USGS OFR 2011-1127: Construction of a 3-Arcsecond Digital Elevation Model for the Gulf of Maine, National Ocean Service Office of Coast Survey, U.S. Bathymetric and Fishing Maps

National Ocean Service Office of Coast Survey, Bathymetric and Fishing Maps

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National Ocean Service Office of Coast Survey, U.S. Bathymetric and Fishing Maps

Figure 10. Map showing the extent of the digitized points for the Gulf of Maine from the U.S. bathymetric and fishing charts of the National Ocean Service’s Office of Coast Survey of the National Oceanic and Atmospheric Administration.

Scanned bathymetry and fishing maps (NK 19-9, NK 19-12, and NK 20-7) were obtained from the National Ocean Service (NOS).

Data Properties:

Vertical Datum: MLW
Vertical Accuracy: Unknown
Coordinate System: Geographic latitude/longitude
Horizontal Datum: NAD83
Horizontal Accuracy: Unknown
Resolution: Varies
Year Data Collected: 1978 (NK1912), 1980 (NK199), 1983 (NK207)
Format: TIFF
Date Data Downloaded: September 2009

The initial processing step for these data was to create a georeferenced image (GeoTIFF). This was done by georeferencing the image in ArcMap 9.2, using the georeference toolbox. Data frame properties were set to decimal geographic coordinate system with a horizontal datum of NAD 83. The TIFF image was then added. Control points were added to the data frame to link known raster positions to known map coordinates. A total of 45 control points were used to georeference each of the scanned images using a first-order polynomial.

The next step was to trace the bathymetry contours on the georeferenced image. Individual contours (250, 500, 750, 1,000, and 1,250 m) were traced to generate a new polyline shapefile. These polylines were converted to points by the "Features to Points" XToolsPro (v. 4.1.0 (Build 375)) extension at randomly spaced intervals and formatted to ASCII XYZ. The Generic Mapping Tool command 'blockmedian' was used to resample the data at intervals of six arcseconds. This interval was chosen to create a coarser resolution so that the Smith and Sandwell would take a higher priority than the digitized contours; the choice of data points was because the Smith Sandwell dataset consists of actual bathymetric and location measurements, whereas the chart data are approximations of depth and location.

The digitized chart data were used to help fill in areas of sparse data on the southern edge of Georges Bank, especially for the canyons leading into the deeper waters (fig. 10).

The last processing step was to adjust the vertical datum from MLW to the VDatum tidal model reference level, within 0.2 m of NAVD88. For more information on the vertical datum transformation, see Common Processing Steps and Data.