Repeat photography is a technique in which a historical and a modern photograph, both having similar fields of view, are compared and contrasted to quantitatively and qualitatively determine their similarities and differences. In precision repeat photography, both photographs have the same field of view, ideally being photographed from the identical location. Since 2000, ground-based, precision repeat photography has been attempted at more than 200 Alaskan locations in Glacier Bay and Kenai Fjords National Parks, northwestern Prince William Sound, and the Coast Mountains to document glacier and landscape change as a result of changing climate. The result has been the production of more than 150 image pairs documenting change over time periods ranging from 26 to 121 years. Airborne-platform-based repeat photography is also being used throughout glacier-covered Alaska to augment the ground-based assessments and to monitor change at geographic scales ranging from individual glaciers to entire mountain ranges. To date, about 40 pairs of comparable airborne images have been produced. They span time periods of about 2 to 75 years. Considering Alaska’s remoteness, its early photographic record is extensive. More than 500 pre-20th century photographs of Alaskan glacier-covered landscapes have been collected. All postdate the purchase of Alaska from Russia in 1867. Several thousand early 20th century photographs have also been found.

Through analysis and interpretation of these photo pairs, both quantitative and qualitative information is extracted to document Alaskan landscape evolution and glacier dynamics for the last century-and-a-quarter on local and regional scales and the landscape response to retreating glacier ice. Information derived from the analysis of repeat photography pairs has been useful in documenting: (1) The post-LIA retreat of more than 99% of the largest glaciers in nearly every part of Alaska, with some retreating more than 50 km; (2) Rapid vegetative succession and the transformation from glacier till and bare bedrock to forest at many locations; (3) The transition of many glaciers from tidewater termini to land-based, stagnant or retreating, glacier termini; and (4) The advance or readvance of about a dozen large glaciers including Harvard, Johns Hopkins, Hubbard, and Taku Glaciers.