Portion of the 1-meter (m) contours in quadrangle 3 of the Stellwagen Bank Survey Area offshore of Boston, Massachusetts based on bathymetry data collected by the U.S. Geological Survey from 1994-1996

Initial data collection and processing is described in Valentine and others 1999, Valentine 2005, and Butman and others, 2007. A summary of that information is presented here. Surveying was conducted aboard the Canadian Hydrographic Service vessel Frederick G. Creed using a hull mounted multibeam echo sounder. These data were collected using a Simrad Subsea EM1000 operating at 95 kHz. SwathEd software (version unknown) developed by the Ocean Mapping Group, University of New Brunswick, was used to process the bathymetric and navigation data. The processing included editing the data to remove spurious points as well as incorporating tidal corrections. Processing took place between 1994 and 1996 by a variety of individuals.

The remaining process steps are a function of incorporating information from the previous publications (noted in cross-references), along with processing notes maintained during data processing. The final SwathEd-produced grid at 10 meters per pixel was converted to an ASCII file using the University of New Brunswick script r4toASCII. The format of the ASCII file is latitude, longitude, and depth. The ASCII file was converted to an ArcInfo ungenerate format using a Perl script. The resulting format is ID, longitude, latitude, and negative depth. Each point has a unique ID value. Using ArcInfo 7.03, the ungenerate format was converted to a coverage format using the 'convert to arc' from 'generate tool' available in ArcTools. The 'build' command with the point option was then used to convert the data to a point coverage. This procedure generates the point locations stored in the ungenerate file but does not incorporate the depth values. To include depth, the ungenerate file - which has location as well as depth - was converted to an ArcInfo table, using 'Tables' in ArcInfo. The structure of the ArcInfo table was defined to include the ID, longitude, latitude, and depth in the appropriate format and size. Longitude and latitude were defined as floating point numbers with six decimal places, while depth was defined as a floating point number with two decimal places. To add the depth value to the point coverage, ArcInfo 'Tables' was used to join the point attribute table (pat) of the coverage to the newly-generated data table based on the ID value. In order to join the tables, the data table ID field had to be renamed to match the automatically assigned coverage ID field. The projection of the point coverage was defined in ArcInfo with the PROJECTDEFINE command and the following parameters: Projection Geographic; Datum NAD83; Units DD; Spheroid GRS80. This process was performed by Jessica Baker in 1997. Then the coverage was projected to the custom Mercator projection using the ArcInfo PROJECT command with the following options: Projection Mercator; Datum NAD83, Spheroid GRS1980; Units Meters; Longitude of Central Meridian -70degrees 19minutes; Latitude of true scale 41degrees 39minutes; False Easting 0; False northing 0. This process was performed by Jessica Baker in 1997. The final step was to convert the points to a grid. This was accomplished using the ArcInfo 'pointgrid' command. The depth attribute was gridded at a cell-size of 13 m, and the background value was set to nodata. This final processing step was performed in 1997, and is the basis for the subsequent work with contours. However, this process was repeated by Tammie Middleton in 2001 and is the grid presented in this data release. The 13-m cell-size was chosen because it reduced stripe artifacts in the data when processing with the SwathEd software.

The original Mercator grid was projected to a geographic coordinate system using the ArcInfo PROJECT command. Once this was done, the Arc/Info routine lattice contour was used to generate the 1-m contours. Extensive editing was done with ArcEdit to remove contours from areas where the seabed is relatively smooth. Although each quadrangle was contoured independently, the contours were manually edited to remove "boundary faults". All editing work was done by Jessica Baker and Tanya Unger from 1997 to 1999.

The 1-m coverage file was added to ArcMap 9.3.1. This dataset was then exported to a shapefile (arc2ctr1mtr_edt.shp). Portions of contours that were not necessary for the interpretation were deleted from the shapefile. This process step and the subsequent process steps were performed by the same person: VeeAnn A. Cross.

Within ArcGIS 10.8.1, the shapefile was incorporated into a file geodatabase with other datasets developed in this data release and additional trimming of the contours was performed as well as removing 0 value contours or contours with length 0. The geodatabase was in a custom Mercator projection which meant including the contours in that geodatabase required they be reprojected to match that coordinate system. The feature class was then exported as a shapefile in the geographic, NAD83 coordinate system. The attributes associated with the original coverage that were not necessary for the shapefile (such as FNODE, TNODE) were removed.

Within QGIS 3.16.7-Hannover the Dissolve tool was used to combine the features of a given contour value into a single feature making each contour value a multipart polyline.