DATA PROCESSING – SINGLE-BEAM BATHYMETRY
The coordinate values of the GPS base station are the time-weighted average of values obtained from the National Geodetic Survey On-Line Positioning User Service (OPUS). The final solution coordinate value (World Geodetic System 1984 (WGS84) (1150)) was used in GrafNav version 8.10 (Waypoint Product Group) as the point-base location. Using GrafNav, the base station GPS session files were then processed to the respective roving survey platform (boat) GPS session files. During this process, steps were taken to ensure that the trajectory produced from the base to the rover was clean and produced fixed positions. From these processes, differentially corrected (DGPS), precise positions were created for the single-beam bathymetry survey at 1 s intervals for each roving GPS session and exported in text format.
The DGPS position points processed in GrafNav were parsed into the bathymetric processing software CARIS® HIPS (Hydrographic Information Processing System (HIPS) and Sonar Information Processing System (SIPS) version 7.1 using the generic data parser within the software. The sounding depths were recorded using HYPAC, version 10 and were imported into CARIS. Bathymetric data were merged with the corrected navigation and then edited for outliers, which were removed and the x,y,z data were exported in their originally acquired ellipsoid datum of WGS84 (1150). The data were then transformed to vertical datum NAVD88, using the National Oceanic and Atmospheric Association (NOAA) VDatum software conversion tool version 3 (reported vertical transformation error is 5.4 cm). The soundings were imported into ArcGIS version 10.0.2 and merged with the swath bathymetry data.
Within ESRI ArcGIS version 10.0.2, the swath and the single-beam soundings were merged together before gridding and soundings were further investigated for outliers, which were removed if found. A 50-m cell grid was created using the ArcGIS Topo to Raster tool. In addition to the sounding data, elevation EAARL lidar data collected for Cat Island by the USGS in 2007 (Smith and others, 2009) and a boundary box polygon for the survey extent were used to constrain the grid. A detailed description of the Topo to Raster (Anudem 4.6.3) algorithm can be found within the help menu of ESRI ArcGIS.