SEMI-CONTROLLED INTERFEROMETRIC MOSAIC OF THE LARGEST EUROPEAN GLACIER

Aleksey I.Sharov (1) and Dmitry B. Nikolskiy (2) 1) Joanneum Research, Wastiangasse 6, 8010 Graz, Austria, E-mail: aleksey.sharov@joanneum.at

2) State University of Geodesy and Cartography, Gorokhovskiy 4, 105064 Moscow, Russian Federation

ABSTRACT A combination of satellite interferometry and altimetry was used for generating and upgrading elevation and evolution models of the Northern Glacial Complex in Novaya Zemlya, measuring heights of ice divides and ice coasts, detecting, measuring, interpreting and mapping glacier changes in linear, areal and volumetric terms, and estimating glacier mass balance on a regional scale. Spatial asymmetry in the present glacier regime was deduced and explained. Six ERS-1/2 overlapping SAR interferometric models were orthorectified and assembled into a semi-controlled seamless mosaic covering a land area of approx. 18,000 km² and containing 8 information layers. These layers provided the basis for the interferometric map series showing structural morphology, ice-flow pattern and glacier elevation changes at 1:500 000 scale in the UTM projection, Zone 41N, WGS84. The map series was presented in the form of a 5.5 min animation. 1. INTRODUCTION Glaciers are retreating in response to global warming. European glaciers shrink by several tens to several hundred meters each year, and publications periodically announce drastic changes in the position of glacier termini in the Alps, Scandinavia, Iceland, Spitsbergen, and Franz Josef Land. There are very few reports about positive glacier changes, which seem to be “anomalous” [1] and negligible in the view of common retreat of numerous albeit relatively small glaciers. Our recent remote sensing studies using satellite SAR interferometry (INSAR) and lidar altimetry revealed significant positive height changes in the accumulation area of the Northern Glacial Complex (NGC) in Novaya Zemlya [2]. At that time, limited terrestrial coverage of single INSAR models and relatively sparse coverage of the extensive glacier summit by ICESat-GLAS data did not allow such an “anomaly” to be studied on a synoptic basis. Several intriguing questions remained open. What is the driving mechanism of positive glacier changes on the NGC? Are there other geophysical processes, apart from the “greenhouse” effect, controlling the present regime of the largest European glaciers? The last IPCC report (2007, draft version) states that “understanding

of these processes is limited and there is no consensus on their magnitude” [3]. This paper presents the main outcomes of the INTEGRAL (EC FP6 GMES), SIGMA (ESA ID.2611) and INTERSTEREO (ESA ID.3582) research projects related to the generation of sufficiently accurate glacier elevation & evolution models and compilation of the INSAR mosaic covering the whole NGC with the aid of altimetric and cartographic control. The polar idea of the present research was to combine the INTERSTEREO method of INSAR and stereometric data fusion, offered e.g. in [4, 5], with the INSARAL approach to upgrading glacier interferometric models with altimetry data described in [6, 7] in order to automate the main procedures of INSAR mosaicking and to improve the reliability, completeness and integrity of glacier interferometric models. The main emphasis was put on - designing a straightforward automatic procedure for joint geometric processing of INSAR, lidar altimetry and cartographic data, - checking the positional accuracy of the INSAR mosaic, - detecting, mapping and interpreting glacier changes that have occurred over the NGC in the past 50 years in both volumetric and fluxometric terms, - studying spatial asymmetry in present glacier regime and estimating glacier mass-balance on a regional scale. The challenging character of the study was defined by the lack of reliable ground control, the local character of previous glaciological studies and the absence of up-to-date ground-truth data on glacier dynamics in the Novaya Zemlya archipelago, which was closed to civilians and foreigners in former times. Apart from comprehensive, but rather obsolete publications [8, 9], the single ground-truth data we had was obtained during the first (since the 1950s) international field campaign carried out in north Novaya Zemlya in IX 2001 [10]. The study is considered as part of the preparation activities for the launch and operation of European GOCE and CryoSat-2 satellites. 2. THE LARGEST EUROPEAN GLACIER Jostedalsbreen in south Norway, the biggest glacier in continental Europe, covers an area of 550 km². Large

