CPP-06. Analog data of Jakobshavn IsbrÓ•: its use improving historical SfM photogrammetric processing

Abstract
Mass loss of the Greenland ice sheet has increased significantly over the past 30-years with an overabundance of those changes occurring at marine terminating glaciers. Observations of the ice sheet pre-dating 1990 are sparse and induce underestimates of historical mass loss in long-term ice sheet mass loss. Sparce data affects our present-day understanding of the long-term dynamics of outlet glaciers both spatially and temporally. For those pre-1990 datasets that do exist, they are often in analog format and one obstacle in working with them is the availability of ground control (e.g., static features like exposed bedrock). Ground control is necessary for digital rendering of historical elevations from aerial photography. XYZ positions of ground control are limited to the terminus regions of outlet glaciers where there is sufficient bedrock distribution for structure-from-motion (SfM) photogrammetric processing. In 1985, aerial photographs were acquired from Jakobshavn IsbrÓ• for the purpose of rendering surface elevations. Initial attempts to process the 1985 images using SfM photogrammetry resulted in extensive elevation blunders due to the coverage of ground control excluding ~70% of the air photos where the captured surface is primarily of the ice sheet. One solution to improving ground control is to use contour elevations from the original analog 1980s processing to expand the coverage of the ground control to the ice sheet. We digitize and georeference film negatives of the isochrons overlain the 1985 terrain-corrected imagery in GIS software. Vector shapefiles of the contour lines are transformed to the WGS-84 ellipsoid after which they are interpolated to gridded elevations. Using co-registration techniques and correcting for tidal fluctuations, we difference these 1985 elevations from a 2020 DEM. The results show a mean surface lowering of ~75 m for the full study-site with the greatest lowering occurring within the present-day grounding zone (~285 m); this mean loss is similar to previous research. The georeferenced film negatives with the interpolated DEM allows us to assign XYZ values to ice sheet features (e.g., crevasses) in the 1985 imagery as ground control for future SfM photogrammetric processing. This study highlights the importance of analog data in producing accurate historical glaciological data for expanding and increasing the resolution of the temporal timeline of observations.