Surface motion of mountain glaciers derived from satellite optical ...

E. Berthier et al. / Remote Sensing of Environment 95 (2005) 14–28 27future work, a more detailed study of the spatial pattern ofthe velocity change will help better understand the causes ofthis ice-acceleration event.5. ConclusionsThe goal of measuring displacements on mountainglaciers with an accuracy of one fourth the pixel size(0.62 m with SPOT5 images) has been achieved. Theuncertainty in the DGPS survey (0.21 cm), the temporalmismatch between the ground surveys, and the acquisitiondates of the SPOT5 images, combined with a suddenincrease in ice-velocity prevent us from confirming definitivelythe accuracy from field observations. Yet, anuncertainty of 0.5 m in each image direction seemsreasonable if satellite images are acquired from a similarpoint of view. This uncertainty of 0.5 m over 26 days isequivalent to 2 cm over 1 day, very close to the accuracyobtained from InSAR using ERS-1/ERS-2 tandem pairs.Our approach is accurate enough to measure a 10% velocityincrease on the Mer de Glace glacier during and just afterthe pronounced heat wave in Europe in early August 2003.Because our approach does not require GCPs, ourmethodology can be routinely and rapidly applied to newpairs of images. The high accuracy obtained without GCPsusing the GTOPO30 DEM suggests the possibility ofmonitoring numerous glaciers, even in remote areas andwithout a contemporaneous DEM. The key step is the relativeorientation of the two correlated images using precisehomologous points. The accuracy of the measurement iscontrolled mainly by the radiometry (low gain is better forglaciers to avoid sensor saturation) and the geometry of theimages. The bshadowingQ effect, highlighted on the motionlessarea surrounding the glaciers, could be a problem for theapplications other than glaciology. The rapidly changingelevation of mountain glaciers in summertime, because ofablation, can create a systematic error in the displacementsalong the image column direction. Addressing this issue,however, requires external information. This systematic biascould be used to monitor the elevation change of glaciersthroughout the ablation season.Images acquired from a similar point of view, withincidence angles differing only by a few degrees, are best.Vertical incidence eliminates the systematic errors caused byablation but such a constraint also reduces the likelihood ofobtaining two good images.Among the limitations of our approach is that a velocitymeasurement during the summer may not be representativeof the annual dynamics of a glacier. Thus, annual surveys ofablation stakes on a few glaciers are needed and complementthe satellite-derived measurements. Another limitationis the difficulty in acquiring two cloud-free images withsimilar incidence angles.Other geological phenomena that deforms the Earth’ssurface, such as earthquakes, landslides, and volcanoescould also be surveyed using the cross-correlation of SPOT5images. Depending on the expected direction of displacements,the incidence angles could be chosen for optimumaccuracy. For mostly horizontal (respectively vertical)displacements, images with vertical (respectively oblique)incidence angles are preferable. For these applications otherthan glaciology, one interesting and promising topic toexplore would be the fusion of displacements obtained byInSAR and optical image cross-correlation to obtain a 3-dimensional surface-displacement map.AcknowledgmentsAurélie Bouilllon provided useful help concerning theSPOT5 algorithms. The comments of R. S. Williams Jr. andan anonymous reviewer led to significant improvements ofthe manuscript. We thank M. Bauer who provided valuableguidance as Editor-in-Chief. SPOT5 images were acquiredthanks to the ISIS program (copyright CNES). This workwas supported by the French national program ACI-OTGlaciers, the GDR STRAINSAR and the French GLACIO-CLIM program. The LGGE and IRD Great Ice Unitprovided support for the field experiment. GMT software(Wessel & Smith, 1998) illustrated the article. The firstauthor gratefully acknowledges a thesis fellowship from theFrench government.ReferencesAl-Rousan, N., Cheng, P., Petrie, G., Toutin, T., & Valadan Zoej, M. (1997).Automated DEM extraction and orthoimage generation from SPOTlevel 1B imagery. Photogrammetric Engineering and Remote Sensing,63, 965–974.Berthier, E., Arnaud, Y., Baratoux, D., Vincent, C., & Rémy, F. (2004).Recent rapid thinning of the bMer de glaceQ glacier derived fromsatellite optical images. 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