Transects with net change results for GPS and Worldview shorelines for the Grand Bay National Estuarine Research Reserve in Mississippi from 2013-2020

For the methods used to derive the shorelines, please refer to the GPS and WorldView metadata records with this data release. Transects were generated by buffering the available shoreline datasets to create inner (land based) and outer (water based) baselines. The baselines form the start and end point for shoreline-perpendicular transects. Baselines were input into the AMBUR statistical package for R (version 4.0.4). AMBUR has several tools, which utilize shoreline parallel baselines to generate transects that are generally perpendicular to the shoreline. Transects were constructed at a sampling distance of 10 meters. Length varied depending on location, and so a length longer than the largest shoreline distance to ensure transects covered all dated shorelines. Once transects were created, the Filter Transects tool was used to adjust and even out the spread of transects. Where shorelines experience sharp bends, such as in small bays and narrow spits, transects may fall at a non-perpendicular angle. Please see the shoreline datasets to determine the estuarine shorelines analyzed. When necessary, final transects were checked and edited in ArcGIS ArcMap (version 10.5.1), to improve analysis results and improve shoreline coverage. Transects were manually edited to reduce errors in the analyses or to provide a more perpendicular intersection with the shorelines. Shoreline points and final statistical analyses were completed in AMBUR to generate the shoreline change rates. The following analysis parameters were used: first intersection (if transect intersects the same shoreline more than once, then by default it selected the first intersection), confidence level 95 (confidence level for the linear regression statistics), unit label m (the units of measure for the map is for Universal Transverse Mercator, in meters), analysis type is advanced (advanced includes additional statistics for a robust linear regression), and time unit for rates is "year" (utilizes years for calculating rates of shoreline change). More information on the AMBUR program can be obtained from Jackson (2010). Transects not covering study area shorelines were removed.

Additional fields were added to the shapefile’s attribute table such as water level (wl) and year fraction (yr_frac). Water level was collected from the National Oceanic and Atmospheric Administration (NOAA) tides and currents webpage using the Dauphin Island station (https://tidesandcurrents.noaa.gov/waterlevels.html?id=8735180). Water level was collected for as close to the time the WorldView image was taken. Year fraction was calculated as the number of days between the GPS collection date and the WorldView date divided by 365 (leap years by 366). Due to having the GPS date as the starting date of the calculation, this often resulted in a negative value for the year fraction.