Bathymetric data during field activity 2021-014-FA offshore Head of the Meadow Beach, Truro MA on February 11, 2021

The bathymetry was collected by launching the SSV into the water from the surf zone. The boat was controlled near the shore by Peter Traykovski and put on auto-pilot using predetermined tracklines for longer transects offshore. The raw bathymetry data was processed with the following steps: 1. Parsed the binary data (BU353s4 GPS Puck, Time, Position and Echosounder range) into MATLAB (v. 2020b) from the EA24 Echosounder using MATLAB Scripts developed by Peter Traykovski at WHOI - see contact below 2. Post Processed Kinematic (PPK) GNSS (Novatel OEM-V2_L1L2) data from the vessel was corrected using the base station (Java Triumph-1) data in Novatel Inertial Explorer (Version 8.70.8722) using the datum NAD83(2011) and then GEOID 12B for the vertical datum NAVD88. A projection for UTM Zone 19N was also specified to get the Easting and Northing information. An antenna height of 0.0 was used and then the actual antenna height is corrected for in Step 5. 3. Refined time alignment to account for any small time delays between GPS data and Echosounder data collection by using time lagged cross correlation of Echosounder range and PPK GNSS altitude in locations with a relatively flat bottom (18 sec). The GPS measures vertical fluctuation of the boat due to waves and these fluctuations were also visible in echosounder data. The equation in Step 5 removed the vertical fluctuation due to waves from the echosounder data that leaves only true bathymetry if GPS and the Echosounder are well synced. The lagged cross correlation process ensures that they were synced optimally. 4. Duplicate times were removed from the PPK GNSS positions to create 2021014FA_Truro_trackline_raw.csv, then GPS values (ppk_Height) were interpolated to time of Echosounder samples (2021014FA_Truro_echo_raw.csv) with the MATLAB function interp1 (linear interpolation) and 'pchip'. 5. Calculated sea floor elevation (referred to as depth) with reference to the NAVD88 datum in meters using the following equation: Depth = ppk_Height - Antenna_Z_Offset + Echo 450kHz; Antenna_Z_Offset = 0.43 m was the offset from the GNSS antenna to the Echosounder transducer on the z-axis of the ASV. 7. Gridded 'Depth' onto 1 m resolution UTM eastings, northings using MATLAB file exchange script regularizedata3d by Jamal (2020), a cubic interpolation with the optimum smoothness coefficient determined by a Monte Carlo optimization procedure for the data regularization. Regularizedata3d entailed removing one cross-shore line of data from the processing and adjusting the smoothness parameter so the overall surface fit best fits the removed data. The process was then repeated for all the lines of data. The optimized smoothing parameter was used with all the data for the final processing. 8. The MATLAB function 'roipoly' returned the mask as a binary image, which sets pixels inside the region of interest (ROI) to 1 and pixels outside the ROI to 0. The boundary was developed so that there were no extrapolated bathymetry data outside the tracklines. The masks extent was the convex hull of the tracklines positions (x,y) and a bounding z coordinate of 0.8 represented the surface in reference to NAVD88 in meters. 9. Exported gridded 1 m data in NAD83(2011)/UTM Zone 19N in NAVD88 meters as a GeoTIFF: 2021014FA_Truro_bathymetry_1m.tif. Note that using NAVD88 as a height, means that the depths are not true-depths but the elevation of the seafloor above or below the geoid.
Jamal (2020). RegularizeData3D (https://www.mathworks.com/matlabcentral/fileexchange/46223-regularizedata3d), MATLAB Central File Exchange. Retrieved September 23, 2020.

Update the publication date to 'go live' date. Modified browse graphic name to be the URL to the browse graphic. This work done on 20220307. Added PID to metadata (20220411). Any edits to the metadata also include an update to the metadata date.