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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

glaciers on European arctic islands are far more extensive: for example, Southern Glacial Complex in the western part of Franz Josef Land occupies 2,150 km², and Vatnajökull in the south-east of Iceland and Austfonna in north-east Svalbard each cover an area of approx. 8,100 km² (Wikipedia 2007). The Northern Glacial Complex (NGC) on North Island in the Novaya Zemlya archipelago covers 22,800 km² (1952) and is thus by far the largest mass of glacier ice in the “Old World”. This glacier is, sometimes, referred to as an ice sheet albeit it is still smaller than 50,000 km². Fig. 1 shows the location of the NGC in the extreme north-east of Europe. It consists of the rounded Northern Ice Cap (1, approx. 2,260 km²) interconnected with the extensive Main Ice Shield (2); the latter stretches for 344 km and has an area of more than 20,000 km². Glacier top heights attain 815 m and 980 m, respectively, and according to indirect geomorphologi-cal estimations the approximate mean thickness of the ice sheet is 350 m. The red dashed line in Fig.1 indicates the boundary between Europe and Asia according to UN standards. There are 41 fast-moving tidewater outlets draining ice from the ice sheet either into the Barents (27) or Kara seas (14). The total length of ice coasts adds up to 200 km. The net mass balance of the whole NGC is controlled to a very great extent by calving and is supposed to have remained negative (- 0.25 m w.e.) during the 20th century [11]. It is worth noting, however, that there are few instrumental records documenting the ice flow speed, the ice discharge through maritime glacier fronts is largely uncertain, and present mass balance calculations for the NGC are very approximate [10].

Fig. 1. Location of the Northern Glacial Complex: Northern Ice Cap (1) and Main Ice Shield (2)

3. SOME REMARKS ON INSAR MOSAICKING The NGC represents both a uniquely fruitful and challenging object for regional remote sensing studies of glacier fluctuations in response to climate change. The immense size, elongated shape and relatively homogeneous topography of the glacier surface make it possible to study spatial variations of glacier changes at macro-level and to determine the universal causes of glacier change, which are usually masked by local topographic, meteorological and ice-motion effects. On the other hand, the huge size of the glaciers makes the analysis of relationships between processes at glacier tops and margins technically difficult. The limited spatial coverage of spaceborne INSAR models requires constructing an interferometric mosaic, i.e. an assembly of individual interferograms geocoded and fitted together systematically to form a continuous, complete and measurable picture of the entire glacial area for subsequent rheology analysis, inventorying of land-ice resources and change mapping. In order to produce a radiometrically homogeneous mosaic with comparatively uniform shadows and highlights, all spaceborne interferograms should be obtained from parallel, e.g. descending orbits. Even then, perfect matching of adjoining fringe images remains difficult because of different fringe rates in separate images, which requires special scaling procedures for equalising the topographic phase in SAR interferograms with different base lines [12]. Dodging or fading neighbouring sections of amplitude, coherence and phase-gradient images presents no essential difficulties. To meet the accuracy requirements of a good map, all the individual interferograms must be orthorectified / geocoded before assemblage so as to remove image distortions caused by glacier relief and orbital inaccuracies. This requires the use of current glacier elevation models and necessitates additional reference, e.g. precise orbital data. The resultant orthomosaic has to be oriented and scaled to accurate ground control. The last extensive aerial and geodetic surveys in Novaya Zemlya were carried out in the 1950s, and up-to-date topographic maps and digital elevation models of the Northern Glacial Complex are either non-existent or of limited quality and coverage. In this case, the geometric constraints needed for precise geocoding and mosaicking of the glacier interferograms have to be derived from other remote sensing and cartographic data. The mosaic constructed in such a way is usually called semi-controlled [13]. In the following, it will be demonstrated that semi-controlled interferometric mosaicking is a rapid and economical method of representing ice motion and strain rate fields, identifying major ice divides and individual drainage basins, and producing glacier change maps.

4. SHORT HISTORICAL REFERENCE & DATA SET DESCRIPTION The exploration history of Novaya Zemlya is closely related to the history of International Polar Years (IPYs), and we wish to mention the following milestones, which provided strong stimulus and valuable information for our studies: • the establishment of the polar station at Malye

Karmakuly during the 1st IPY (1882-1883), which is presently the oldest operational station in the Barents Sea region;

• first airborne surveys of the NGC from a Zeppelin airship (Fig.2), inland topographic surveys and systematic glaciological observations during the 2nd IPY (1931-1933);

• extensive aero-geodetic surveys and generation of standard topographic map series of the NGC during the 3rd IPY (1957-1959);

• satellite geodesy and remote sensing applied to glacier change analysis and mapping during the 4th IPY (2007-2009).

