SEA FLOOR TOPOGRAPHIC CONTOUR MAPS AND PERSPECTIVE-VIEW
IMAGERY OF QUADRANGLES 1-18, STELLWAGEN BANK NATIONAL MARINE SANCTUARY
OFF BOSTON, MASSACHUSETTS
Includes Encapsulated PostScript, PostScript,
Arc export, and Portable Document File formats of 18 individual maps
U.S. Geological Survey Open-File Report 98-138
Prepared in cooperation with the National Oceanic and Atmospheric Administration
February 9, 1998
Link-repaired version online, 14 March 2001
By
Page C. Valentine
U.S. Geological Survey
384 Woods Hole Road
Woods Hole, MA 02543
Jessica L. Baker
U.S. Geological Survey
384 Woods Hole Road
Woods Hole, MA 02543
Tanya S. Unger
U.S. Geological Survey
384 Woods Hole Road
Woods Hole, MA 02543
Christopher Polloni
U.S. Geological Survey
384 Woods Hole Road
Woods Hole, MA 02543
Introduction - The Stellwagen Bank National Marine Sanctuary Mapping Project is a cooperative effort of the U.S. Geological Survey and the National Oceanic and Atmospheric Administration, with support from the University of New Brunswick and the Canadian Hydrographic Survey. The survey was conducted on four cruises over a two-year period from the fall of 1994 to the fall of 1996. The project is mapping an area of 1100 square nautical miles. The maps show a series of 18 quadrangles (see location map) in which sea floor topography is presented at a scale of 1:25,000.
Survey methods - The survey was conducted with a multibeam echo sounder aboard the Canadian Hydrographic Survey vessel Frederick G. Creed, a SWATH (Small Waterplane Area Twin Hull) ship that surveys at speeds up to 16 knots. During the survey, the ship's position was determined with an accuracy of 10 m or better using a geographic positioning system (GPS) receiver in conjunction with differential GPS corrections transmitted by U.S. Coast Guard radio beacons. The multibeam data were collected utilizing a Simrad Subsea EM 1000 Multibeam Echo Sounder (95 kHz) that is permanently installed in the hull of the Creed. In water depths between 5 and 200 m, the EM 1000 produces 60 aimed beams spaced at intervals of 2.5 degrees that insonify a strip of sea floor measuring in width approximately 7.5 times the water depth; horizontal spatial resolution is on the order of 10 percent of the water depth at 16 knots; and vertical resolution is approximately 1 percent or better. Software developed by the Ocean Mapping Group, University of New Brunswick, was used to process and edit the bathymetric and navigation data; and tidal corrections based on NOAA's Boston tide gauge were used to reference depth data to mean lower low water.
Topographic contour mapping - Bathymetric data were contoured with Arc/Info geographic information system software (Environmental Systems Research Institution, Inc., version 7.03). Processed data were formatted into a point file using the Arc/Info "point generate" routine. The point file was transformed to a Mercator projection with the longitude of the central meridian at 70o 19'W and the latitude of true scale at 41o 39'N. The "point grid" routine was used to create a grid from the point file and to assign depth values to individual grid cells. The cell size of the output grid was 13 m. Topographic contours at 5-meter intervals were generated using the "lattice contour" routine. Most of the contour lines are displayed here uneditted. However, in areas of very smooth sea floor, some contours displayed distortions that are due to problems encountered during data acquisition at nadir (directly below the vessel's keel) and to refraction effects at the outermost edge of the beam pattern. These distortions were smoothed by using a user-defined low-frequency "focal median" filter routine on the grid created by "point grid". Square focal median filters varying in size from 5 x 5 to 21 x 21 cells were tried, and a 9 x 9 cell size was selected for Quadrangles 1-4 and 6-18; an 11 by 11 cell size was chosen for Quadrangle 5. The resulting contours were compared with features displayed in sun-illuminated seabed imagery of the same data and edited manually with "Arc/Edit" to remove small artifacts that remained after filtering. Each of the quadrangles was contoured independently, and contours that extend into adjacent quadrangles were edited manually to match at the boundary.
Contents of CD-ROM - Included in this CD-ROM are four file types: (1) Encapsulated PostScript; (2) PostScript; (3) Arc export; and (4) Portable Document File. The Encapsulated PostScript and PostScript files were generated from Corel Draw 7.0 and contain maps in a Mercator projection with the longitude of the central meridian at 70o 19'W and the latitude of true scale at 41o 39'N. The Encapsulated PostScript files are included for the purposes of viewing. The PostScript files are included for printing and require a page size of 26 inches by 29.5 inches (66 cm x 74.9 cm). The Arc export files are coverages in a Geographic projection with Geodetic Reference System 1980 and North American Datum 1983. The attributes which accompany these coverages are depth and depression. The depth field contains positive values for each contour line. The depression field contains values representing topographic lows or depressions (identified by 1) and regular topographic contours (identified by 0). The Portable Document Files are included for viewing and printing. In addition, the CD-ROM contains 3-d perspective views of all quadrangles, color-coded in 10-meter contour intervals.