Accuracy assessment for mapping glacier flow velocity and detecting flow dynamics from ASTER satellite imagery: Tasman Glacier, New Zealand

Image matching via correlation of remotely sensed imagery is an increasingly popular technique for measuring glacier flow because it can provide spatially distributed measurements while mitigating many of the challenges accompanying in situ measurements of glacier velocity. Despite the increasing use of such techniques, and the requirement for well understood uncertainties where analysis of temporal variability is sought, studies that incorporate an assessment of accuracy based on in situ data remain rare. This study presents a rigorous methodology to quantify the uncertainties that apply to glacier flow velocities derived from optical satellite imagery. This method is applied on three successive ASTER images of Tasman Glacier, New Zealand. Evaluation of the quality of co-registration between image pair constituents allows unique uncertainties to be calculated for individual velocity measurements. These uncertainties have magnitudes that correspond to sub-pixel co-registration errors and account for anisotropic co-registration variance. Compared to estimated velocities, uncertainties on the order of 2–30% are achieved. Flow-fields for the 2009–2010 and 2010–2011 periods were derived using various spectral bands and validated with in situ data obtained by GPS survey over a nearly coincident period. This revealed the importance of considering the spectral characteristics of the target surface when selecting bands for image correlation, while validation demonstrated the success of the implemented methodology. Uncertainties derived for the flow-fields permitted statistical significance of velocity change between measurement periods to be assessed. Significant velocity changes, observed across the glacier surface, suggest that the Tasman Glacier behaves in a more dynamic way than previously recognised, highlighting the advantages of the methodology described here.