Hasholt, Bent: A new map of the Mitdluagkat glacier - a preliminary
report. Geografisk Tidsskrift 87: 19-21, Copenhagen June
1987.

A new map covering the whole Mitdluagkat Glacier has been elaborated. The map is based on air photos from Geodetic Institute in scale 1:150,000 taken July 30, 1981. The scale of the new map is 1:20,000 with 10-m contour intervals. Preliminary field tests indicate that the map is so accurate that it might be used for comparative studies of long-term variations in mass balance of the glacier.

Bent Hasholt, M.Sc., associate professor, Institute of Geography,
Øster Voldgade 10, DK-1350 Copenhagen K.

Keywords: Mapping, glaciology, air photos.

The terminus of the Mitdluagkat Glacier has been observed since 1933. Maps from Geodetic Institute (1:250,000 and 1:50,000 surveyed 1932-33 and 1943, respectively) exist. A detailed map based on air photos from 1972 has been evaluated together with the maps from Geodetic Institute by Hasholt (1986).

The evaluation showed several differences between the maps with contradicting consequences for the computation of a mass balance of the glacier. It is most likely that some of the discrepancies are due to errors in the old maps from Geodetic Institute.

Another difficulty is that the map showing the 1972-situation is not covering the whole glacier, partly due to inadequate overlap between air photos and partly because of lack of photos of the southern part of the glacier.

There was therefore a need for a map covering the whole glacier so that a hypsographic curve could be drawn to facilitate studies of precipitation distribution and of mass balance. Part of the glacier was photographed by

Aerokort 1980, but the photos covered only the southeasternpart
of the glacier and could therefore not be used.

These years Geodetic Institute is mapping the northern and eastern part og Greenland on the basis of aerial photography. Photos covering the Ammassalik Island were taken 1981, and it was obvious to try to use these photos for elaboration of a more detailed map of the glacier.

Purpose

The purpose of this investigation was to elaborate a map covering the whole glacier using the aerial photos from 1981, which are available in a rather smalle scale (1:150,000), however. It was therefore also part of the purpose to evaluate how detailed a map it is possible and meaningful to elaborate. The new map could also indicate the reliability of the 1972 map compared to the old maps from Geodetic Institute. To facilitate studies of the water balance in the area it was also the intention to find a more exact location of the watersheds on or near the glacier.

Elaboration of the map

It was first planned to use the Wild B8 at the Institute of Geography in Copenhagen for the map-drawing. The optics of the Wild B8 could not cope with the wide angle photos from 1981, however. Therefore the firm Aerokort A/S, which owns a Zeiss Planicomp, was contacted; it was agreed upon that it was realistic to try to draw in a map scale of 1:20,000 with contour intervals of 10 meters.

The aerial photos from Geodetic Institute to be used were: 878 G 329 and 878 G 331 taken July 30, 1981. The main problem was, however, that there were no triangulation points or other signals visible on the 1981 aerial photos. It was therefore not possible to orientate the photos in a stereoscopic model immediately. To facilitate comparisons with the map based on the 1972 aerial photos, it was decided that the triangulation points from 1972 should be included in the new map. It was first tried to transfer these to the 1981 photos manually by identification of characteristic landforms near the fixed points. A test computation showed, however, that this method was not sufficiently accurate.

The next step was to use the 1972 photos on which the triangulation points are visible, to create new points by an aerotrigulation. Furthermore, due to the favourable location of the glacier near the sea, it was possible also to use the sea-level for Orientalin of the stereoscopic model. The results of the aerotriangulation and the transfer of points are shown in table 1 together with a theoretical evaluation of the accuracy.

The map was drawn on the Zeiss Planicomp by cartographer J. Johansen at Aerokort. The author helped to define borders of the glacier and interpret watersheds, watercourses and lakes. The map is shown in black and

white in plate 1. To faciliate the interpretation also a coloured version of this map was drawn with contours in green, border of the glacier in black, watersheds in red and water in blue.

A preliminary evaluation of the map.

