USGS Geologic Division Coastal and Marine Geology Program GOMAINE

Construction of Digital Bathymetry for the Gulf of Maine

Assembling the Input Data

 Data was collected from available sources on the Web and from CD-ROM products. The data included digital sounding data, digitized contour line data and previously gridded products from a variety of sources.

The following is a brief description of the datasets identified for inclusion into the composite dataset and the techniques used to process each of them. FGDC Metadata for all of the data sets are provided in the Appendix of this document:

1. NOAA Hydrographic Survey Data and NGDC Marine Trackline Geophysics Data

Description: The largest digital dataset available was the National Ocean Service Hydrographic Data Base (NOSHDB) distributed by the National Geophysical Data Center. This digital dataset contains survey data from the National Ocean Service, the Defense Mapping Agency, Hydrographic/Topographic Data Center, and international sources. The NOSHDB contains data digitized from smooth sheets of hydrographic surveys completed between 1930 and 1965, and from survey data acquired digitally on NOS survey vessels since 1965. The vertical datum is mean low water and the horizontal datum is NAD27. The data is extremely dense in many regions (greater than 0.5 km resolution), but there are large gaps in the coverage due to surveys currently in non-digital form. The coverage is limited to U.S. waters.

NGDC’s Marine Trackline Geophysics database contains bathymetric, magnetic, gravity and seismic navigation data collected during marine cruises from 1953 to the present. Data sources include both U.S. and foreign oceanographic institutions and government agencies. The trackline bathymetric data available for the Gulf of Maine was collected using a variety of vertical datum's and unfortunately, much of this data turned out to be unsuitable for inclusion. Only a high resolution survey of the canyons south of Georges Bank was included.

Processing: All the surveys within our latitude/longitude bounds were extracted from the NOS Hydrographic Survey Data Vol. 1, Ver. 3.2 CD-ROM using the supplied GEODAS software. The GEODAS software was used to retrieve data from the NGDC National Ocean Service Hydrographic Survey data and Marine Trackline Geophysics CD’s. This software enabled the geographic search and retrieval of the NOS surveys and was able to export ASCII files of longitude, latitude, and depth.

Identification of bad NGDC survey points was accomplished by using ESRI's ArcView. The depth values of the raw soundings were coded by colored dots, with the hue indicating depth. Errors where tracks crossed were thus readily apparent, and erroneous data were quickly identified by clicking on the point to determining the point attributes (survey ID, numeric depth value). If the problem was obvious (e.g. fathoms coded as feet), the depth values were corrected. Otherwise, the erroneous values were not used. The depth values in survey H08938 (northern Massachusetts Bay) were too large by nearly a factor of six, suggesting that these data had been originally coded as feet instead of fathoms. The depth values for this survey were therefore divided by a factor of six, upon which they blended smoothly with values from neighboring surveys. Depth values from surveys H08602 and H08603, south of Nantucket, were also clearly incorrect. These were eliminated since there was sufficient coverage in this region from other surveys.

For all of the good NOSHDB data, the longitudes and latitudes were converted from NAD27 to NAD83 horizontal datum using the NAD2NAD program from the freely-available PROJ map projection software package. The depths were converted from a mean low water datum to an approximate mean sea level by adding the mean tidal amplitude at each sounding location. The mean tidal amplitude was determined by interpolating results from a high resolution numerical model of  the M2 tide in the Gulf of Maine (Lynch and Naimie, 1993).

2. Naval Oceanographic Office DBDB-V gridded bathymetry

Description: The Naval Oceanographic Office (NAVOCEANO) Digital Bathymetric Data Base - Variable Resolution (DBDB-V) is a collection of varying resolution bathymetry grids ranging from a global 5 minute grid (equivalent to DBDB5 and ETOPO5), to 30 second grids in certain regions. In the Gulf of Maine, several of the 30 second DBDB-V grids provided crucial coverage in the interior of the Gulf and in Canadian waters. These grids were "constructed from a variety of public and classified source data" (Sandy, 1996). The horizontal datum is WGS84 and the vertical datum is mean sea level.

