## 2 Historical Data
### Aerial Photography in the Polar Regions  
The initial impetus for early aerial photography (or airphot) of the polar regions was strategic topographic mapping by national mapping agencies. For regional, strategic mapping programs, the focus was on systematically covering large geographical areas as efficiently as possible. Therefore, oblique aerial photography was commonly used, especially for what was really reconnaissance mapping of largely unknown areas. Following further developments in aviation, photography and plotting techniques driven by reconnaissance and military mapping during World War II (WWII), topographic mapping from vertical aerial photography became the norm. National mapping and military agencies began to acquire systematic coverage of polar regions, initially often using ex-military cameras from the WWII era. Some of this aerial photography was acquired using near-infrared film, originally developed for military use as camouflage detection film; this preserves a record of vegetation coverage at the time. Large areas of both the Arctic and Antarctica were covered by aerial photography flown by the U.S. Army Map Service in the late 1940s and 1950s as part of Cold War strategic information gathering. The value today of this legacy of hundreds of thousands of historic aerial photographs for studying environmental changes is incidental to the original reconnaissance and topographic mapping purpose; photos provide a baseline for environmental variables purely because these features happen to be part of the topography. Following WWII, there was separately an intentional effort to use aerial photography for Arctic and lower-latitude glacier monitoring. Regular measurements of mass-balance for selected glaciers in Scandinavia, the Alps, Alaska and the former Soviet Union, some of which continue to this day, were initiated in the 1940s [3]. Some of these selected glaciers also have an archive of repeat coverage, even though there may not be regional coverage with the same frequency. While these glaciers were targeted for monitoring studies, the majority of polar aerial photography did not have a scientific focus. As a consequence, the photographs can often be difficult to use quantitatively for detailed studies and are an under-utilized resource. Flight lines are usually aligned for efficient regional coverage rather than optimally for landscape features, for example along the central flow-line of a glacier. In the decades since the aerial photographs were acquired, some datasets have lost metadata about the cameras and lenses that were used, which hinders precise photogrammetric work. Additionally, remote polar areas often have geodetic survey networks that are too sparse to accurately georeference the photography, and acquiring adequate ground control points is difficult and expensive for large areas in accessible regions. Fortunately, applying modern photogrammetry and image analysis methods can overcome these problems, unlocking the legacy of aerial photographs for detailed measurements of landscape change over decadal scales. For example, some studies have used calibration from military cartographic cameras [4,5] and linking or surface matching to modern satellite or airborne images to improve measurements of historic glacier volume change on the Antarctic Peninsula [6,7]. Others use modern GPS or LiDAR data to fix ground control points in historical photos for photogrammetric processing [8, 9, 10]. These new techniques and ancillary data can bring new life to historical remote sensing data. Analogue aerial photographs from a half-century ago cannot match the resolution and radiometric depth of modern digital camera systems and can have problems with image saturation and lack of surface detail on snow surfaces. Nevertheless, aerial photography camera, lens and film technologies were evolving rapidly during the 1950s and 1960s, and aerial photographs from this period that have been well stored can be of surprisingly high quality.