This section describes the methods used to compile shoreline data and to calculate rates-of-change for the north coast of Alaska between the U.S.-Canadian border and Icy Cape (figs. 1-4).

Shoreline Data

Historical shoreline data were acquired from multiple sources and have been documented in the shoreline attribute table and metadata files in this report. Shoreline data from the 1940s era were digitized from National Oceanographic and Atmospheric Administration (NOAA) topographic sheets (T-sheets) from 1947 and 1949. Shoreline data from the 2000s era (1997–2012) were digitized from aerial photographs, satellite images, and a lidar topographic survey.

The historical shorelines are based on an interpreted high water line (HWL) as mapped on and digitized from topographic maps (1940s) and an interpreted land-water interface (LWI) digitized from aerial photographs, satellite images, and lidar topographic survey (2000s).

Additional information regarding the shoreline-compilation methods and measurement uncertainties, and a summary of the results are available in Gibbs and Richmond (2015).

Calculation of Shoreline Change Rates

Rates of shoreline change were generated within ESRI ArcMap™ version 10.1 using the Digital Shoreline Analysis System (DSAS) version 4.3, an ESRI ArcGIS™ tool developed by the USGS (Thieler and others, 2009). The tool is a freely available application designed to work within the ArcGIS™ software. The DSAS was used to generate orthogonal transects starting from a reference baseline and intersecting the shoreline positions at 50-m intervals. The distance measurements between the transect-shoreline intersections and the baseline were then used to calculate the rate-of-change statistics. An end-point method was used to calculate rates of change.

End-Point Rates

Rates of shoreline change were calculated at each transect using an end-point rate calculation between the shoreline position from time period 1 (1947 and 1949) and time period 2 (1997 to 2012) to provide an approximate 60-year rate. The end-point rate is calculated by subtracting the difference in shoreline position between the two survey years and dividing it by the time between surveys to give a rate in meters per year. The shoreline change rate was not assumed to be linear between the two survey years; this rate simply represents the net change between the two shorelines. The transect metadata files provide descriptions of the two fields associated with the end-point rate calculation. Additional information is available in Section 7 of the DSAS 4.0 Installation Instructions and User Guide.

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