2018 mean high water shoreline of the coast of MA used in shoreline change analysis

Explanation of the methods used to delineate shorelines that are a part of this update for the Massachusetts Office of Coastal Zone Management Shoreline Change Project: This data release contains a new 2018 MHW shoreline extracted from lidar data that covers the North Shore, South Shore, Cape Cod Bay, Outer Cape, South Cape, a small portion of Buzzard’s Bay, Martha’s Vineyard, and Nantucket. Lidar data for 2018 were not available for Boston or the Elizabeth Islands region.
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 profile open-ocean coast, the elevation of MHW was obtained from Weber and others (2005). Since Weber and others (2005) only covers open-ocean coast, all MHW elevations for profile not open ocean-coast shorelines come from NOAA's vdatum (version 3.8; https://vdatum.noaa.gov/).
Historical vector shorelines digitized from georegistered T-sheets using standard editing tools in ArcMap provide a proxy-based high water line (HWL) feature that is not tidally-referenced. Individually these shorelines are stored as polyline shapefiles. All shorelines have an uncertainty value listed in the attribute table that provides the horizontal uncertainty associated with the shoreline, regardless of the method used. The shoreline database contains both MHW and HWL shorelines, which can lead to incorrect estimates of rates of change since visually identified HWL-type proxy shorelines are virtually never coincident with datum-based MHW-type shorelines. In fact, HWL shorelines are almost universally estimated to be higher (landward) on the beach profile than MHW shorelines. Not accounting for this offset will cause shoreline change rates to be biased toward slower shoreline retreat, progradation rather than retreat, or faster progradation than in reality (for the typical case where datum-based MHW shorelines are more recent data than the proxy-based HWL shoreline dates). A proxy-datum bias (PDB) describes the unidirectional horizontal offset that occurs between the MHW shoreline and the HWL shoreline, and is stored as a line feature class appended to the reference baseline, with an attribute for the bias value as well as the bias uncertainty. For more information about the proxy-datum bias, see Ruggiero and List (2009)
All the following process steps were completed by the same person: Marie K. Bartlett

Whenever possible, a profile method was used to extract the operational MHW shoreline from the lidar point cloud data, using a Matlab-based approach (Matlab version 2019b) similar to the one developed by Stockdon and others (2002). Elevation values for the height of MHW were determined from vdatum (version 3.8) provided by NOAA (https://vdatum.noaa.gov/). We continued the practice set out by Weber and others, (2005) of using one MHW value for a continuous section of coast (as opposed to using a continuously varying value). We chose this value such that it is always within 15 cm of the value returned by vdatum at any point along the coast. For example, we used MHW = 0.98 m for the entire Outer Cape even though vdatum shows it varying slightly over the basin. For the south shore of Cape Cod we used an average MHW elevation of 0.2 meters from Falmouth to approximately Waquoit and 0.50 meters from approximately Waquoit to Chatham.
This profile method uses a coast-following reference line with 20 m spaced profiles. All lidar data points that are within 1 m of each profile line are associated with that profile. All work is done on the 2 m 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. Foreshore beach slope is defined as the slope of the regression line.
Each MHW shoreline point that was extracted using this profile method has an uncertainty associated with it. For the new 2018 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 lidar data’s 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 uncertainty for the 2018 shoreline was calculated slightly differently than the uncertainty for the previous lidar shorelines. For details on the uncertainty for lidar shorelines prior to the 2018 shoreline, see pp.12-13 under the section titled Lidar-Derived MHW Shoreline Position Uncertainty in Hapke and others (2011).
There is a known horizontal offset between the datum-based lidar MHW shoreline and the proxy-based historical shorelines on open-ocean sandy beaches that nearly always acts in one direction (Ruggiero and List, 2009). In order to estimate the bias between these two types of shorelines, wave data from a nearby buoy is needed to calculate wave run-up. Many of the sheltered areas within the Massachusetts study area do not have nearby buoy data and a proxy-datum bias value cannot be calculated in these areas. Therefore, when this new datum-based MHW shoreline is compared to historic proxy-based HWL shorelines in these sheltered areas, there may be some artificial landward offset of the historic data that has not been quantified and could potentially influence the rates. The north facing shoreline of Martha's Vineyard tends to have lower wave energy, which would result in less run-up and yield a smaller offset between a proxy shoreline and a datum shoreline than would occur along open-ocean coasts

Contour shoreline extraction: While the profile method of shoreline extraction is best for open-ocean coastlines, a contour method is used along sheltered sections of the coast that were too crenulated for the profile method. The elevation of MHW was used from Weber and others, 2005 when available. In areas not covered by Weber and others (2005), NOAA's vdatum (version 3.8; https://vdatum.noaa.gov/) is used to determine MHW. 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. Each contour shoreline was given an average horizontal positional uncertainty of 2 m. 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 shoreline file for the state.

The profile method shoreline and contour method shoreline were merged into a single feature class using Esri's ArcToolbox > Data Management > General > Merge. 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)

The exported shoreline shapefile was projected in Esri's ArcToolbox > Data Management Tools > Projections and Transformations > Project. Parameters: input projection - UTM zone 19N (WGS84); output projection - geographic coordinates (WGS84); transformation = none.