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Brenva Glacier, Mont Blanc Massif, Italy) Using Indirect Sources: Methods, Results and Validation ⁎ Carlo D'agata A, , Antonio Zanutta B

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Brenva Glacier, Mont Blanc Massif, Italy) Using Indirect Sources: Methods, Results and Validation ⁎ Carlo D'agata A, , Antonio Zanutta B Global and Planetary Change 56 (2007) 57–68 www.elsevier.com/locate/gloplacha Reconstruction of the recent changes of a debris-covered glacier (Brenva Glacier, Mont Blanc Massif, Italy) using indirect sources: Methods, results and validation ⁎ Carlo D'Agata a, , Antonio Zanutta b a “Ardito Desio” Earth Science Department, University of Milan, Via Mangiagalli, 34, 20133 Milan, Italy b DISTART, Faculty of Engineering, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Received 21 September 2005; accepted 21 July 2006 Available online 5 October 2006 Abstract A quantitative analysis was performed with the aim of identifying changes in the volume and thickness of the Brenva Glacier tongue (Mont Blanc Massif, Italy) in the second half of the 20th century. This analysis was based on the comparison of digital elevation models (DEMs) derived from historical records, specifically maps (1959, 1971, 1983, 2003) and photogrammetric surveys (1991, 1997). The DEMs were generated by means of a digital photogrammetric workstation, with semi-automatic and automatic procedures. Problems relating to the identification of the control points in the photos had to be resolved in order to define the external orientation. An unconventional photogrammetric methodology, based on the identification of homologous points in zones considered outside of the glacier area, was adopted to insert the surveys into a single reference system. Furthermore, along with the photogrammetric data, DEMs derived from digitized historical maps were generated and compared to define changes in the geometry of the glacier tongue. The results indicated a positive long-term glacier tongue balance. In fact, between 1959 and 2003, there was an increase of 22.6×106 m3, equal to an average thickness of ca.+34 m (+0.7 m a−1 w.e.). Validation of the data obtained from comparison of the DEMs and the reliability of the results were discussed as well. © 2006 Elsevier B.V. All rights reserved. Keywords: digital elevation model; accuracy; debris-covered glacier; Mont Blanc Massif; recent glacier changes 1. Introduction and study area are only a few examples of such glaciers in the Alps, but they have increased in number in recent years, with the Debris-covered glaciers comprise a significant fraction widening and thickening of the debris cover in the lower of the global population of glaciers (Nakawo et al., 2000) part of the ablation zone of many Alpine glaciers. and are widespread in the mountain chains of Asia, such The current warming period and the consequent as the Karakoram, the Himalaya and the Tien Shan; they delaciation have heightened mass wasting and macro- are also common in New Zealand and the Andes. There gelivation processes, which deliver larger volumes of debris to the glacier surface. Therefore, the study of the recent evolution of debris-covered glaciers is of global ⁎ Corresponding author. concern to gain a better understanding of this type of E-mail address: [email protected] (C. D'Agata). glacier, which will probably become a more frequent 0921-8181/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2006.07.021 58 C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68 Fig. 1. The debris-covered tongue of the Brenva Glacier. phenomenon in the Alps in the near future. In Italy, we its terminus reaches 1415 m a.s.l., the lowest glacier find only a few examples of such glaciers. Brenva altitude in the Italian Alps. The tongue, which is ex- Glacier (on the Italian side of the Mont Blanc Massif) is tensively mantled with granitic debris mainly deriving one of the best known (Figs. 1 and 2). It is a valley from rock avalanches (1920 and 1997), has been com- glacier that is about 7 km2 wide and about 6.7 km long; pletely separated from the upper accumulation basin Fig. 2. Brenva Glacier location. C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68 59 Fig. 3. A view of orthophoto of the Brenva Glacier terminus (copyright of Regione Autonoma Valle d'Aosta, aerial flight of August 9, 1991. Italian Ministry of Defense and Aeronautics, permission no. 1116 of November 20, 1991. Flight by Compagnia Generale Riprese Aeree S.p.A.-Parma). since 2004. The glacier's terminus fluctuations and its 2.1. Digital elevation models obtained from historical departures from the general trend of Alpine glaciers are maps well known (Orombelli and Porter, 1982; Cerutti, 1992), while the tongue thickness and volume variations re- The following historical maps were used: main relatively unknown (D'Agata et al., 2005). The aims of this paper are: 1) The 1959 map, scale 1:5000, by EIRA (Ente Italiano Rilievi Aerofotogrammetrici), from a terrestrial pho- – to present the variations in the volume and thickness togrammetric survey, published in the Bollettino del of the glacier tongue in the last half century, as Comitato Glaciologico Italiano, II, 19, 1971 (the obtained by processing and comparing large-scale glacier tongue only; Capello, 1971); maps and aerial photographs by GIS; – to discuss the methods applied and result reliability. 