Processing: The NAVOCEANO DBDB-V bathymetry dataset was retrieved in its native binary format. This format required the use custom software that was supplied by NAVOCEANO for a SGI workstation. The SGI binary was used to convert the DBDB-V data to ASCII triplets (lon,lat,z).  No datum corrections were made, since the data were already in WGS84 and relative to mean sea level.

3. Supplemental Datasets from Bedford Institute of Oceanography and Brookhaven National Laboratory

Description: Dave Greenberg from Bedford Institute in Canada supplied a bathymetry grid that had nominal 5 km spacing over most of the region. This data filled gaps in the interior of the Gulf between the NOSDB data and the DBDB-V data as well as providing coverage of the Scotian Shelf. The depths are relative to mean sea level. The horizontal datum is NAD27. In addition, bathymetry collected by Charles Flagg from Brookhaven National Laboratory during ADCP cruises for the Globec Georges Bank program were obtained to fill gaps on the eastern flank of Georges Bank. Depth values are relative to sea surface at the time of measurement. The horizontal datum is WGS84.

 Processing: The Greenberg data were converted to WGS84. No vertical datum conversions were performed, since the Greenberg data were relative to MSL and the Flagg data were unknown. Because of the relatively small tide range (< 1 m) in the Georges Bank region, however, the depth anomalies relative to MSL were considered insignificant.

4. NOAA Medium resolution digital Shoreline and DMA World Vector Shoreline

Description: The Medium Resolution Digital Shoreline is a high quality (1:80,000), GIS-ready, general use digital vector dataset for US shoreline created by the Strategic Environmental Assessments Division of NOAA's Office of Ocean Resources, Conservation and Assessment. The horizontal datum is NAD83 (equivalent to WGS84) and the shoreline is based on mean high water. The DMA’s World Vector Shoreline (WVS) dataset is available on NGDC's "Global Relief" CD-ROM and is suitable for scales close to 1:250,000 . Ninety percent of all identifiable shoreline features are accurate to within 500 meters.

Processing: NOAA’s Medium resolution digital Shoreline and DMA World Vector Shoreline were edited in ArcInfo to match at the US Canadian border. Horizontal datum correction was not necessary. The shorelines were converted from an elevation of 0 above mean high water datum to elevations relative to mean sea level by adding the mean tidal amplitude at each sounding location by interpolating results from a high resolution numerical model of the M2 tide in the Gulf of Maine (Lynch and Naimie, 1993). The shoreline data were then used as elevation values in the composite dataset for the purpose of better defining the topography in the vicinity of the coast.

5. Defense Mapping Agency ETOPO5 Digital relief of the Surface of the Earth

Description: ETOPO5 was generated from a digital data base of land and sea- floor elevations on a 5-minute latitude/longitude grid. The original source of the data in the ocean areas in ETOPO5 is from the U.S. Naval Oceanographic Office.

Processing: All depth values less than 500 m were eliminated, since we were only interested in using ETOPO5 to describe the regions of the slope and rise where we had no other data.

6. GEBCO General Bathymetric Chart of the Oceans

Description: Digitized bathymetry for the World Ocean at a scale varying from 1:10 million to 1:500,000 depending on data density. GEBCO bathymetry is available from the British Oceanographic Center on behalf of the International Hydrographic Organization.

Processing: The 2000 and 3000 m isobaths were extracted from the GEBCO dataset in the region of interest and were used as digital soundings to enhance definition of the seamounts south of Georges Bank.

7. USGS North American 30 arc-second Digial Elevation Model (DEM)

Description: The USGS North American 30 arc-second DEM was used for land values in the Gulf Of Maine in the US and Canada. The North American 30 arc-second DEM was obtained from the  USGS EROS Data Center. The horizontal datum is WGS84. The vertical datum is mean sea level.

Processing:  No datum conversions were necessary, since the data was already using the WGS84 and MSL horizontal and vertical datums.

For comments and questions, contact Rich Signell



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