DATA PROCESSING – INTERFEROMETRY SWATH BATHYMETRY
Soundings
SWATHplus serves as both acquisition software and initial processing software. Roll calibration data were collected and processed using SWATHplus and Grid Processor 3.7.10 (both products of Systems Engineering and Assessment Ltd.). Raw data files (.sxr) were then processed using system configuration files (.sxs) which contain roll calibration values, measured equipment offsets, acquisition parameters, navigation and motion from the F190R, and speed of sound. Any calibrations and filtering conducted in SWATHplus are saved to a processed data file (.sxp) which can then be imported into other software for secondary processing (for example, CARIS HIPS and SIPS). The acquisition datum for OmniSTAR HP position and navigation is ITRF2005, and data were processed to the ellipsoid.
All processed data files were imported into CARIS HIPS and SIPS, version 7.1 where the data were edited for outliers using depth filters, reference surfaces, and manual editing of data points. A CARIS BASE (bathymetry with associated statistical error) surface was created at 10-m resolution for coarse CUBE (Combined Uncertainty and Bathymetry Estimator) surface hypothesis editing in CARIS Subset Editor, and a 1-m resolution surface was created for detail editing in Subset Editor. A CUBE hypothesis surface represents the soundings weighted by uncertainty and proximity to the grid nodes. This algorithm allows for multiple grid node hypotheses to be verified or overridden by the user. The x,y,z data were exported as ASCII text at a 10x10-m sample resolution in the ellipsoid datum of ITRF 2005. The data were then transformed to vertical datum NAVD88 using NOAA VDatum software conversion tool version 2.3.3 (reported vertical transformation error is 5.4 cm).
Gridding
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.
