provides a review of the mapping of the eight glaciers in the LangjökuII Group, from the Danish Atlas/Quarter sheets to the 28 August 1980 Landsat image of the region.

Table III

Langjök.Ull Group. Area in Square Kilometers of the Nine Glaciers in the Group From Historic and Modern Maps of Iceland

Fig. 3

Index map to the 5 principal glaciers (ice caps) in the LangjökuII Group. Historic and modern geographic place-names of the glaciers, margins of Langjökull, and other features, such as nunataks (Thursaborg), have been culled from various published Icelandic sources. Outlines of glaciers from Landsat MSS image (22045–12131) of 28 August 1980.

Langjökull

Figure 4 is an overlay of maps of LangjökuII from three different sources: Danish maps, AMS maps, and 19 August 1973 Landsat MSS image. The greatest changes between the glacier margins are in the recession of the Thristapajökull, Sudurjökull, Hagafellsjökull eystri, and Hagafellsjökull ytri outlet glaciers. The positions of the termini of the last two outlet glaciers in the mid-1930s, mid-1940s, and 1973 are consistent with field reports of fluctuations (Reference SigbjarnarsonSigbjarnarson 1967, Reference RistRist 1974). Figure 5 is an overlay of two maps of LangjökuII based on two Landsat images, 19 August 1973 and 28 August 1980, showing the advance of Hagafellsjökull ytri resulting from its 1980 surge (Reference TheódórssonTheódórsson 1981) and the major advance of Hagafellsjökull eystri, resulting from two surges, one in 1975 (Reference SigbjarnarsonSigbjarnarson 1977) and the other in 1980. The map positions of the termini on the Landsat images are consistent (±100 m) with the 600 m surge of Hagafellsjökull

Fig. 4

Outline of LangjökuII, the principal glacier in the LangjökuII Group, from two map series and a 19 August 1973 Landsat MSS image, indicating that the fluctuation in the margin of the ice cap has been mainly in the recession of the outlet glaciers, Thristapajökull, Sudurjökull, Hagafellsjökull eystri and Hagafellsjökull ytri (see Fig.3.).

Fig. 5.

Outtine of Langjökull, from two Landsat MSS images: 19 August 1973 and 28 August 1980. Note especially the 2.2 km advance of the terminus of Hagafellsjökull eystri and the lesser (about 0.5 km) advance of Hagafellsjökull ytri, during this period (see Fig.3.). The transient snow lines of 19 August 1973 and 28 August 1980 are also indicated, yielding an Accumulation Area Ratio (AAR) of 0,70 and 0.65, respectively.

ytri and the combined 2200 m surge of Hagafellsjökull eystri into lake Hagavatn, from which it had retreated about 2600 m between the late 1930s and late 1973.

Figure 5 also shows the transient snow lines on the 19 August and 28 August 1980 Landsat images. Although not quite at the end of the summer melt period, the accumulation area ratio (AAR) was calculated to be 0.78 for the 1973 image and 0.65 for the 1980 image.

Ok

Figure 6 shows the margins of the glacier on Ok, during four stages in its recession to about 4 km2 in 1978 from 15 km2 in 1910. Figure 7 is a graph of the declining area of the glacier on Ok, showing a rapid decline in area until about 1960 and a diminishing decline since that time. The 28 August 1980 Landsat image of the glacier shows approximately the same size as depicted on the 5 September 1978 aerial photograph, If the wastage of the glacier on Ok had continued at the same rate as between 1910 and 1950, it would have completely disappeared by about 1980.

Fig. 6.

Outlines of the glacier on the lava shield, Ok, from two map series and two vertical aerial photographs, for a 68-year period (1910 to 1978) οf recession (see Table IV), From 1910 to 1945, the ice cap was reduced in area by 8.2 km2 or to 55 % of its 1910 area. The loss in volume of ice, during this 35-year period, is estimated at 0.6 km3, from comparison of the two topographic maps.

