Measuring Small Glaciers in Slovenia

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Connecting Geodetic Measurements and Non-metric Imagery for Glacier Measurements

Measuring Small Glaciers in Slovenia

Figure 1, Triglav glacier in 1959 (a) and 2012 (b).

Small glaciers in the Alps and elsewhere are important indicators of short-term local, and even global, climatic changes. Monitoring them usually involves measurements of the terminus retreat, the reduction of area and volume, and the velocity of glacier movements. Measurements of glaciers began in the Eastern Alps as early as 1878, when the first measurements were conducted on the Pasterze glacier (Austria) using a meter band. Regular tachymetric measurements were introduced on the same glacier as early as in 1928. Long-term glacier measurements show not only glacier changes over time but also how geodetic measuring techniques have evolved from classical to remote sensing technologies.

While the new technologies enable more accurate measurements, they cannot cover the past glacier changes. This raises the important question of how to bring the old images into a common reference frame of new measurements. This article examines past and emerging approaches for glacial monitoring. The aim is to understand how these techniques and data might be integrated to support both historical and contemporary understanding of glacier changes.

Dr Mihaela Triglav-Cekada is a researcher at the Geodetic Institute of Slovenia. She is involved in national and international projects which deal with the application of different remote sensing methods in geodetic practice. She is especially interested in the study of ways to acquire 3D data from old non-metric images.

Slovenia is home to two very small glaciers, and these are the most south-eastern glaciers in the Alps. Regular monitoring has been conducted on them for more than 60 years. Both glaciers, the Triglav glacier (Figure 1) and the Skuta glacier (Figure 2), are located at relatively low altitudes; their lowest points lie at the altitudes of 2,400m and 2,020m respectively.

The Skuta glacier is an example of a small cirque glacier conserved in a very deep cirque, which protects the glacier by providing shade almost all year around. The Triglav glacier can be characterised as a small mountain

Dr Matija Zorn is a researcher at the Scientific Research Centre of the Slovenian Academy of Sciences and Arts.

He holds the position of assistant director of the Centre’s Anton Melik Geographical Institute and is also the head of the Institute’s Department of Physical Geography. matija.zorn@zrc-sazu.si

mihaela.triglav@gis.si

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By Mihaela Triglav-Čekada, Geodetic Institute of Slovenia, and Matija Zorn, Scientific Research Centre, Slovenian Academy of Sciences and Arts feature

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Figure 2, Skuta glacier in 1982 (a) and 2003 (b).

glacier in the form of ice apron.

Today, these two glaciers no longer display typical glacier characteristics such as glacier crevasses which result from the glacier’s movement on uneven ground. Therefore, they can be regarded as glacier remnants.

Crevasses were last observed on the Triglav glacier in 1955 and on the Skuta glacier in 1973. Then, the Triglav glacier measured over 10ha and the Skuta glacier a little less than 3ha. In comparison, at the end of September 2012, the Triglav glacier measured 0.6ha and the Skuta glacier 1.4ha (Figure 3).

Manual measurements and imaging

Systematic annual measurements of the Triglav and Skuta glaciers started in 1946. The measurements were conducted manually using meter band, rope and a compass to measure each glacier terminus’s distances from and directions towards permanent points marked on the rocks around the glaciers (Figure 4a).

From some permanent points, it was also possible to measure the vertical thinning of the glaciers.

During these expeditions, some non-metric images were also made

from various standpoints using different cameras. Over the years, these standpoints became fixed and are still used today for annual imaging of the glaciers at the end of the melting season.

An additional regular monthly imaging of the Triglav glacier started in 1976 using a fixed panoramic Horizont camera from two fixed standpoints. A panoramic camera was chosen as it enabled the whole glacier to be covered in one single photograph. Unfortunately, classical photogrammetric stereovision cannot be applied since the views of the glacier from these standpoints are convergent.

Geodetic measurements

The first geodetic tachymetric measurements on the Triglav glacier were made in 1952. The activity was not to be repeated until as late as 1995. Since the start of regular photogrammetric measurements in 1999, tachymetric measurements have been used for control and sometimes also for glacier boundary delineation.

In 2001, 2005 and 2012, global navigational satellite system (GNSS) measurements of control points were

taken on the Triglav glacier. Due to the non-existence of a permanent GNSS network in Slovenia in 2001, only the time-consuming static GPS method could be used for very accurate GPS measurements. This enabled the acquisition of just eight photogrammetric control points. The 2001 GPS measurements enabled previous local measurements to be transformed into the global coordinate system. With the establishment of a fully operating Slovenian permanent GNSS network in 2006 and the development of more accurate GPS instruments, highly accurate and less time-consuming RTK-GNSS measurements could be used to measure more points in a shorter time. Hence, the detailed glacier boundary was measured with GNSS measurements for the first time in 2012.

On the Skuta glacier, tachymetric measurements were first conducted in 1997 and again in 2003. The glacier has been tachymetrically measured annually since 2007. Because the GNSS signal is almost absent in the glacier’s very deep cirque, successful GNSS measurements of its boundary have not yet been may 2013 |

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Figure 3, The area reduction of the Triglav and Skuta glaciers, determined using different measuring methods.

Only those years are presented when the glaciers were not completely covered by snow from the previous winter.

realised. It is hoped that it will also become possible to obtain GNSS measurements in such demanding mountain environments when the Galileo system is fully operational.

