file: readme.htmSUN-ILLUMINATED SEA FLOOR TOPOGRAPHIC MAPS AND PERSPECTIVE-VIEW
U.S. Geological Survey Open-File Report 99-363
Page C. Valentine, Tanya S. Unger, Jessica
L. Baker, and Christopher F. Polloni
September 24, 1999
U.S. Geological Survey
384 Woods Hole Road
Woods Hole, MA 02543
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This Compact Disc-Read Only Memory (CD-ROM) publication was prepared by members of the US Geological Survey. Neither USGS or the United States Government nor any agency thereof, nor any of their employees, make any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed in this report, or represents that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. Any views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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 Service. Multibeam echo sounder surveys were conducted on four cruises over a two-year period from the fall of 1994 to the fall of 1996. Results of these cruises are presented here in a series of 18 mapped quadrangles (see location map) in which sea floor depth information is depicted in sun-illuminated (or shaded relief) view at a scale of 1:25,000, with topographic contours overprinted in blue (Valentine and others, 1999a-r). The shaded relief images shown here were created using a light source that is 45 degrees above the horizon from an azimuth of 350 degrees. The vertical exaggeration in the images is four times. In effect, topographic relief is enhanced by illuminating the sea floor from a position 10 degrees west of north so that shadows are cast on the southern flanks of seabed features. Some features in the images are artifacts of data collection. They are especially noticeable where the seabed is smooth and include small highs and lows and unnatural-looking features and patterns that are oriented parallel or perpendicular to survey tracklines. For a depiction of the topographic contours alone, and for an explanation of survey and topographic data processing methods, see the companion maps by Valentine and others (1997a-r). Topographic contour maps of all 18 quadrangles of the map series are available on a CD-ROM in EPS, PS, Arc export, and PDF file formats (Valentine and others, 1998). Blank areas represent places where no data exists.
The survey was conducted using a multibeam echo sounder installed aboard the Canadian Hydrographic Service 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 global positioning system (GPS) receiver in conjunction with differential GPS corrections transmitted by U.S. Coast Guard radio beacons. The multibeam echosounder 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 generates 60 aimed beams spaced at intervals of 2.5 degrees that insonify a swath 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; 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 echosounder bathymetric and differential GPS navigation data; 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 using the 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 swath. 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 shaded relief 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
The 18 mapped quadrangles are included in this CD-ROM in five different formats.
1. Encapsulated PostScript (eps)
2. PostScript (ps)
3. Arc export (e00)
4. Registered Tifs (tif)
5. Portable Document File (pdf)
There are eight directories on this CD-ROM:
This directory has the Arc export (e00) files. 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).
This directory has the auto-start uitility files.
This directory has the logo graphic files for some of the html files.
This directory has the html files used for documentation on this CD-ROM.
This directory has 3-d perspective views of all quadrangles.
The Encapsulated PostScript (EPS) and PostScript files (PS) were generated from Corel Draw version 7.375 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 EPS files have been compressed with WinZip and require ~25Mbytes of disc space when uncompressed.
The PostScript files are included for printing and require a maximum page size of 26 inches by 36 inches (66 cm x 91.4 cm).
The Portable Document Files are included for viewing and printing using Adobe Acrobat Reader.
The Tif and associated world file (tifw) can be incorporated into a GIS database for use with ArcView.
Go to Discussion
Go to Location Map
Go to Front Page
Paper copies of the maps can be obtained from the source listed below. See individual quadrangle pages for map titles.
USGS Information Services
Box 25286 Denver Federal Center
Denver, CO 80225-0046