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

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 photogrammetry 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 photogrammetric 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. The DEMs were Riprese Aeree), flight RAVDA (Regione Autonoma first generated automatically, with post-editing by the Valle d'Aosta), scale 1:17,000; operator to correct errors deriving from the correlation 2) The 1997 stereopairs, CGR (Compagnia Generale procedure. Accurate ground control points (GCPs) were Riprese Aeree), flight RAVDA (Regione Autonoma available only for the 1991 flight. Therefore, to deter- Valle d'Aosta), scale 1:17,000. mine the external orientation of the 1997 model, an C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68 63 archival photogrammetric technique was defined, based absolute orientation. Residuals from external param- on the detection of homologous points located on the eter estimation that are always lower than 15 cm outcrops outside the glacier. demonstrate the overall good quality of photogram- The method consisted of the following steps: metric surveys. a) Identification of a set of ground points clearly defined All the photographic models were then registered in on the 1991 model; the ground control points were the UTM grid. The reference system adopted allowed kindly provided courtesy of the Regione Autonoma for comparison of historical maps and DEMs to detect Valle d'Aosta; multitemporal changes in the glacier tongue. The SV b) Measurement of the 3D coordinates of these points; OrthoPhoto module was also used in the orthophoto c) Using the 1991 control point coordinates, the exter- processing to obtain metric products and, at same time, nal orientation parameters of the 1997 model were realistic multitemporal photo visualizations of the area calculated. In order to detect and eliminate points (Figs. 3 and 4). possibly affected by errors, the procedure was re- The use of a GIS software package permitted manage- cursively applied by analyzing the residuals of the ment of the maps, DEMs and ortophotos, and thus, also

Fig. 6. (A) Brenva glacier tongue: location of longitudinal and cross profiles. (B) Longitudinal profiles. (C) Cross profiles. 64 C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68

Fig. 6 (continued). allowing for DEM comparisons and surface change (50–70 m). The average increase was ca. 23 m (equal to analysis. 1.7 m w.e. a− 1). In the 1983 and 1991 period, however, there was a 3. Results loss of volume amounting to ca. 8.2×106 m3 (considering a comparison surface area of 758.7×103 m2) with The comparison of all the DEMs obtained served to a considerable reduction in thickness, especially in the produce several thickness variation maps. upper and central sectors, where the negative variations For the first comparison period (1959–1971), the reached values of −40 and −50 m (Fig. 5C). Thickness isovariation map (Fig. 5A) clearly shows an increase decreased on the snout (maximum value: −20 m). The of volume over a period lasting slightly over 10 years average variation in thickness was ca. −11 m (equal to (ca. 15.3×106 m3 considering a comparison surface −1.3 m a− 1 w.e.). area of 663.8×103 m2). The increase affected the en- The reduction in volume affecting the Brenva Glacier tire length of the ablation tongue, with peaks of 40– tongue continued on through the 1991–1997 period 50 m concentrated on the snout and in the middle (−9.3×106 m3, considering a comparison surface area sector of the tongue (especially on the right hydro- of 758.7×103 m2). The average variation in thickness graphical side). Most of the tongue area, ca. 48%, was −12 m (equal to −1.7 m a− 1w.e.). underwent an increase of 20–30 m in thickness, The isovariation map (Fig. 5D) illustrates the whereas on the outer side boundaries of the glacier and distribution of the thickness reductions, which reached in the areas at about 1750 m in altitude, the increases values of −20 and −10 m, with peak losses exceeding were less substantial (between 0 and 20 m). The −50 m. The avalanche of January 18, 1997 spread 1– average thickness variation was ca. +23 m (equal to + 1.5×106 m3 of debris over an area 700,000 m2 wide, 1.8 m w.e.a− 1). equal to a mean thickness of 1–1.5 m (Barla et al., 2000; During the 1971–1983 period, the Brenva glacier Deline, 2002). Note that the 1997 map was produced tongue experienced a positive volumetric change as well starting from the aerial photo of August 31, 1997. (ca. 17.8×106 m3 considering a comparison surface Therefore, the volume and thickness decreases cited area of 787.2×103 m2). The isovariation map (Fig. 5B) above may be slightly underestimated. reveals a 20–30-m increase over ca. 45% of the tongue In the 1997–2003 period, the tongue expanded by area. A considerable increase occurred on the snout 9.05×106 m 3, considering a comparison surface area of C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68 65

