Massachusetts Bay, MA (N170) Bathymetric Digital Elevation Model (30 meter resolution)
Derived From Source Hydrographic Survey Soundings Collected by NOAA |
Bathymetry for Massachusetts Bay was derived from thirty-eight surveys containing
297,628 soundings. Two older, less accurate, overlapping surveys were omitted before
tinning. The average separation between soundings was 57 meters. The thirty-eight
surveys used dated from 1940 to 1970. The total range of soundings was 3.0 meters
to -71.0 meters at mean low water. Mean high water values between 2.7 and 2.9 meters
were assigned to the shoreline. Fifty-six points, predominantly from two surveys,
were found that were not consistent with the surrounding data. These were removed
prior to tinning. DEM grid values outside the shoreline (on land) were assigned null
values (-32676). Massachusetts Bay has fifteen 7.5 minute DEMs and two one degree
DEMs. The 1 degree DEMs were generated from the higher resolution 7.5 minute DEMs
which covered the estuary. A Digital Elevation Model (DEM) contains a series of elevations
ordered from south to north with the order of the columns from west to east. The DEM
is formatted as one ASCII header record (A- record), followed by a series of profile
records (B- records) each of which include a short B-record header followed by a series
of ASCII integer elevations (typically in units of 1 centimeter) per each profile.
The last physical record of the DEM is an accuracy record (C-record). The 7.5-minute
DEM (30- by 30-m data spacing) is cast on the Universal Transverse Mercator (UTM)
projection. It provides coverage in 7.5- by 7.5-minute blocks. Each product provides
the same coverage as a standard USGS 7.5-minute quadrangle but the DEM contains over
edge data. Coverage is available for many estuaries of the contiguous United States
but is not complete. |
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