Unlocking the time capsule of historic aerial photography to measure changes in Antarctic Peninsula glaciers

Recent studies have reported widespread retreat and acceleration of glaciers on the Antarctic Peninsula, attributed to regional warming. The loss of ice is a contributor to sea level rise, but its volume and impact on sea level is poorly known. There are few ground measurements of ice thickness change and existing satellite altimeters are ineffective over the mountainous terrain. An accurate assessment of changes in surface height, and hence ice volume, of glaciers on the Antarctic Peninsula over past decades is needed to aid better estimates of their past impact on sea level rise and predictions of their future contribution. There is an archive of over 30 000 aerial photographs going back to the 1940s for parts of the Antarctic Peninsula. Photogrammetry of time series of these historic photographs is now the only way to reconstruct changes in glacier surface height over the past 60 years. However, the historic aerial photographs are difficult to employ for detailed measurements owing to inadequate ground control, unfavourable sortie characteristics, incomplete calibration data and the unavoidable use of paper prints only. This paper describes a method of providing control for historic photos without fieldwork on the ground, by linking them to a newly acquired, highly accurate photogrammetric model adjusted through direct kinematic GPS positioning of the camera. It assesses the achievable accuracy through a worked example using a glacier on the Antarctic Peninsula with typical aerial photography at five dates from 1947 to 2005. Overall measurement accuracy of better than 2 m rmse in X, Y and Z was achieved for all the photography types, which is accurate enough to allow meaningful measurement of changes in ice thickness for the glacier over periods of the order of decades. The principal constraints are image quality of the historic photographs and the availability of paper prints only.