710.4×103 m2. The average change in thickness was increases in volume during the second half of the 20th +12.7 m; the annual increase rate was +2.1 m. (Fig. 5E). century (Smiraglia et al., 2001; Diolaiuti et al., 2003). The accumulation proved to be concentrated in the lo- The main problem in reconstructing changes in wermost part of the tongue, whereas the upper part was glacier geometry by DEM comparisons lies in the va- experiencing losses. lidation of the data obtained. In this study, the DEMs Therefore, considering the entire study period (1959– were produced using two different data acquisition 2003), we can observe that the Brenva Glacier tongue techniques: cartographic digitizing of contour maps and gained in volume by 22.6×106 m3 with respect to the photogrammetry. comparison surface area of 664.5×103 m2, with an In order to evaluate errors concerning multitemporal average increase in thickness amounting to ca. +34 m variations in volume, the first step consists of calculat- (+0.7 m a− 1 w.e.). The upper part of the tongue under- ing DEM errors. This evaluation is not an easy task went slight losses, whereas the gain in accumulation was owing to a variety of reasons, but it is fundamental concentrated in the lower part (Fig. 5F). because DEM errors constitute uncertainty that is Intercomparison of the DEMs also made it possible propagated with manipulation of the elevation data. to derive profiles of the tongue surface (Fig. 6A). The DEM quality is the root mean square error between longitudinal profiles (Fig. 6B) show an advance and an the true elevation and the DEM value. This is the ac- increase in the thickness of the snout between 1959 and curacy resulting from many parameters (accuracy of the 1991 (ca. +360 m of advance). Moreover, there was an source data; characteristics of the terrain surface; method appreciable increase in thickness until 1983, a decrease for DTM surface generation), which can be evaluated by during subsequent decades, and an increase in the years theoretical analysis or by experimental investigation at the end of the 20th century. The cross profiles (Fig. 6C) (Ackerman, 1980; Li, 1992; Florinsky, 1998). indicate the same changes with small differences in In this study, simple approaches (such as computation magnitude due to their location in the upper sector of the of variance propagation and empirical assessments of the glacier tongue. quality of results) were used to estimate the precision of the DEMs acquired in stereoscopy and derived from maps 4. Discussion and thus to evaluate error in the variations of volume.

The data presented here on the changes in the geo- 4.1. Map-derived DEMs metry of the Brenva tongue during the second half of the 20th century, as determined by indirect sources (maps In order to evaluate the error to assign to the map- and aerial photographs), show a first phase of volume derived DEMs, mathematical models proposed by Pilouk increase (1959–1983), followed by a phase of volume (1992) and Li (1992; 1994) were adopted. decrease (1983–1997), and another increase in volume The accuracy of elevation measurements in DEMs between 1997 and 2003. These variations resulted in a derived from contour data can be estimated by means of positive long-term balance: between 1959 and 2003, an r ¼ bd CI þ r d tgðaÞð1Þ increase of 22.6×106 m3 was recorded, equal to an h r average thickness of ca. +31 m (+0.7 m a− 1 w.e.). This where value diverges considerably from those obtained by applying the same methods to other Italian debris-free σh =root mean square error of DEM elevation; glaciers. The Lys (Monte Rosa, Valle d'Aosta) and Forni σr =map reading error (0.2 mm of the scale factor); (Ortles-Cevedale, Lombardy) glacier tongues lost ca. b=empirical number commonly within the 0.16–0.33 18 m between 1953 and 1994 (Rota and De Lotto, 2001) range; and ca. 24 m between 1953 and 1988 (Merli et al., CI=contour interval; 2001), respectively. These differences must be attributed α=local slope of the DEM. to the well-known insulating effect of debris cover, which, if it is thicker than the critical value (Østrem, First, it was necessary to calculate the slope at any 1959; Fujii and Higuchi, 1977; Mattson et al. 1993; given grid node of DEMs, based on the direction of Rana et al., 1997), reduces ice ablation. Behaviour steepest descent or ascent. The local slope at a point is similar to that of Brenva Glacier, however, has been the magnitude of the gradient at that point (Moore et al., observed in the cases of other Italian debris-covered 1991; Moore et al., 1993). glaciers, such as Miage (Mont Blanc Massif) and A mean slope value of the tongue of Brenva glacier Belvedere (Mount Rosa Group), which witnessed was used for the calculation of σh. As is observable in 66 C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68