Photogrammetry

Detailed photogrammetric measurements have been carried out on the Triglav glacier only. In 1999, 2001 and 2003, a combination of manual aerial photogrammetric imaging from a helicopter and oblique imaging from the ground was performed with the purpose of glacier surface and boundary delineation. The camera used was a medium-format photogrammetric camera, Rolleiflex 6006. In 2005, standard aerial photogrammetric imaging was conducted using a large-

format Leica RC 30 photogrammetric camera mounted on an aeroplane. One of the most important results of the 2005 photogrammetric measurements was the digital terrain model (DTM) with a 2m x 2m grid, which was later applied for archive imagery processing. Since 2007, tachymetric measurements with supplementary oblique photogrammetric imaging using the Rolleiflex 6006 have been made annually from the ground.

These too have been used mainly for glacier boundary delineation. Figure 4 shows the different types of control points used.

Both glaciers can also be seen on stereoimagery from the national Cyclic Aerial Survey of Slovenia (CAS). Since the 1970s, this survey has been carried out every 3 to 4 years by a large-format aerial photogrammetric camera. The CAS images are taken in image scales suitable for cartography of the scale 1:5,000. Because the CAS images are rarely captured at the end of the melting season, not all images featuring the glaciers are useful for glacier monitoring purposes. For the Triglav glacier, for instance, only old CAS imagery from the years 1975, 1992, 1994 and 1998 can be used.

Aerial laser scanning

Figure 4, Different control point signalisations used: aerial photogrammetry (a), terrestrial oblique photogrammetry (b), tape measurements (c) and aerial laser scanning (d).

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Aerial laser scanning (Lidar) and imaging for orthophoto production were performed on both glaciers twice in 2012 using a Riegl LM5600.

The first aerial laser scanning session was conducted in mid-May with the purpose of snow-depth determination.

The second was done in late

September, at the end of the glaciers’ melting season. A Digital Surface Model (DSM) of snow-covered terrain and a DTM with a 1m x 1m grid were produced from aerial laser scanning data. Both sets of aerial laser scanning data had an average laser point density of 8pt/m2. The difference between the DSM and DTM presents the detailed local snow distribution. This can be used to check the hypothesis that very small glaciers owe their existence mainly to the area’s topographic features, which locally enhance avalanches and wind drift.

Ground-penetrating radar

The real glacier thickness can be only measured by a ground-penetrating radar survey (GPRS). The maximal thickness of the Triglav glacier measured by GPRS was 9.5m in 2000.

Thanks to GNSS development in the last decade, as mentioned above, it will be possible in the future to repeat GPRS measurements on the same locations as measured in 2000.

The Skuta glacier has not yet been measured with GPRS. In 2006, its maximal thickness of 11.7m was measured using a steam ice drill.

3D data acquisition from archive imagery

As glaciers are a very prominent topographic feature, they can often be discovered in old mountaineering books and archives; old images, drawings and rare maps represent the only source of knowledge about the glaciers prior to the start of hand measurements in 1946. With the help of modern photogrammetric and aerial laser scanning DTMs, it is possible to obtain relatively good 3D glacier boundary information from this non-metric archive imagery. This has enabled the area of the Triglav glacier to be reconstructed until as far back as the 1880s, and in the case of the Skuta glacier as far back as around 1910.

Additionally, monthly variations of the Triglav glacier can be studied from 1976 onwards based on series of non-metric panoramic Horizont images.

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Conclusion

The combination of different geodetic and remote sensing methods enables reconstruction of the glacier area until as far back as 1952 for the Triglav glacier, and the 1970 for the Skuta glacier. The sets of geodetic/ photogrammetric data enable detailed surface and boundary measurements useful for the area and volume studies.

However, since these techniques are not enough for detailed long-term annual studies, manual measurements and non-metric imagery are applied every year too.

The advancements in aerial laser scanning and GNSS measurement in the last decade are enabling more accurate and detailed data acquisition in demanding mountain environments where glaciers exist.

Smaller GNSS instruments also reduce logistical problems and make glacier expeditions less expensive.

Consequently, with a more accurate DTM available, archive imagery can also give more detailed results presenting past stages of the glaciers. Hence, the advancement

More information - Gabrovec, M. 2008, Il ghiacciaio del Triglav (Slovenia) [‘The Triglav glacier’]. Ghiacciai nontani e cambiamenti climatici nell’ ultimo secolo. Terra glacilis – Edizione speciale, pp. 75-87.

- Triglav-Cekada, M., et al., 2011, Acquisition of the 3D boundary of the Triglav glacier from archived non-metric panoramic images, The Photogrammetric Record, 26/133, pp. 111-129.

- Triglav-Cekada, M., et al., 2012, Measurements of small alpine glaciers: Examples from Slovenia and Austria, Geodetski vestnik, 56/3, pp. 462-481.

of photogrammetric and GNSS measuring techniques is also enabling a more detailed look into the past.

Acknowledgement

This work has been supported by the Slovenian Research Agency (ARRS) through different projects. The latest projects are L6-7136, Z2-4182 and L6-4048.

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The comparison of meteorological data and long-term glacier monitoring enables research into worldwide climatic change. The studies of very small glaciers also enable a detailed look at the regional (e.g. comparison with glaciers of similar sizes in south-eastern Europe) and local

climatic trends (e.g. the Slovenian Environment Agency already uses such data as indicators of climate change). The geodetic and remote sensing methods enable not only the study of the current glacier status but also, combined with the use of archive imagery, a detailed look into the glacier’s past.

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