In August 1986 the author was head of an expedition to the Mitdluagkat Glacier; one of the aims was to establish triangulation points in solid-rock near the upper part of the glacier to facilitate future computations of mass balance by aerial photography and to evaluate the maps based on the 1972- and 1981-photos. Measurements were carried out with a Kern T2 Theodolite and a DM 502 electro-optical distancemeter from a nunatak situated 515 m above sea-level and from a peak on the watershed south of the glacier. Unfortunately, we were not able to connect the new triangulation points with the triangulation network at the coast because of bad weather (a piteraq, a gale with wind speeds from 32-60 m/sec.). It is therefore only possible to compare relative differences in height. On the map the approximate position can be identified of the westernmost top of the nunataks giving name to the glacier (Mitdluagkat meaning the breasts), the peak at the watershed and the nunatak 515m above sealevel. The level of these points could be computed by use of trigonometric levelling relative to nunatak 515. Also a traverse across the glacier has been measured; this could, however, only be used as a rough estimate of the level on the map, because the surface of the glacier varies due to accumulation and ablation. The height difference between the 515 nuntak and the Mitdluagkat nunatak was 455.6 m found by trigonometric levelling, and by barometric levelling the value was found to be 465 m. The difference found from the map was 416.7 m. The difference between the 515 nunatak and the peak at the watershed was found to be 395 m by trigonometric levelling and 402 m from the map. The signifacant and non-systematic differences are thought to be caused by the fact that the cartographer has not been able to locate the highest points exactly in the stereoscopic model as the peaks have a very small horizontal extension, the traverse of the glacier shows that the deviations between the 1986 surface relative to the 515 nunatak and the surface on the new map from 1981 are 5-10 m.

The new map could also be compared with the map based on the photos from 1972. The base levels differ by 1.5-2.0 m so that the zero contour line on the new map lies approximately 1.9 m below the mean sea-level found by Hasholt (1986). The lakes situated along the western margin of the glacier could easily be compared on the two maps. The height difference between the water-surface levelsof the two lakes is 129.2 m on both maps. When a correction to mean sea-level is made, the water-level of the northernmost lake is 147.4 m a.s.l. on the 1972 map

and 146.1 on the 1981 map. The locations of lakes and comparable nunataks and rock outcrops have the same coordinates, which indicates that there is no distortion in the x-y plane.

As the threshold of the two lakes have been the same both in 1972 and in 1981, the water-level of the lakes could be used as a relative base-level, when the two maps are used for comparative studies of the glacier surface.

Finally, it is possible to compare the south-eastern part of the new map with a map drawn by Aerokort for GTO on the basis of aerial photos from 1980. Mean sea-level was used as base level in the 1980 map. When differences

in base-level on the two maps are taken into account, the water-levels of the lakes differ only 0.5-1.1 m. A comparison between peak levels on the two maps also shows good accordance. This comparison indicates that, although the fixed points only are available in the westernmost part of the new map, no serious distortion of the height scale is present in the map.

Summary and perspectives

A new map of the Mitdluagkat Glacier based on aerial photographs from 1981 in scale 1:20,000 has been elaborated. It is shown that it is possible to draw a fairly accurate map in the scale 1:20,000 with 10-m contour intervals although a very limited number of fixed points are at hand. This indicates that the aerial photos from Geodetic Institute in scale 1:150,000 could be used for elaboration of more detailed maps in other areas of scientific interest and for tourism purposes.

The new map represents a significant improvement of the knowledge of the Mitdluagkat Glacier. The good accordance with the 1972-map supports the hypothesis that there are significant errors in the topography on Geodetic Institute's old maps (from 1932-43). It is intended to make a more detailed evaluation of the map in the future; thus the triangulation points in the upper part should be connected with the points near the coast, and the level should be determined of the characteristic points that could be identified on the aerial photos. Such points will also be useful in case of future photo flights or availability of satellite scenes with high resolution. The glaciological and hydrological application of the new map will be described in a later report.

Acknowledgments

The Commision for Scientific Research in Greenland and the Danish Natural Science Research Council are thanked for supporting the project; also thanks to director O. Valbjørn and cartographer J. Johansen, Aerokort, for help and valuable discussions.

References

Geodetic Institute, aerial photographs 878 G 329 and 878 G 331,
July 30, i9Bi.

Greenland Technical Organization (GTO), map of Ammassalik
1:20,000.

Hasholt, B. (1986): Kortlægning af Mitdluagkat Gletscheren og
nogle hydro-glaciologiske observationer. Geografisk Tidsskrift
86: 9-16. Copenhagen, June 1986.