Multibeam bathymetric data collected in Little Egg Inlet and offshore the southern end of Long Beach Island, NJ, during USGS Field Activity 2018-001-FA, using a dual-head Reson T20-P multibeam echo sounder (32-bit GeoTIFF, UTM Zone 18N, NAD 83, NAVD 88 Vertical Datum, 4-m resolution)

PROCESSING STEP 1: CARIS HIPS DATA PROCESSING. Multibeam bathymetry processing within CARIS HIPS (version 10.4.1) during the survey consisted of the following flow:
1) Vessel configuration files were created in CARIS for the port ("M"=main, or primary) and starboard ("S"=secondary) sonars (Petrel_dual_T20P_s7k_M.hvf, and Petrel_dual_T20P_s7k_S.hvf) which included relevant, linear and angular installation offsets for each T20-P unit as well as vendor specified uncertainty values for each of the survey sensors.
2) A CARIS HIPS project (version 10.4.1) was created with projection information set to Universal Transverse Mercator (UTM) Zone 18N, WGS 84. Separate HIPS projects were created for the Port (M), and Starboard (S) line files using the two vessel configuration files in #1 above.
3) Each Reson s7k file (M and S) were imported to the new CARIS projects (M and S respectively) using the Import/Conversion Wizard.
4) Delayed heave data from raw POS MV files were used to update HIPS survey lines using the import auxiliary data function.
5) Navigation was reviewed and edited as needed using the Navigation Editor tool.
6) Sound velocity correction was applied using the CARIS algorithm, a master SVP file containing all the sound velocity profiles collected during the survey and specifying the nearest in distance method, delayed heave source, and use surface sound speed.
7) Data were merged selecting no tide and the delayed heave source.
8) 4-m resolution Swath Angle Weighted (SWATH) surfaces were created to incorporate all the files (M and S) as they were processed, and the SWATH surfaces were reviewed for inconsistencies and anomalies.
9) The swath and subset editors were used to remove spurious points through manual editing and filter application, and the refraction editor was used to adjust sound speed values in areas where sound velocity data did not adequately correct depth profiles, which were obviously influenced by local anomalies in speed of sound through the water column.
10) Survey lines adjusted for refraction anomalies were remerged, and the respective SWATH surfaces were recomputed to reflect the changes. Processing during the survey was primarily focused on QA/QC during acquisition. Editing processes did require trial and error and were at times iterative. This process step was completed over the course of several months beginning with work done during the survey in June 2018; the date on this process step indicated below is the last day described processing was done. The contact person for this and all subsequent processing steps below is Seth Ackerman.

PROCESSING STEP 2: APPLY POST PROCESSED SBET FILES AND EDIT SOUNDINGS.
Post-survey processing within CARIS HIPS (version 11.3.0) consisted of the following flow: 1) Delayed heave data from raw POS MV files were re-imported. Post-processed navigation, vessel attitude, and GPS height data from POSPac SBET files, and post-processed RMS attitude error data from POSPac smrmsg files were used to update HIPS survey lines using the import auxiliary data function.
2) Navigation source was set to Applanix SBET, and navigation was reviewed and edited as needed using the Navigation Editor tool.
3) Data were remerged. In CARIS version 11 the merge function is now called Georeference Bathymetry. In this step, the sound velocity correction is applied, the GPS tide is computed, the delayed heave is selected as heave source, and a single datum value of 0 m (vertically referencing the data to the WGS 84 Ellipsoid) is chosen.
4) A 4-m resolution SWATH surface using both the M and S files was created using the Swath Angle method.
7) Additional editing was conducted using the swath and subset editors to minimize inconsistencies and artifacts, and a final SWATH surface was created to reflect the changes. Finally, small "no data" holidays were filled using two iterations of the "Fill Raster Holiday" tool and a 5x5 cell filter using data from a minimum of 4 neighboring cells.

PROCESSSING STEP 3: EXPORT AND TRANSFORM TO NAVD 88.
The CARIS HIPS SWATH surface was exported as 4-m per pixel ASCII files referenced to UTM Zone 18N, WGS84 and ellipsoidal heights. The National Oceanic and Atmospheric Administration's Vertical Datum Transformation tool (VDatum v.4.1.2) was used to transform surfaces to the North American Vertical Datum of 1988 (NAVD 88). The process required transformation of the horizontal and vertical reference frames from WGS84 (using ITRF2008) and ellipsoidal heights to UTM Zone 18N, North American Datum of 1983 (NAD 83) and NAVD 88 orthometric heights (all in meters) using the default geoid model. The resulting ASCII raster was opened in Global Mapper (version 19.0) and converted into a 32-bit floating point GeoTIFF (2018-001-FA_T20P_Bathymetry_4m_NAVD88.tif) using Export, Export Elevation Grid Format, GeoTIFF.