Multibeam bathymetric data collected in Cape Cod Bay, Massachusetts during USGS Field Activity 2019-002-FA, using a dual-head Reson T20-P multibeam echo sounder (32-bit GeoTIFF, UTM Zone 19N, NAD 83, MLLW Vertical Datum, 5-m resolution)

PROCESSING STEP 1: CARIS HIPS DATA PROCESSING. Multibeam bathymetry processing within CARIS HIPS (version 11.3) post-survey consisted of the following flow:
1) A vessel configuration file was created in CARIS for the HSX sonar files (WarrenJrHSX_4-24-2020.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 11.3) was created with projection information set to Universal Transverse Mercator (UTM) Zone 19N, WGS84 using the vessel configuration file from step 1.
3) Each HSX file was imported to the new CARIS project 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) 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.
6) Navigation source was set to Applanix SBET, and navigation was reviewed and edited as needed using the Navigation Editor tool.
6) Georeference Bathymetry was done to apply the sound velocity corrections from the final SVP file (2019-002-FA_SVP_MASTER_v2.svp) containing all the sound velocity profiles collected during the survey, specifying the nearest in time method, 'delayed' heave source, and use surface sound speed.
8) 5-m resolution Swath Angle Weighted (SWATH) surfaces were created to incorporate all the files 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.
The steps described above were performed by Brian Andrews and Bill Danforth. The contact person for this and all subsequent processing steps below is Seth Ackerman.

PROCESSING STEP 2: CREATE FINAL BATHYMETRY SURFACE.
Bathymetry surfaces were created for several separate areas (Leg1_5m_extract, Leg1_1.8X_cutoff_5m, Leg2, and then mosaicked into a single surface in CARIS 11.3. Two rounds of interpolation were applied to the mosaicked surface to fill small gaps.

PROCESSSING STEP 3: EXPORT AND TRANSFORM TO MLLW.
The final CARIS HIPS SWATH surface was exported as a 5-m per pixel ASCII file referenced to UTM Zone 19N, 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 Mean Lower Low Water (MLLW). The process required transformation of the horizontal and vertical reference frames from UTM Zone 19N, WGS84 (using ITRF2008) and ellipsoidal heights to UTM Zone 19N, North American Datum of 1983 (NAD 83) and MLLW 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 2019-002-FA_T20P_Bathymetry_5m_MLLW.tif) using Export, Export Elevation Grid Format, GeoTIFF.

PROCESSSING STEP 4: POST-PUBLICATION DATA FIX.
Three weeks after publication, it was noticed that additional, unintentional interpolation was done during the conversion from ASCII raster to 32-bit GeoTIFF in Global Mapper (see PROCESSING STEP 3 above). To fix the problem, the bathymetry surfaces: Leg2_V3_5m_Interp2, CCBayWest_Leg2_5m_Interp2, Leg1_5m_fbp_interp2, and PtownLine_5m_interp2, were combined in CARIS HIPS version 11.3 and re-exported as described above. The same process to transform the surface to MLLW was done using VDATUM version 4.4.2 and the same conversion to GeoTIFF was done using Global Mapper version 21.0. The new filename is named the same as the original filename.