Abstract
The Tasman Glacier is New Zealand’s longest and largest, representing almost a third of New Zealand’s glacier ice by volume. A relatively long observational record exists for the Tasman Glacier. Velocity measurements are present throughout this record, and generally reflect the large mass loss that has occurred through the twentieth century. Recent studies have applied digital image matching techniques to measure flow velocities on the surface of the Tasman Glacier from repeat satellite imagery. These studies have, however, utilized temporally limited data sets. Additionally, precise quantification of uncertainties is not common; with an accuracy of ± 1 pixel (15 m where ASTER imagery is used) assigned in earlier studies, while no previous work has accounted for inevitably anisotropic uncertainties. Large and ambiguous uncertainties make significance assessment of small inter-annual velocity changes difficult.
This thesis provides a decade long (2000–2010) record of flow velocities, derived for the Tasman Glacier from optical satellite imagery. This record has been derived from a series of annually acquired Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery (2000–2010). Flow-fields were derived from pairs of consecutive images using Correlation Image Analysis Software (CIAS). Repeat GPS measurements of markers installed on the glacier surface were used for optimisation of image matching and flow-field validation. Co-registration of each image pair was carefully quantified at a sub-pixel level, enabling the calculation of confidence intervals for each map of flow velocity. Uncertainties presented here are calculated so as to be unique to each individual velocity measurement. Additionally, uncertainties are anisotropic, accounting for unequal co-registration variance between images in the x and y direc tions. The significance of velocity changes could thus be assessed and the spatial distribution of these changes be interpreted.
The results show two major units of flow, with an apparent disconnection between the upper Tasman and Hochstetter Glaciers. Furthermore, the high temporal resolution velocity record revealed marked oscillations between significant acceleration and deceleration of the glaciers surface flow. Analysis of climatic data suggests that observed velocity changes may be driven by climatic and hydrological forcings. The observations indicate that while a discontinuity exists between the upper and lower parts of the glacier, a previously suggested, de-coupling between ice masses at the Hochstetter Confluence is non-existent. The techniques employed by this study reveal new insights into the dynamic behaviour of the Tasman Glacier, previously regarded as a stagnating glacier undergoing slow thinning and retreat. This thesis highlights the advantages of long term and ongoing monitoring, and the development of continuous records, rather than intermittent and isolated measurements, for the detection and interpretation of variability in the flow regime of large temperate valley glaciers. The findings presented here offer improvements to the measurement of glacier flow velocities from optical satellite imagery, and provide new insights into the response of the Tasman Glacier to climatic and environmental change. Such insights may help improve understanding of temperate, debris covered, valley glacier behaviour, and strengthen predictions of their response to future climate change.