1998_Glacier margins derived from 1998 aerial imagery for the named glaciers of Glacier National Park, MT and environs

These polygon features represent digitization of the glacier margins for the 37 named glaciers of Glacier National Park (GNP) and the 2 glaciers along the border of GNP derived from aerial images acquired on September 11, 12, 14, 15, 1998. The polygons represent only the main body portion of the glacier as they appeared in the 1998 imagery. Disconnected patches are not included as this dataset represents only the main body features. Polygons were digitized from orthorectified aerial images with initial digitization completed in 2001 by Michelle Manly, University of North Dakota graduate student. This set of polygons represents a thorough review with revisions to the initial dataset based on local knowledge and improved satellite imagery acquired in 2015. In several cases, the much higher resolution 2015 imagery revealed features, such as debris covered ice, where 1998 image analysis had deemed bedrock and actual margins had to be re-evaluated. A Wacom Pro digital tablet was used by USGS staff to trace outlines and make revisions to the original margins. Glaciers were digitized at 1:2000 scale. Since multiple images in time series contribute to this analysis, if previous image showed perennial snow that was absent from the glacier (bedrock visible), then that portion was deemed "seasonal/perennial snow" in subsequent photos and not included in the digitization of 1998 glacier margins.