The bathymetry was collected by launching the ASV 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 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. Interpolated PPK GNSS positions and altitude (ppk_Height) to time of Echosounder samples with the MatLab function interp1 (linear interpolation).
4. Refined time alignment to account for any small time delays between GPS data and Echsounder data collection by using time lagged cross correlation of Echosounder range and PPK GNSS altitude in locations with a relatively flat bottom. The GPS measures vertical fluctuation of the boat due to waves and these flucations are also visible in echosounder data. The equation in Step 5 removes the vertical fluctuation due to waves from the echosounder data leaving only true bathymetry if GPS and the Echsounder are well synced. The lagged cross correlation processing ensures they are synced optimally.
5. Calculated sea floor elevation (referred to as depth) with reference to the NAVD88 datum in meters by the following equation: Depth = ppk_Height - Antenna_Z_Offset - Echo_altitude; Antenna_Z_Offset = 0.43 m is the offset from the GNSS antenna to the Echosounder transducer on the z-axis of the ASV.
6. Exported Trackline data to text file: 2020015FA_Truro_trackline.txt to be read into Matlab and then copied TXT file into Excel and saved as a CSV file for publication.
7. Gridded Depth onto 1 m resolution UTM eastings, northings using Matlab file exchange script regularizedata3d (cubic interpolation) with smoothness of 0.001 (used for data with low noise where the input points nearly coincide with the output surface) by Jamal (2020).
8. The Matlab function 'roipoly' returns the mask as a binary image, setting pixels inside the region of interest to 1 and pixels outside the ROI to 0. The boundary is developed so that there is no extrapolated bathymetry data outside the tracklines. The masks extent is the convex hull of the tracklines positions (x,y) and a bounding z coordinate of 0.8 representing the surface in reference to NAVD88 in meters.
9. Export gridded 1 m data in NAD83(2011)/UTM Zone 19N in NAVD88 meters as a GeoTIFF: 2020015FA_Truro_bathymetry_1m.tif
Jamal (2020). RegularizeData3D (https://www.mathworks.com/matlabcentral/fileexchange/46223-regularizedata3d)
, MATLAB Central File Exchange. Retrieved September 23, 2020.