Historic Glacier Change using a GIS: Progress and problems

We are exploring the potential wealth of information on glacier extent and topography contained in historic maps. If successful, we can examine the spatial and temporal patterns of historic glacier change where glaciological studies are typically absent. Our project is focused on the American West and data on glacier extent and topography is derived from aerial and ground-based photographs, paper maps, and digital maps. Our initial conception was that digital maps would provide a reliable base on which new data can be added. However, we encountered many errors when compiling the glacier outlines from the 100K scale USGS digital line graph (DLG) coverage. We resorted to overlaying the DLG coverage over the scanned and geo-registered paper maps (digital raster graphics) to error check the DLG coverage. At the time of this writing we are working on the 24K scale coverage, which we obtained from the US Forest Service that uses a different approach to digital map presentation. One important challenge is to define the date of the mapping. Because glaciers change in time, we need to know the date of photography used to make the USGS map. However the data accompanying the maps do not specify what part of the map was updated at the time of last printing. Experience shows that the glacier features are not updated and their outline dates to the original photography from which the first map was made. Although our immediate goal is to assess changes in area, changes in volume are important as well. We found that that historic evaluations of topography (prior to aerial photography) appear reasonable low on glaciers in the ablation zone, higher on the glaciers the topography can be way off and suggests caution in relying on such data sources. While we have found fairly good results in using historic maps, one must carefully examine every map to evaluate its reliability.