Abstract
The Tasman Glacier is New Zealand’s largest body of ice comprising several tributary glaciers. Since the early 1990s it has undergone a rapid frontal retreat associated with the expansion of a proglacial lake. In this study, digital photogrammetric processing of vertical aerial photographs were used to derive two high-resolution Digital Elevation Models (DEM) of the Tasman Glacier. Additionally, in order to account for the volume loss due to lake expansion, the subaqueous lake topography was generated from a recent bathymetric survey and appended to the latest DEM. Detailed analysis of the multitemporal DEM enabled the quantification of geodetic surface elevation and mass balance changes. Calculations show that the main Tasman Glacier and its tributaries have lost 19.72 ± 0.05 × 10^8 m^3 of ice between 1986 and 2008. The proglacial Tasman Lake occupied ca. 28% of this volume loss. The glacier-wide volume loss corresponds to a geodetic balance of 0.87 ± 0.002 m w. eq. yr^-1. Differential DEM analysis revealed diverse spatial patterns of thickness and volume change, varying between the tributaries and within and between elevation bins. Furthermore, reworked and transported material from recent rockfall deposits produced localised areas of apparent positive elevation changes. Previous ice volume change estimates may have underestimated mass loss from the Tasman Glacier, partly due to a lack of bathymetric data to account for the glacier-wide volume variation and omission of the tributary glaciers. The methods developed in this study offer a potential way forward for glacier monitoring in New Zealand, where old aerial photographs are available but have not previously been processed in this manner to obtain accurate assessment of geodetic mass balance.