2. Methods and data sources Historical maps and aerial photographs are of fundamental importance not only for qualitative analyses of land areas, but also for quantitative assessments. In fact, recent advances in information technology have led to the development of automated digital photogramme- try techniques, allowing for rapid and cost-effective data collection. Within the framework of this study, two indirect survey methods (photogrammetry and digitizing maps) were used to evaluate the recent evolution of Brenva glacier. More specifically, four maps and two aerial stereo- Fig. 4. Three-dimensional view of the Brenva Glacier tongue. It is pairs were employed, covering time intervals of about possible to observe the debris-covered tongue and the small ice 10 years, from 1959 to 2003. connection with the upper basin. 60 C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68 2) The 1971 map, scale 1:5000, from aerial photogram- 4) The 2005 map, scale 1:5000, by the Aosta Valley metry (scale 1:10,000) by Alifoto (Turin), published Region (Carta Tecnica Regionale–Regional Techni- in the Bollettino del Comitato Glaciologico Italiano, cal Map) from a 2003 aerial photogrammetric survey II, 20, 1972 (the glacier tongue only; Lesca, 1972); (scale 17,000). 3) The 1988 map, scale 1:10,000, by the Aosta Valley Region (Carta Tecnica Regionale–Regional Techni- The data sources were digitized using a scanner for cal Map) from a 1983 aerial photogrammetric survey the hard copy maps (GTCO CalComp ScanPlus III S3- (scale 1:13.000); 400T, 600 dpi resolution). Only the 1988 and 2005 maps Fig. 5. (A) Changes in surface elevation on the tongue of Brenva Glacier, 1959–1971. The white line indicates the perimeter in 1959; the outer line indicates the perimeter in 1971. (B) Changes in surface elevation on the tongue of Brenva Glacier, 1971–1983. The light grey line indicates the perimeter in 1971; the dark grey line indicates the perimeter in 1983. (C) Changes in surface elevation on the tongue of Brenva Glacier, 1983–1991. The light grey line indicates the perimeter in 1991; the black line indicates the perimeter in 1983. (D) Changes in surface elevation on the tongue of Brenva Glacier, 1991– 1997. The perimeters in 1991 and 1997 (the outer line) are practically identical. (E) Changes in surface elevation on the tongue of Brenva Glacier, 1997– 2003. The dark grey line indicates the perimeter in 2003. The outer line indicates the perimeter in 1997. (F) Changes in surface elevation on the tongue of Brenva Glacier, 1959–2003. The black line indicates the perimeter in 1959. The outer line indicates the perimeter in 2003. C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68 61 Fig. 5 (continued). were digital versions (for these two maps the high each different map scale, along with a constant length resolution was guaranteed by the map publisher). The between digitizing points. In addition, we preferred to 2005 version was already in vector format. use manual on-screen vectorization, rather than digitizer The raster maps were then georeferenced by using table vectorization, because it permitted direct manage- specific software (ENVI 3.2). All the maps were geo- ment in GIS environment, utilisation of semi-automatic referenced using the UTM grid coordinate system points vectorization and direct access to the virtual reality by existing on the maps. Then the contour lines on the maps overlaying (o: overlaying) the maps on the DEM. were digitized as polylines in a semi-automatic pro- DEMs generated by measuring terrain points along cedure by an operator using GIS software (AUTOCAD contour lines were interpolated with a contour-specific MAP 2002 with the Raster Design version). The irre- algorithm to attempt to specify the topological and mor- gularly shaped curves were approximated by straight- phological properties of contour lines. line segments connecting the acquired points. We used The volume and thickness variations of the glacier the highest possible density of registration points for were quantified by comparing the DEMs. Thematic maps 62 C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68 Fig. 5 (continued). of the variations and longitudinal and transversal profiles The 1991 and 1997 stereopairs were digitized using a were also processed. photogrammetric scanner (RasterMaster photogrammet- ric scanner at a resolution of 2116 dpi, equal to a pixel 2.2. Digital elevation models obtained from aerial size of 12 μm, corresponding to approximately 20 cm on photographs the ground), sufficient to ensure a high level of detail. A digital photogrammetric workstation (StereoView The following aerial photographs were used: Suite, Menci Software, Arezzo, Italy) was adopted for the generation of the digital elevation models, using the 1) The 1991 stereopairs, CGR (Compagnia Generale semiautomatic and automatic modes.
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