Bathymetry of the Hudson Canyon region (100-m resolution Esri binary grid and 32-bit GeoTIFF, Mercator, WGS 84)

A suite of processing software (called SwathEd) (Clarke, 1998; see cross reference) developed by the Ocean Mapping Group at the University of New Brunswick, Canada, was used to process, edit, grid, display, and archive the multibeam data. These routines are designed to be run on a Unix and/or Linux operating system.
The initial processing and editing steps carried out were:
1. Demultiplex (unravel) the SeaBeam data files using the program unravelSB2100 to generate separate files for a given raw file (filename.mb41), including depth soundings (filename.mb41.merged) and sidescan sonar backscatter values (filename.merged.mb41.ssdata).
Command line: unravelSB2100 -declin 0.0 filename.mb41
2. Generate a navigation file from the soundings file.
Command line: stripNav -comp filename.mb41.nav filename.mb41.merged
3. Edit the navigation data on-screen using the SwathEd routine jview to remove undesirable points, including turns at the ends of survey lines. Jview also rejects stray GPS fixes outside of the survey area, as set by the operator.
Command line: jview -rejectnav -navedit filename.nav
4. Make a copy of the edited navigation, and then create a new navigation file that has the bad fixes deleted from it.
Command line: mv filename.nav filename.nav_all Command line: appendNav -goodonly -comp filename.nav filename.nav_all
5. Merge the edited navigation back into the file containing the depth soundings using just the prefix of the filename. The location of the antenna relative to the motion reference unit is also entered here, along with a maximum time gap (in seconds) allowed between navigation fixes.
Command line: mergeNav -time_limit 300 filename
6. Edit the multibeam soundings for each trackline. SwathEd displays blocks of data across and along the trackline. Anomalous points were identified by comparison to other points and by an understanding of the sea floor geology and morphology. Anomalous soundings were removed.
Command line: swathed filename.merged

Map the bathymetric soundings from each processed data file onto a digital terrain model (DTM) in the custom Mercator projection using weigh_grid. The bathymetric data are presented at a resolution of 100 m/pixel. The weigh_grid program creates a DTM by summing up the weighted contributions of all the provided data into the DTM. The weighted contributions only extend to a user defined region around the true location of the estimate (the cutoff). The weight of an individual point contribution decays away from the node location based on the order of the Butterworth (inverse weighting) filter (-power n), the width of the flat topped radius of the weighting function (-lambda n.n), and the absolute cutoff limit (-cutoff) that will be used for data points contributing to a node. In addition, each beam/other value can be pre-weighted if required to account for its reliability. A custom weight file for each beam is created by hand and input into the weigh_grid program using the -custom_weight option. Three files are created which are used by the weigh_grid program. An ".r4" file, which contains the grid node values in binary format, an "r4_weights" file, and an "r4_weight_depth" file. To do the summing of the weights, the .r4_weights file contains the sums of the weights for each data point contributing to the node, and the .r4_weight_depth file contains the sum of the weights x the depths. Naturally the solution is: weight_depth/weights for each node. For more information, visit the OMG website at <http://www.omg.unb.ca>. The values computed for each grid node are an average of the soundings within an inner 75 m circle, and a weighted average of the soundings within an outer circle that increased in size with water depth: between 500 and 1,500 m, the outer circle radius was 100 m, between 1,500 and 2,500, the radius was 200 m, and for depths greater than 2,500 m, the radius was 300 m.
First a blank binary grid and the weights and weight_depth files must be created:
Command line: make_blank -float gridFile
This command commences a dialog to enable an 8 bit image and input the map boundaries and resolution. The program also prompts for the projection type and parameters to be used creating the binary map file (custom Mercator projection, central longitude of -75 degrees, latitude of true scale 40 degrees north).
The next command creates the weights and weight_depth files associated with the binary grid: Command line: tor4 gridFile
These two steps create all three files with a gridFile prefix.
Next, each individual data file (.merged suffix) is added to the grid using weigh_grid.
Command line: weigh_grid -omg -tide -coeffs -beam_mask -beam_weight -custom_weight RonBrownGrid_weights -butter -power 2 -cutoff 300 -lambda 75 gridFile filename.merged

Processing steps 1 and 2 produced a preliminary bathymetry dataset. Processing was carried out to further edit and correct the data and to produce final grids and images of the data. Processing and editing steps included:
1. Correct errors in soundings due to sound refraction, caused by variations in sound velocity profile, using the SwathEd refraction tool. These artifacts can be recognized in a cross-swath profile of a relatively flat patch of sea floor. When viewing the swath data across a profile, the sea floor will appear to have a "frown" or "smile" when in fact the data should be flat across the profile. Insufficient and/or erroneous sound velocity information, which is usually due to widely spaced or non-existent velocity profiles within an area, results in an under or over-estimate of water depth which increases with distance from the center of the swath. For a discussion of how this effect can be recognized in a swath bathymetric data file, see < http://www.omg.unb.ca/AAAS/UNB_Seafloor_Mapping.html>.
2. Remove erroneous soundings that were not edited in the field using the SwathEd program.
3. Create a new 100 meter grid of the bathymetric soundings using the SwathEd routine weigh_grid, using different cutoff parameters based on water depth.
Grid these files using a cutoff radius of 300 meters, lambda of 75 meters. Times: 08/30-31/2002 all, 09/01-06/2002 all, 09/07/2002 0000-0833 Times: 09/13/2002 0345-2219
Command line: weigh_grid -omg -tide -coeffs -beam_mask -beam_weight -custom_weight RonBrownGrid_weights -butter -power 2 -cutoff 300 -lambda 75 gridFile filename.merged
Grid these files using a cutoff radius of 200 meters. Times: 09/07/2002 0921-2359, 09/08/2002 0000-1117 Times: 09/14/2002 0813-2359, 09/15/2002 all
Command line: weigh_grid -omg -tide -coeffs -beam_mask -beam_weight -custom_weight RonBrownGrid_weights -butter -power 2 -cutoff 200 -lambda 75 gridFile filename.merged
Grid these files using a cutoff radius of 100 meters. Times: 09/08/2002 1152-2359, 09/09/2002 all, 09/10/2002 0000-1854 Times: 09/14/2002 0543-0743Command line: weigh_grid -omg -tide -coeffs -beam_mask -beam_weight -custom_weight RonBrownGrid_weights -butter -power 2 -cutoff 100 -lambda 75 gridFile filename.merged
4. Convert binary bathymetric grid to Esri ASCII raster format:
Command line: r4toASCII gridFile.r4
This creates a file called gridFile.asc.
Note: Errors in the UNB gridding software were identified in 2003 and 2007. These data were processed with corrected software.

Create an Esri bathymetry grid by importing data from Esri ASCII raster format into Esri grid format.

Added keywords section with USGS persistent identifier as theme keyword.