Fig. 2. Vigorous Outlet Glacier, NGC, Novaya Zemlya (handheld camera, airship “Graf Zeppelin”, 29.07.31)

Apart from historical glaciological and meteorological data, our set included six ERS-1/2 (1996) and one JERS-1 (1998) SAR overlapping interferometric models of north Novaya Zemlya, all obtained from different descending orbits under cold and steady weather conditions. Since the construction of a uniform scale mosaic from SAR interferograms requires the generation of new or upgrading of available glacier elevation models, we scanned and stitched together 41 sheets of Russian topographic maps 1:100 000 (26, CI = 20 m, 1971) and 1:200 000 (15, CI = 40 m) representing the state of the NGC and surrounding areas in the 1950s. Coastlines, contours and height spots were vectorised and stored in a contour file, and the raster (“old”) glacier elevation model with 50 m posting was

generated using the MicroStation 7, ERDAS 9 and ArcGIS 9.2 software. Nearly 200 ICESat-GLA06 altimetric transects (release 28) obtained over the NGC in the cold seasons of 2003–2006 were collected and visualised; clearly erroneous data were removed. ICESat ellipsoid heights were transformed to geoid heights. Typical height errors of corrected altimetric data were estimated as being 20 times smaller than those in standard INSAR products and it was concluded that the lidar altimetry data can be applied as an additional or even alternative control to precise geocoding and mosaicking of glacier interferograms. Five cloud-free optical images from LANDSAT-7 satellite and 12 ASTER scenes, all obtained in the 2000s, were gathered for the sake of additional control and glacier change visualisation. Two maps of free-air gravity anomalies in the Arctic derived from satellite altimetry data [14, 15] were used for the interpretation of glacier changes. 5. JOINT GEOMETRIC PROCESSING OF ALTIMETRIC, INTERFEROMETRIC AND CARTOGRAPHIC DATA In contrast to our previous studies described in [6, 7] we concentrated on merging both lidar altimetry and cartographic data with standard INSAR products for the precise topographic modelling and change detection over the NGC. The enhanced approach to multisource data processing includes the following basic procedures: - co-registering altimetric and cartographic (contour)

data with standard INSAR products; - determining and removing phase offsets in INSAR

data (also those due to radar penetration into snow); - determining present glacier elevations between the

altimetric transects along the old contour lines; - generating a modern glacier elevation model; - determining natural changes in glacier elevations

and generating a glacier evolution / change model (optionally);

- geocoding, scaling, fading and mosaicking of SAR interferograms.

ICESat altimetric transects and corresponding sections of contour maps were co-registered to SAR interferograms using a straightforward transformation. In order to fit SAR image geometry, all map contours, height spots and altimetric points were shifted towards the sub-satellite SAR master track depending on their height Δ h and the local look angle θ. The shift value can be calculated from the next equation

⎟⎠⎞

⎜⎝⎛ +⋅Δ=

2αθctghD , (1)

where the angular measure of foreshortening α is usually less than 0.1° and can be neglected, which means that the following simplified equation can be used

θctghD ⋅Δ≅ . (2) The transformation requires little computing time and the relative error introduced by such simplification does not exceed 0.2%. Fig. 3 illustrates the result of co-registration. The big advantage of such composition is that it allows + reliable estimation of interferometric phase distortions and removal of phase offsets over homogeneous and slow-moving glacier surfaces characterised by high coherence of the interferometric signal, + determination of modern heights of target points lying on glacier contours or coinciding with cartographic height spots from the ICESat transects and the phase information in between using the algorithm proposed in [6, 7], + automatic measurement and control of glacier elevation changes at all coherent points spatially coinciding with those digitised in old topographic maps, + mitigation of some local problems related to interferometric phase unwrapping at precipitous ice coasts, inland borders of outlet glaciers, etc. If high-coherence interferograms are used, the new glacier elevation model has practically the same spatial parameters as the old one and has smaller interpolation errors in the areas with rear contouring covered by altimetry data. The 50 m posting interval in both new and old glacier elevation models is a trade-off between the pixel size of 20-look INSAR products (40 m) and the footprint of lidar altimetry data (70 m). Note that the glacier evolution model can be built independently from the generation of both new and old raster elevation models. This reduces the number of error-prone procedures, such as gridding and height interpolation, and improves the accuracy of measuring glacier elevation changes at target points. In low-coherence glacial areas, some kind of interpolation is still required for estimating height values between target points, which does not introduce?? essential errors over short distances. In real data processing, we assumed a spatially homogeneous distribution of glacier elevation changes during the 10-year interval between interferometric and altimetric surveys, and supposed that the residual cumulative influence of random ablation-accumulation processes on height measurements did not exceed several meters. The geocoding of available SAR interferograms was performed in a standard way using the new glacier elevation model, precise orbits and the ERS- or JERS-

