Public domain data from the U.S. government are freely redistributable with proper metadata and source attribution. Please recognize the U.S. Geological Survey (USGS) as the source of this information.
Atlantic--mean low water; Gulf of Mexico--low water datum; Pacific--mean lower low water; Hawaii--mean lower low water; Alaska--mean lower low water; Great
Lakes--low water datum. Data are archived under entry ID FE00125; data inventories are archived under entry ID FE00125B.
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.
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).
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).
Note about NAD83 and WGS84 horizontal datums; The horizontal differences between NAD83 and WGS84 were determined to be insignificant at the resolution of the 15 and 30 arc-second grids. With cell sizes of ~500 and 1000 meters any displacement of data points because of differences in horizontal datum would have no effect on the outcome of the grid cell value after pre-filtering through the GMT block median procedure.
Note about grid cell sizes: we produced the 15 and 30 arc-second grids because of varying source data resolution in the near-shore and offshore areas. The 15 arc-second grid has much higher source data resolution(NOS data) in the nearshore areas and could possibly be recompiled at a higher resolution in certain areas. The 30 arc second grid was produced for a more uniform fit of the data since the DBDB-V, and the USGS DEM contained data spaced at this resolution.
The contour lines were exported and saved in the Arc export format.
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