2013–14 lidar elevation data were downloaded from NOAA’s online data access viewer (
https://coast.noaa.gov/dataviewer/#/) in LAS format (NAD83 2011, UTM Zone 19N (meters) and NAVD88 (meters)) for ten regions along the Massachusetts coast. These regions include: the North Shore, South Shore, Cape Cod Bay, Outer Cape, South Cape, part of the South Coast along Buzzard’s Bay, the southwestern, southern, and eastern coastlines of Martha’s Vineyard, the Vineyard Sound-facing coastline of Martha’s Vineyard, the Atlantic Ocean-facing coastlines of Muskeget, Tuckernuck, and Nantucket Islands, and the Nantucket Sound-facing coastlines of Muskeget, Tuckernuck, and Nantucket Islands. Each of the ten regions has its own coast-following reference line along which the lidar data are processed, and each reference line is divided into segments.
The lidar point data were processed segment by segment along each region’s reference line to extract the seaward-most dune toe following methods developed by Stockdon and others, 2012. The elevation data for each segment were interpolated in MATLAB 2017a to create a shore-parallel grid with a resolution of 10 m in the longshore direction and 2.5 m in the cross-shore. Each 10 m wide grid row is a cross-shore profile along which dune features are extracted. Each segment grid begins at profile number one, and for each profile, the MATLAB algorithm identifies the dune toe as the location of maximum slope change between the shoreline and the dune crest. The vertical error, which is based on the scatter of the data, is also calculated for each point. See Stockdon and others, 2012 for details.
The MATLAB algorithm was run for all segments in each region. Within the program, the geographic coordinates (WGS84) were calculated for each point and the automatically-extracted dune toe data, which includes dune toe elevation, error, and location in both eastings/northings and latitude/longitude, were written to ArcGIS shapefiles using MATLAB’s shapewrite.m script and Keyhole Markup Language (kml) files.
The dune toe locations were manually examined to ensure that an appropriate alongshore feature was identified. This was done by simultaneously viewing the data in three ways: dune toe kml files were viewed on 2014 Google Earth imagery, the shapefiles were viewed on DigitalGlobe world imagery in ArcMap, and the dune crest, toe, and lidar data (interpolated and point data) were viewed for each grid profile in a MATLAB Graphical User Interface (GUI). The GUI allowed for the deleting or moving of points as necessary to produce a final shapefile of dune toe points for each region. In the absence of a dune, the beach berm or seaward edge of the headland, cliff, bluff or hard structure (e.g., road, parking lot, seawall) was extracted since those features would serve as the first line of defense during a storm. Where appropriate, the dune toe was placed at the base of these proxy features. Much manual editing and deleting was required in developed regions as well as in regions with complex topography. The dune toe shapefiles for each of the ten regions were combined in ArcMap (ArcToolbox >> Data Management Tools >> General >> Merge) to create a final shapefile of dune toe position and elevation for the entire state.
A coordinate system transformation of the dune crest data was required for the points to correspond with the beach and marsh shoreline data that are part of this update to the Shoreline Change Project. Using ArcGIS version 10.5, the dune crest shapefile was transformed from the UTM zone 19 (NAD83 2011) coordinate system to the WGS84 geographic coordinate system (ArcToolbox >> Data Management Tools >> Projections and Transformations >> Project) using the “NAD_1983_HARN_To_NAD_1983_2011 + NAD_1983_HARN_To_WGS_1984” transformation. The DBF file of the shapefile was then saved as a CSV file using Microsoft Excel (version 2016).