SAR sensor model implemented in the RSG 5.1 software. The high accuracy of the procedure was indirectly proved by overlaying the geocoded C- and L-band interferograms and getting a sharp picture. Six geocoded ERS-SAR interferograms of the NGC were cut, faded and jointed along the altimetric transects so that image borders became invisible and the resultant mosaic appeared seamless (Fig. 4). The interferometric mosaic was oriented and scaled to the map assembly 1:200 000. The resultant interferometric mosaic covers a land area of approx. 18,000 km² (45,000 km² in total) and contains 8 information layers: intensity orthoimage, coherence orthoimage, fringe orthoimage, phase-gradient orthoimage (Fig. 4), composite orthoimage, 2-pass differential orthointerferogram (DEM 1950s), upgraded differential orthointerferogram (DEM 2000s) and orthoimage of glacier elevation changes in the past 50 years.

Fig. 3. The result of co-registering altimetric and cartographic data with a composite image of the NGC

Fig. 4. Phase-gradient image mosaic of the NGC

6. ACCURACY ANALYSIS OF INSAR MOSAIC The positional accuracy of the INSAR mosaic and glacier elevation models was tested by comparing planimetric coordinates of 18 steady check points, 17 arbitrary distances and 3 hypsometric profiles between the steady check points situated in flat ice-free areas with known elevations, which were identified in both the mosaic and the topographic map assembly 1:200.000. The r.m.s. difference between measured planimetric coordinates was ± 84 m and that between measured distances was ± 96 m. Mean differences in the distances measured along and across the SAR track direction were nearly equal. The r.m.s. difference between ice-free spot elevations in the upgraded elevation model and those given in available maps did not exceed 5 m. The vertical accuracy of the old elevation model and that of the evolution model is supposed to be approx. ± 10 m r.m.s., i.e. one third of the average contour interval of 30 m (20:2 + 40:2), excluding several erroneous areas at glacier tops. The analysis of the cross-differential C-L interferogram provided no evidence of the geometric impact of radar penetration into dry snow. The root mean square difference between altimetric and cartographic heights was estimated at ± 0.7 m. Careful inspection of multitemporal altimetric transects proved high repeatability of altimetric measurements obtained over the NGC with a time gap of 3 days in cross-over areas. Remember that ICESat footprints of 70 m are spaced 170 m apart, and the pointing accuracy is better than 2 arcsec or within ± 35 m on the ground. It is worth noting that, at a latitude of 76°N, the available transect spacing is about 10 km, and the maximum distance between neighbouring cartographic contours at the top of the glacier surface is up to 5 km. The vertical accuracy of output elevation models in such locations might be lower. The positional accuracy of the semi-controlled INSAR mosaic was thus concluded to be sufficient for glacier (change) mapping at 1:500 000 scale. 7. MAPPING & INTERPRETING RESULTS The mosaic layers served as a base for the interferometric map series showing structural morphology, ice-flow pattern and mass-balance characteristics of the Main Ice Sheet at 1:500 000 scale in the UTM projection, Zone 41N, WGS84. In the interferometric maps, all fore-shortening effects at precipitous glacier margins are accounted for, thus allowing precise glacier change detection in multitemporal data sets. A small-size copy of one image map is given in Fig. 4. The research framework, the processing chain and the output results were

presented in the form of a 5.5 min animation. The resultant maps and the animation can be accessed at http://dib.joanneum.at/integral (cd results). Practical mapping work revealed essential changes in glacier elevations and termini positions. The position and present heights of ice coasts and main ice divides were determined by joint interpretation of altimetric transects and different maps from the series. The present top heights of the NGC were determined as 817 m a.s.l. on the Northern Ice Cap and 982 m a.s.l. on the Main Ice Shield. These values differ significantly from the corresponding heights of 590 m and 1173 m given in old topographic maps. Old topographic map sheets show a remarkable elevation on the glacier surface close to the Main Ice Divide with a height of 1173 m, which appears in the form of 7 concentric fringes in the original version of the 2-pass differential interferogram. This conic feature could not be observed in fringe and amplitude layers, however, and was absent in spaceborne optical images. Hence, we concluded that this was a map mistake. In the upgraded glacier elevation model, the Northern Ice Cap appears to be 227 m (= 817 m – 590 m) higher than it was in the 1950s. The consequent character of glacier contours in available maps and several photogrammetric height spots (the highest is at 556 m) placed nearby does not allow this difference to be interpreted as a topographic mistake. We therefore assumed that, apart from the modelling errors, this height difference might be explained by the natural accumulation of snow in the past 50 years. This would mean that the net mass balance of Northern Ice Cap remained positive during the past 50 years. The accumulation of snow was generally higher on northern slopes so that the position of the Main Ice Divide changed and the upper part of Northern Ice Cap “shifted” to the north. This finding indicates that at glacier tops the relation between accumulation and ablation was influenced rather by direct insolation than by the greenhouse effect. Large positive elevation changes seemed to be exclusive and we tried to find similar places in other areas of the NGC. For the sake of better visualisation, the altimetric transects were overlaid with a perspective view of the old glacier elevation model. All transect sections located “inside” (below) the model were masked. All transects running “outside” (above) the model remained “visible” (Fig. 5). In this picture, we detected 3 other glacial areas with evidently positive elevation changes up to + 110 m alternating with areas of negative changes up to – 50 m. These distinct elevation changes in the topographically homogeneous upper part of the NGC looked strange and gave us the idea of comparing the map of glacier

