Vectorized marsh shorelines derived from global positioning system data for the Grand Bay National Estuarine Research Reserve in Mississippi from 2013-2020

From 2013 to 2020, coastal managers’ survey shoreline position using Real-time Kinematic (RTK) GPS at eleven locations in GBNERR. GPS data were collected using a Trimble R8 Model 3 Global Navigation Satellite System (GNSS) and TSC3 data collector from 2013 to 2018, or a Trimble R10 GNSS system and TSC3 data collector from 2018 to 2020. Each were attached onto a 2-meter (m) graphite rod with a mounted foot to obtain both horizontal and vertical shoreline position. The positional accuracy of Trimble R8 Model 3 GPS points was ± 10 millimeters (mm) + 1 parts per million (ppm) root mean square (RMS) horizontal error and ± 20 mm + 1 ppm RMS vertical error. The horizontal error of the Trimble R10 GPS points was ± 8 mm + 0.5 ppm RMS and vertical error was ± 15 mm + 0.5 ppm RMS. The points were collected roughly 5 to 10 meters apart along the vegetation-water boundary, which typically represented the top of an erosional scarp. In instances where an erosional scarp was not visible, the most suitable shoreline position based on dense shoreline vegetation is mapped.

GPS points were processed and turned into an ArcGIS shapefile. The points were converted into vector shorelines using XTools Pro (v.17) "polylines from points" tool. The location between each point was interpolated so the specific shoreline location between each point might not represent the actual shoreline position. To see the GPS points see the GPS point shapefile (GPS_pts_with_near_dist_2013_2020.shp) of this data release (https://doi.org/10.5066/P9W8TNQM).

Shorelines were compiled from all available data sources as both previously published shorelines or ones created for this study. Regardless of the data source, shorelines were edited in ArcGIS (version 10.5) so that only the desired shorelines were included in the final dataset. Once shorelines were edited and checked, all dated shorelines were merged into a single dataset. Attributes were edited to include information regarding the date and source of the original data. Compiled shorelines were processed using the AMBUR statistical package for R (version 4.0.4) tool to add required missing attributes. AMBUR performs shoreline change analysis on vector digital shorelines. All necessary attribute fields must be present in order to run various AMBUR functions; the following fields are required for boundaries: "Id", "DATE_", "ACCURACY", "SHORE_LOC", "CLASS_1", "CLASS_2", "CLASS_3", and "GROUP". In order to determine if all the required fields were present in the merged boundary datasets, an AMBUR sub-routine (ambur.check) was used to check all shoreline shapefiles for missing attribute fields. For more information about the AMBUR shoreline requirements see the AMBUR user manual (Jackson, 2010). Some fields created by AMBUR ("SHORE_LOC", "CLASS_1", "CLASS_2", "CLASS_3", and "GROUP") are required and have null values as placeholders, thus they should not be used for any other analysis outside of AMBUR. Null fields were left in the results as they are required by AMBUR and are needed to replicate our study. Users are advised to review the data to determine if it is suitable for their studies.