Table 1 errors in models derived from stereoscopic imagery Accuracy in digital elevation models derived from maps and from (Table 1, case C). digital photogrammetry. m =scale factor; σ =map reading error b r The normal case in photogrammetry is a theoretical (0.2 mm of the scale factor); σh =root mean square error in elevation h; (A) b=empirical number commonly within the 0.16–0.33 range; method for obtaining photographs, where the base line CI=contour interval; α=mean slope of the DEM; (B) h=relative flight (distance between the camera centres) and the camera height; c=focal length of the camera; B=photobase (distance between axes relative to two stereo pairs are at perfect right angles. central points concerning subsequent photograms); σ =error Pξ This method permits simplification of the colinearity concerning horizontal parallax measurement=12 μm; (C) a=empirical constant ranging within 0.1–0.15; h=relative flight height. All σ values equation, and simple application of the law of variance are in meters propagation to obtain a theoretical mean square deviation value expected for elevation. 1959 1971 1983 2003 1991, 1997 The equation is Source Map Map Map Map Aerial h2 models r ¼ d r ð2Þ h cd B Pn mb 5000 5000 10,000 5000 15,000 σr 112 1– (A) b=0.16 1 1 2 1 – where b=0.33 2 2 4 2 – rh ¼ bd Cl þ rrd tgðaÞ h=relative flight height; c=focal length of the camera; ––– (B) 0.37 B=photobase=distance between central points on sub- h2 sequent photograms; r ¼ d r h d Pn σ μ c B Pξ =horizontal parallax measurement error=12 m.

(C) a=0.1 ––– –0.24 The errors in DEM restitution can be estimated by a=0.15 ––– –0.36 means of the following empirical formula (Kraus 1994; h r ¼ ad Table 1, case C): h 1000 h rh ¼ ad ð3Þ Table 1, errors from this model were comparable with 1000 the map reading errors (0.2–0.4 mm of the scale factor). where 4.2. DEMs from stereoscopic imagery a=empirical constant within the 0.1–0.15 range; h=relative flight height. In this case, DEM accuracy depends on the flying height of the photographs from which the elevations were com- As is observable in Table 1, the errors coming from piled and the compilation method/equipment (Table 1). case B are quite similar to the mean value obtained with In this study, two methods were adopted to calculate case C. Thus, in the computation, the case B values were DEM accuracy. The first one consisted of an evaluation adopted to represent errors in the photogrammetric DEMs. of the variance of the elevations, applying the law of variance propagation to the so-called Normal Case 4.3. Estimation of volume differences (Table 1, case B). The second method consisted of an experimental equation obtained from the literature While the 10×10 grid DEMs, derived from the contour (Kraus, 1994) and that permits an estimation of plotting data, were generated by interpolation, the 1991 and 1997

Table 2 Statistical parameters concerning multitemporal 3D surface comparison: Vs=volume difference between the upper (more recent) and lower (oldest) surface; σVs =error in Vs using σh from Case A for the 1959–1983 surfaces and σh from Case B for the 1991–1997 comparison Differences 1959–1971 1971–1983 1983–1991 1991–1997 1997–2003 Vs (m3) +15.3×106 +17.8×106 −8.2×106 −9.3×106 +9.05×106

σVs (%) ––– 4.4 b=0.16 ±7.1 ±11.4 ±21.8 ±9.7 b=0.33 ±12.2 ±19.7 ±37.2 ±16.2 C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68 67