elevation changes with the combined map of free-air gravity anomalies in the Arctic. The results of the comparison were astonishing (Fig. 6). The locations of three of four glacial areas with positive height changes detected in the upper part of the NGC were highly correlated with the locations of positive gravity anomalies (+50 to +60 mGal) in the region (Figs. 6, a, b). The asymmetric shape of the NGC, especially in its northern part, also correlated well with the outlines of gravity anomalies nearby. The high asymmetry in the ice flow pattern and calving characteristics with much faster ice flow (up to 200 m/a) towards the Barents Sea than in the opposite direction (See Fig. 4) can also be explained by the existence of a strong (+ 70 mGal) gravity anomaly 50 km offshore from the western coast of north Novaya Zemlya.

Fig. 5. Perspective view of the NGC with ICESat transects overlaid showing areas of snow accumulation

a)

b) Fig. 6. Glacier elevation changes (a) and combined map

of free-air gravity anomalies (b) in Novaya Zemlya

Although an inverse relation associating the emergence of positive gravity anomalies with the glacier ice load should not be forgotten, we assume that there is a direct impact of gravity anomalies on the accumulation of snow, topography and evolution of large glacial complexes. This conjectural hypothesis will be either disproved or confirmed by new gravity data to be obtained from GOCE satellite and from ship surveys during the 4th IPY 2007 - 2009. If this concept is verified, it could essentially contribute to a synoptic understanding and better forecasting of glacier changes in response to climate change in the European Arctic, which hosts a large cluster of areas with anomalous gravity. Frontal velocities of tidewater outlets and corresponding values of the ice flux at seaward glacier margins were estimated by analysing the horizontal shift of the coastal sea ice forced by glacial flow. Quantitative integral estimations of ice flux were provided for 5 glacial provinces of the NGC and compared with those published in [8, 10, 16]. Furthermore, it was determined that, apart from the slightly positive mass balance in the northernmost part, the NGC lost at least 200 km³ of ice in the period between 1952 and 2003, i.e. nearly 4 km³/a. The comparison with previous estimates made by other researchers showed that land ice loss in Novaya Zemlya has accelerated by approx. 10 %. 8. CONCLUSIONS It has been demonstrated that the combination of satellite interferometry and altimetry offers a particularly potent solution for precise topographic modelling of large glacier complexes in the case of absence or insufficient quality of ground control / truth data. New glacier elevation and change models were generated and the INSAR mosaic containing 8 information layers and covering practically the whole area of the NGC was compiled. Typical height errors of the upgraded elevation model were estimated as being nearly 5 times smaller than those in standard INSAR DEMs. The high planimetric accuracy of the INSAR mosaic was proved as well. Interpretation of the mosaic provided a wealth of previously unknown information on structural morphology, ice-flow pattern, spatial changes and mass-balance characteristics of the Northern Glacial Complex. The data set collected, the processing techniques designed, the INSAR mosaic compiled and glacier elevation models generated provide a solid foundation for further methodological research, causal analysis of glacier changes and studying geoid undulations on the sub-continental scale. Two conjectural hypotheses relating spatial asymmetry in snow accumulation, glacier extent and ice flow to the existence of gravity anomalies and direct insolation will be verified during field surveys in 2007 – 08.

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ACKNOWLEDGEMENTS The research was carried out under the EC FP6 GMES, Contract No.SST3-CT-2003-502845 (INTEGRAL). ERS-1/2 and JERS-1 SAR data were provided by ESA to C1P.2611 (SIGMA) & AOP.3582 (INTERSTEREO). ICESat altimetry data were made available by NSIDC. REFERENCES

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