![]() ![]() Although elevation change estimates have been obtained for some areas, as mentioned above, in many cases no historical cartographic data exist. To use these data, however, to determine mass balance requires, as mentioned, the addition of estimates for accumulation, ablation and ice thickness: data that are rarely available for more than a handful of glaciers. For example, velocity data can be obtained using either feature tracking or InSAR techniques and these methods have been used to monitor the development of a surge in an Arctic glacier (Luckman et al., 2002). Thus, although the instruments used may be common, the approach may not be. The problems associated with monitoring the behaviour and mass balance of these types of ice mass are, therefore, often different compared with the ice sheets of Greenland and Antarctica. 1000km2 and can be partially (or even wholly) debris covered, snow covered or bare ice. Subpolar glaciers are clustered within specific mountain regions. For subpolar glaciers, the flux divergence approach has been less useful as there is, in general, no well-defined gate across which the flux can be measured, ice thickness is rarely known and difficult to infer, as is the net mass balance. Unfortunately, the northern limit of SRTM was 60°N, missing all of the major glaciated areas of the Arctic such as Svalbard, Greenland, Severnaya Zemlya and so on. Thus, these data could have considerable value for use in elevation change measurements for other low-latitude glaciers, where reasonable historical cartographic data exist. In this study, the accuracy of the SRTM data was found to be around ☒m. Again, the results showed thinning rates far greater than expected, based on previous mass-balance estimates. A similar approach was taken for the Southern Patagonian Icefield but using a DEM derived from SRTM data instead of laser altimetry and comparing this with historical cartography (Rignot et al., 2003). The results suggested a mass wastage much higher than previous estimates, equivalent to twice the loss from the Greenland ice sheet over the same period. Airborne laser altimetry was combined, for example, with cartographic maps, derived from aerial stereo photogrammetry from the 1950s, to estimate dh/dt over a period as long as 40 yr for some 67 Alaskan glaciers (Arendt et al., 2002). The two methods discussed above for determining ice-sheet mass balance can be, and have been, used to determine the mass balance, not only of ice sheets, but also glaciers. ![]()
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