DEMs derived from the photogrammetric images were cannot be attributed to climatic factors. In fact, 1997– not interpolated to maintain the measured data. 2003 was an extremely negative period for glaciers, in The differences in volume (Vs) were calculated by terms of reduced accumulation and in terms of very high GIS software (Surfer 8.0), comparing the more recent summer temperatures, particularly in 2003 (Cerutti, surface to the oldest one. 2004). It is possible that the expansion of the tongue As result, the net volumes were calculated as the mean may linked to the large avalanche that occurred on value of three numerical models generally adopted to January 18, 1997, falling from the Brenva Spur. Ac- estimate volume (Extended Trapezoidal Rule, Extended cording to Cerutti (2005), the part of the avalanche Simpson's Rule and Extended Simpson's 3/8 Rule). debris that accumulated in the accumulation basin may Thus, the errors in differences in volume were have contributed to increasing the glacier's advance rate estimated (Table 2) by means of (about 300 m/year). This phenomenon is thought to have triggered continuous large-scale ice avalanches, Xn r2 ¼ ðr d Þ2 ð Þ with the ice accumulating at the apex of the tongue, Vs i A 4 i¼1 below the Pierre a Moulin chasm. A rough estimation of the material deposited on the tongue owing to icefalls where showed values that are comparable to those obtained through the comparison of data from the indirect sources. … … I=1 n, n=6 number of DEMs (1=1959, 2=1971, We note, however, that these are merely hypotheses that 6=2003); require confirmation with more detailed studies. Taking σ2 Vs =variance of the Vs; into account the statistical significance of the comparison σ I = root mean square error in each DEM; between the 1997 orthophoto and the 2003 map, we can A=surface area. state that the Brenva tongue registered an increase in volume and in thickness during that time interval. 5. Conclusions In order to ensure the meaningfulness of the data obtained and to apply the same methods to other debris- The evaluation of the changes in the geometry of the covered glaciers, quantification of errors regarding the Brenva Glacier tongue showed a net increase in volume multitemporal changes in volume was required. With this (+22.6×106 m3) between 1959 and 2003, resulting aim, simple approaches such as the computation of vari- from three main phases: a strong positive increase in ance propagation and empirical assessments were used to both volume and thickness (1959–1983), a negative estimate the precision of the DEMs and the volume variation in the 1983–1997 period, and an increase in variation DEMs. As concerns the accuracy of the map- volume between 1997 and 2003. The long-term vari- derived DEMs, the height errors ranged between 1 and 4 m, ation resulting for the Brenva Glacier tongue contrasts while for the DEMs obtained from photogrammetry the with the variations of Italian debris-free glaciers (which errors proved to be lower than 0.4 m. Errors in volume at the end of the nineteen nineties had lower thicknesses changes obtained from cartography were between 7.1% and volumes), but is similar to the variations observed and 21.8% with b=0.16, and between 12.2% and 37.2% on other Italian debris-covered glaciers. These differ- with b=0.33, whereas the error in volume changes obtained ences are due to the action of the debris cover, which, if from photogrammetry was equal to 4.4%. The higher errors it is thicker than the critical value will reduce the ice from the cartography sources can be attributed to the use of ablation rate, delaying the effects of the global warming. a smaller scale map (survey 1983, scale 1:10,000). The glacier's evolution since the end of the nineteen In conclusion, the changes in geometry obtained by nineties is particularly interesting. The 1983–1997 pe- processing the DEMs (derived from maps and aerial riod, characterized by a loss in volume, seems to indi- photographs) proved to be sufficiently reliable (although cate that Brenva Glacier is also beginning to feel the with rather high error margins when drawn from maps strong effects of the global warming phase in progress. with relatively small scales). This approach is the only one On the debris-free glaciers of Mont Blanc, global applicable for past periods to evaluate the volume and warming has accelerated the magnitude of the negative thickness variations of debris-covered glaciers, which mass balances and also increased the retreat rates since very rarely are investigated in the field to measure their the mid nineteen eighties (Cerutti, 2001). However, the mass balances. For these kinds of glaciers, the data ob- comparisons of maps and aerial photographs indicate tained from indirect sources represent the only informa- that the Brenva Glacier tongue increased in volume tion available to reconstruct their past conditions and to between 1997 and 2003, a phenomenon that certainly learn their present size. Moreover, considering that these 68 C. D'Agata, A. Zanutta / Global and Planetary Change 56 (2007) 57–68 glaciers are becoming more and more diffuse in mountain in the second half of the 20th century. Arctic, Antarctic, and Alpine – regions, the role of indirect survey methods will probably Research 35 (2), 255 263. Florinsky, I.V., 1998. Accuracy of local topographic variables derived become more important in the near future, with high- fom digital elevation models. International Journal of Geograph- resolution data (such as satellite imagery) taking a sup- ical Information Science 12 (1), 47–62. porting role. Fujii, Y., Higuchi, K., 1977. 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