greater, some areas such as the Bahamas, Nicaragua Rise, Cuba, and Florida have
values as low as 70 and were not further resolved in order to maintain the mini-
mum element size of 1 km. The majority of the TLS values equal 0.25 or less,
however, along the continental shelf break and other areas with steep bathymetric
gradients the TLS values were closer to the set target of 1.0. The final Eastcoast
2001 grid contains 254,629 nodes and 492,182 elements.
Bathymetry
It is essential that bathymetry be accurately represented to ensure accurate
results. In fact, bathymetry controls physical processes ranging from wave
propagation, reflection and refraction, the 2-D structure of the waves as well as
dissipation. Bathymetry for the Eastcoast 1991 computations was based on the
ETOPO5 database while bathymetry for the Eastcoast 1995 computations was
based on the ETOPO5 and NOS (National Ocean Service) databases.
ETOPO5, shown in Figure 11, is a 5-min gridded Earth topography database,
which was put together by the U.S. Naval Oceanographic Office in 1985 and
revised again in 1987. The ETOPO5 database, which has been widely used in
the past, provides values throughout the entire WNAT domain. However, the
ETOPO5 database is somewhat limited in resolution and in some areas can be
unreliable, particularly on the continental shelves. Nonetheless, ETOPO5 is the
best and sometimes the only general database to which nonmilitary applications
have access.
The NOS bathymetric data values, Figure 12, are derived from the NOS' raw
sounding database (National Oceanic and Atmospheric Administration (NOAA)
Digital U.S. Coastal Hydrography Sounding Database). NOS' sounding database
includes bathymetric values on most of the U.S. continental shelf, from the
shoreline and within selected estuaries to the continental shelf break. Since the
data is in raw sounding track values form, the NOS bathymetric data values are
filtered onto the Eastcoast 2001 grid using an element-based gathering/averaging
procedure instead of a direct interpolation procedure. The gathering/averaging
procedure searches for all available sounding/topographic survey values within
the cluster of elements connected to one specific node, averages these values and
assigns the average value as the depth/topographic elevation to that node. This
gathering/averaging procedure essentially implements grid scale filtering to the
bathymetric/topographic data and ensures that bathymetry/topography is con-
sistent with the scale of the grid. The NOS database has significant higher
density to represent critical features and is also more reliable than the ETOPO5
database. Therefore, NOS' values are used instead of the ETOPO5's where
possible.
Recently the Digital Nautical Charts (DNC) bathymetric database, shown in
Figure 13, by the National Imagery and Mapping Agency has been made
available. This database provides bathymetry throughout most of the WNAT
domain with significantly more precision than the ETOPO5 database. In some
regions within the WNAT domain, such as in parts of the Atlantic Ocean and the
northern coast of South America, an insufficient number of data points were
12
Chapter 3 Eastcoast 2001 Grid Development
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