National Hydrography Dataset Alaska Glacier Inventory Acquisition
By Cynthia Miller-Corbett and Jeffrey D. Simley
Figure 1. Primary glacier complex locations.
The National Hydrography Dataset (NHD) is incorporating a new geospatial dataset to update delineation of glaciers included as a subset of the NHD hydrography waterbody feature class. The new geodata will significantly improve the existing NHD representation of ice mass features through the integration of current, authoritative glacier data provided in the Alaska Randolph Glacier Inventory (RGI).
Using a Geographic Information System, comparisons of existing NHD ice mass data with the Alaska RGI for glaciers in the Brooks Range, Alaska Range, Wrangell Mountains, Chugach Mountains, St. Elias Mountains, Coast Mountains, and the Aleutian Range were used to visually assess and measure differences in the areal extent of glaciers and the relation of NHD Stream/River vectors and Lake/Pond polygons to new glacier outlines (Figure 1). Where possible, the agreement between RGI glacier outlines and the 2012 Systeme Pour l’Observation de la Terre (SPOT) Imagery for US Topo maps was also evaluated.
Originally produced as a supplement to the Global Land Ice Measurements from Space initiative (GLIMS), the RGI is a combination of both new and existing published GLIMS glacier outlines that were developed based on analysis
Figure 2. Summary of approximate change for glacier extent.
These differences may be a result of temporal changes, differences in interpretation of the glacier extent, or both. The majority of historical NHD ice fields were delineated from topographic maps compiled in the 1950s and 1960s. Generally speaking, the largest glacier recessions are for large glaciers proximal to the southern Alaska coast. The estimated changes in glacial extent reported here only provide simple, approximate values. It would require a complex solution that incorporates atmospheric, solid earth geophysics, and ice mass behavior algorithms to attempt a valid assessment of change trends.
Figure 3. Changes in extent of Bering Glacier, St. Elias Mountains and Columbia Glacier, Chugach Mountains.
Research and assessment of the Alaska RGI geospatial data indicate the RGI meets NHD revision criteria for integrating data from an accurate, authoritative source. Comparison of RGI glacier outlines with current SPOT imagery for US Topo maps confirms good agreement, indicating compatibility for integrating the geospatial data in new US Topo map production. The representative sampling of glaciers suggests that areas where the Alaska RGI has delineated glacier outlines using coarse raster cell boundaries account for a small percentage of the inventory and may be resolved through the Alaska RGI or through acceptable digital map smoothing techniques.
As could be expected for some areas, differences in the areal extent of an NHD ice mass results in disconnections between previously associated NHD stream/river flowlines or lake/pond polygons. Changes to these NHD features will be identified and addressed in NHD data updates as required for accuracy and association of surface water flow features.