A GIS compilation of vector shorelines for the Virginia coastal region from the 1840s to 2010s

Historical shoreline data covering the coast of Virginia from the previously published National Assessment of Shoreline Change: Historical Shoreline Change along the New England and Mid-Atlantic Coasts (USGS Open-File Report 2010-1119) were compiled and merged into a single feature class within a personal geodatabase in ArcMap v 10.7.1. The data were projected in ArcToolbox v10.7.1 > Data Management Tools > Projections and Transformations > Feature > Project. Parameters: input projection = geographic coordinates (WGS84); output projection = UTM zone 18N (WGS 84); transformation = none. Historical shoreline data covers date range 1849-1997 and includes one lidar-derived shoreline (1997). Some shoreline segments from 1962 were removed because they represented the wrong feature. See associated source metadata (https://pubs.usgs.gov/of/2010/1119/title_page.html) for complete process steps for historical shoreline delineation.

MHW shorelines are datum-based and in the case of the shorelines used here, were extracted from lidar data using a profile method, whenever possible, or a contour method. For both methods, a MHW elevation of 0.26 m was obtained from Weber and others (2005).

The profile method used to extract the operational MHW shoreline from the lidar point cloud data utilizing the Matlab-based approach (Matlab version 2019b) is described in Farris and others (2018). The profile method used a coast-following reference line with 20-meter spaced profiles. All lidar data points that were within 1 meter of each profile line were associated with that profile. All processing was done on the 2-meter-wide profiles, working on a single profile at a time. For each profile, a linear regression was fit through data points on the foreshore and the regression was evaluated at the MHW elevation to yield the cross-shore position of the MHW shoreline. If there was a data gap at MHW or if the MHW elevation was obscured by water points, the linear regression was simply extrapolated to the MHW elevation. For each profile, the foreshore beach slope was defined as the slope of the regression line. This slope is stored and later used for the calculation of the proxy-datum bias feature. Each MHW shoreline point that was extracted using this profile method has an uncertainty associated with it. For the new 2010 and 2017 lidar shoreline, this uncertainty includes four components: 1) the 95% confidence interval on the linear regression estimate of the shoreline position; 2) the vertical error of the raw lidar data as reported in the metadata; 3) a 15 cm vertical error in our chosen value of MHW, and; 4) the uncertainty due to extrapolation (if the shoreline was determined using extrapolation). These four components of uncertainty were added in quadrature to yield a total error for each shoreline point. The 2010 shoreline extraction was performed by Amy S. Farris The 2017 shoreline extraction was performed by Marie K. Bartlett

One small (~6k) section of the 2017 lidar shoreline was extracted using a contour method, with a MHW elevation of 0.26 m determined from Weber and others (2005). To extract the MHW shoreline from each DEM surface, the operational MHW elevation was entered into the ArcGIS tool Contour List as the contour value. This step was iterated for each individual DEM in the study area. As a result, individual shapefiles were created for each DEM. The shoreline was then manually edited with the lidar data displayed with categorized elevation values to highlight the MHW values. Segments of the contour that did not fall near MHW were removed. Sometimes the lidar data did not extend down to MHW, resulting in gaps in the contour. Each contour shoreline was generalized using the Smooth Line tool (ArcToolbox > Cartography > Generalization > Smooth Line), applying the PAEK algorithm with a 5 m tolerance. The shoreline features were then merged (ArcToolbox > Data Management > General > Merge) into one feature class. The contour shoreline segments were given the positional uncertainty of the adjacent profile shoreline segments, which was 2.15879. The resulting contour shoreline was merged with the profile method shoreline using the Merge tool in Esri’s ArcToolbox > Data management > General > Merge to generate one complete 2017 shoreline file for the state.

The profile method shorelines, contour shoreline segment, and historical shorelines were confirmed to have the same attribute fields required for use in DSAS before they were merged into a single feature class using Esri's ArcToolbox > Data Management > General > Merge. After rate calculations, the feature class was exported out of the geodatabase to shapefile format by right-clicking on the feature class name in the table of contents of the ArcMap project and choosing Export > Shapefile (single); Coordinate System: UTM Zone 18N (WGS84)