2018 Mean High Water Shorelines of the Puerto Rico coast used in Shoreline Change Analysis

Overview of the methods used to extract shoreline features in Puerto Rico and the islands of Vieques and Culebra: This data release has two methods of shoreline extraction: the contour method and the profile method. Both methods use the MHW elevation for shoreline extraction. The average MHW for Puerto Rico was calculated using NOAA’s vdatum tool (version 4.1.2; https://vdatum.noaa.gov/) to model the regional surface using local MHW values based on the PRVD02 vertical datum.
The contour method was the primary method in the shoreline extraction process. Described in Farris and others (2018), the contour method extracts the elevation of average MHW value from DEM data using ArcMap tool “Contour List” with the MHW value chosen for the contour. These shorelines are polyline shapefiles.
Also described in Farris and others (2018), the profile method produces a datum-based mean high water (MHW) shoreline. The profile method extracts the MHW shoreline point from the lidar point cloud data, using a cross shore transect in a Matlab-based approach.

Definition of average MHW for Puerto Rico: The height of MHW was determined from vdatum 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). This value is always within 6 cm of the value returned by vdatum at any point along the coast. The MHW value ranges from 0.09 m to 0.19 m from all tide observation stations in Puerto Rico, Vieques, and Culebra. Taking the mean from each of those MHW observations results in 0.13 m. This is the contour value used to delineate the shoreline and is recorded as the contour attribute in this shapefile.

Contour Shoreline extraction: A suite of ArcGIS tools (ArcGIS version 10.7.1) were compiled into a series of in-house models with ArcGIS ModelBuilder and used to help automate the contour method of shoreline extraction. First, the MHW shoreline elevation (0.13 m NAV88) was extracted from the DEMs using the ArcGIS Contour List tool (ArcToolbox > Spatial Analyst > Surface > Contour List). This step was iterated for each individual DEM in the study area. As a result, individual shapefiles were created for each DEM.

Contour Shoreline smoothing: 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.

Contour Shoreline date calculation: The date for each shoreline is required to identify the dataset, and is used for shoreline change analysis with DSAS (Himmelstoss and others 2018). The date for each contour shoreline was calculated using the corresponding flight paths layer provided by the United States Army Corps of Engineers (USACE) Joint Airborne Lidar Bathymetry Technical Center of Expertise (JALBTCX).
The flight paths were downloaded from the “JALBTCX 2018 Flight Lines: Dates Flown” ArcGIS Online layer (https://usgs.maps.arcgis.com/home/item.html?id=4e70d40f9c8340b58c58935e99b0d960). The flight lines from Puerto Rico were selected and saved as a line shapefile. The flight lines were also projected to match the same projection as the contour shorelines (ArcToolbox > Data Management > Projections and Transformations > Project).
The dates associated with the flight lines in the attribute table were used to apply the date to the contour shoreline features. In some instances, however, there were multiple flight lines to one shoreline feature as multiple flight line paths overlapped with each other. In this case, the average date was calculated and applied to those shoreline features.
To do this, the date field from the flight lines was converted to the Julian day (in this case, Julian day refers to the day of the year number).
In order to spatially join the flight lines to the contour shorelines, the flight lines were converted from a line to polygon shapefile using the Minimum Bounding Geometry Tool (ArcToolbox > Data Management > Features > Minimum Bounding Geometry). A “Count” field was added and calculated with a value of 1 in the flight line boundary polygon shapefile. Then, the contour shorelines were split where they intersected with the flight line boundaries. This was done by creating a separate point shapefile where the contours intersected with the flight line boundary (ArcToolbox > Analysis > Overlay > Intersect) and then split the contours at these intersection points (ArcToolbox > Data Management > Features > Split Line at Point).
Each of these split contours were then spatially joined to the corresponding flight line boundary or boundaries within which they fell (ArcToolbox > Analysis > Overlay > Spatial Join). The sum of the Julian day and Count fields was designated as the merge rule when spatially joining the flight line boundaries to the contours.
The summed Julian day field was divided by the summed Count field to derive the average Julian day for each of the contour shorelines. The average Julian day was then converted back to a date as a text field, compatible with DSAS.

Contour Shoreline attributes: Additional attributes are added UNCY (see Horizontal Positional Accuracy Report), PROXY (MHW), AGENCY, TYPE (lidar), METHOD (contour).

Contour Shoreline edits: The shoreline was then manually edited with the lidar data displayed with categorized elevation values to highlight the MHW values and a modern base map image for reference. Segments of the contour that did not fall near the open ocean MHW were removed. As noted areas with insufficient coverage due to data gaps in the DEM at or near the mean high water line, were flagged for additional analysis using the Profile method (Farris, and others 2018).

Profile Shoreline extraction: The profile method of extraction was used for over 31 kilometers of shoreline where data gaps were present in the contour shoreline. The a profile method extracts the operational Mean High Water (MHW) shoreline from the lidar point cloud data, using a Matlab-based approach (Matlab version 2015b) similar to the one developed by Stockdon and others (2002). Elevation values for the height of MHW were determined from vdatum (version 4.1) 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 6 cm of the value returned by vdatum at any point along the coast. For example, we used MHW = 0.13 m for all of Puerto Rico even though the MHW varies across the island from 0.09m to 0.19m from all tide observation stations in Puerto Rico. Profile shorelines were not extracted for the islands of Vieques and Culebra.
This profile method uses a coast-following reference line with 20 m spaced cross shore profiles. All lidar data points that are within 1 m of each profile 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. See the Horizontal Positional Accuracy Report for details.
The final output is a shoreline comprised of the MHW shoreline points, with attributes added for Shoreline Type (MHW), Date, and Uncertainty.

Combined the Profile and Contour Shorelines and updated attributes for profile segments (PROXY, AGENCY, TYPE, METHOD).

Final coordinate transformation from "NAD_1983_NSRS2007_StatePlane_Puerto_Rico_Virgin_Isls_FIPS_5200" to "GCS_WGS_1984".