Table IV

Area of Glacier On Ok From Historic and Modern Sources

The glacier on Ok is one of only 5 glaciers in Iceland, for which the 1:50 000 scale Quarter sheet plane-table topographic maps can be compared with 1:50 000 scale AMS Series C762 aerial photogrammetric topographic maps, to show reduction in volume during a 30- to 40-year interval. The others are Eyjafjallajökull, Tindfjallajökull, Snaefelfsjökull, and Drangajökull. The 1910 Quarter sheet, 36 Botnsheidi N.A., shows the maximum elevation on the ice cap of 1198 m, with the summit crater of the lava shield Ok not visible. The 1945 AMS Series C762 map, Thórisjökull, shows a maximum elevation on the ice cap of 1127 m or a reduction of 71 m. When the elevation differences are computed for the two maps, for that part of the ice cap shown on the 36 Botnsheidi N.A. map (72.2 per cent of the glacier mapped in 1910), the glacier was found to be reduced by 0.45 km3 in volume. Assuming a reasonable symmetry to the ice cap, the loss from the full 15 km2 in 1910 to the 6.8 km2 in 1945 is 0.62 km3.

Fig. 7.

Graph, showing the decrease in area of the glacier on Ok between 1910 and 1978. The recession was most rapid between 1910 and about 1960, after which it slowed appreciably; otherwise, the glacier would have disappeared by about 1980. The earlier two values of area are from maps (crosses), the latter from aerial photographs (triangles).

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Figure 0

Fig. 1Index map to the 8 groups of glaciers included in an inventory of Iceland’s glaciers. The drainage divides between the 12 sectors of the Hydrological Network of Iceland are shown with a dashed line (Iceland National Energy Authority, 1972). The 34 numbers associated with many of the glaciers are International Hydrological Decade (IHD) index numbers assigned by Rist (1967 and 1977) to margins of glaciers monitored annually.

Figure 1

Table IArea of Iceland Covered By Glaciers (from published and unpublished sources)

Figure 2

Table IIMaps and Map Series Used to Compile Inventories of Iceland’s Glaciers

Figure 3

Fig. 2Index map to the 87 1:100 000 scale Atlas Sheets (Atlasbiöd) (light solid lines) and 118 1:50 000 scale Quarter Sheets (Fjórdungsblöd) (dashed lines) of Iceland, mapped between 1902 and 1939 by the Danish General Staff (1900–1920), the Danish Geodetic Institute (1930–1944), and the Iceland Geodetic Survey (1944 to present) and the years in which field surveys were carried out (heavy solid lines). Modified from Nørlund (1944).

Figure 4

Table IIILangjök.Ull Group. Area in Square Kilometers of the Nine Glaciers in the Group From Historic and Modern Maps of Iceland

Figure 5

Fig. 3Index map to the 5 principal glaciers (ice caps) in the LangjökuII Group. Historic and modern geographic place-names of the glaciers, margins of Langjökull, and other features, such as nunataks (Thursaborg), have been culled from various published Icelandic sources. Outlines of glaciers from Landsat MSS image (22045–12131) of 28 August 1980.

Figure 6

Fig. 4Outline of LangjökuII, the principal glacier in the LangjökuII Group, from two map series and a 19 August 1973 Landsat MSS image, indicating that the fluctuation in the margin of the ice cap has been mainly in the recession of the outlet glaciers, Thristapajökull, Sudurjökull, Hagafellsjökull eystri and Hagafellsjökull ytri (see Fig.3.).

Figure 7

Fig. 5. Outtine of Langjökull, from two Landsat MSS images: 19 August 1973 and 28 August 1980. Note especially the 2.2 km advance of the terminus of Hagafellsjökull eystri and the lesser (about 0.5 km) advance of Hagafellsjökull ytri, during this period (see Fig.3.). The transient snow lines of 19 August 1973 and 28 August 1980 are also indicated, yielding an Accumulation Area Ratio (AAR) of 0,70 and 0.65, respectively.

Figure 8

Fig. 6.Outlines of the glacier on the lava shield, Ok, from two map series and two vertical aerial photographs, for a 68-year period (1910 to 1978) οf recession (see Table IV), From 1910 to 1945, the ice cap was reduced in area by 8.2 km2 or to 55 % of its 1910 area. The loss in volume of ice, during this 35-year period, is estimated at 0.6 km3, from comparison of the two topographic maps.

Figure 9

Table IVArea of Glacier On Ok From Historic and Modern Sources

Figure 10

Fig. 7.Graph, showing the decrease in area of the glacier on Ok between 1910 and 1978. The recession was most rapid between 1910 and about 1960, after which it slowed appreciably; otherwise, the glacier would have disappeared by about 1980. The earlier two values of area are from maps (crosses), the latter from aerial photographs (triangles).