Change in glacier extent is a good indication of climate change. Inventories for glaciated areas should therefore be made at certain intervals. For New Zealand a digitized glacial inventory including both the two main islands’ glaciers was made from aerial photographs recorded in 1978. This inventory needed an update.
One Aster scene (60*60km) recorded 14. February 2002 covering the central Southern Alps of New Zealand is used for the updating. The area covered by the image contained, in 1978, 41% of the glaciated areas in New Zealand. The image was orthorectified using a combination of points collected in field and points from the New Zealand topographic database and DEM made from 20-meter contour intervals in PCI. Due to the problems often involved in automatic methods for glacier extraction, the glacier areas on the image were manually digitized. For validation of the digitizing, field work was conducted during late summer of 2005 (Feb-April). Glacier outlines were mapped for 9 individual glaciers using a differential GPS. These data were later corrected to a base antenna giving sub-meter accuracy. 5 of these 9 glaciers were so called ‘Index glaciers’ that had been annually photographed since 1977 as a part of the New Zealand Annual Snowline Survey. Aerial photographs of the 5 field work Index glaciers, were used to adjust for eventual changes in the glacier outlines between image acquisition and the in-situ recordings. Automatic classifications were tested on the entire image and on 3 zoomed in study areas to give an estimate of the efficiency of these automatic methods in the New Zealand setting. Band ratio of ATER3/ASTER4 proved to be the most efficient automatic classification method, with the threshold set around 2.0. However, as a result of the large debris cover on many glaciers in New Zealand, automatic glacier extraction would require significant manual post processing. The manually digitized glacier map was used to calculate the change in glacier area since 1978. An overall reduction of 16.6% was found, more specifically 14.3% for the western and 18.3% eastern side of the Main Divide of the Southern Alps. The large and fast flowing western glaciers, Fox and Franz Josef Glaciers, were pulsing back and forward in the study period, but showed an overall advance, whereas the large low-gradient heavily debris covered valley glaciers developed proglacial lakes and have shown a rapidly increasing retreat due to calving. The smaller high elevated alpine glaciers in the area show only slight changes
