A seamless Bathymetric-Topographic digital elevation model (DEM) was developed by merging the "best available" NOAA NOS bathymetric data and USGS land elevation data.  Each of the datasets was initially processed independently to apply the “best available” criteria to select the data to be merged.  Prior to merging, the data were transformed to a common reference coordinate system, both horizontally and vertically.

The best available bathymetric data were selected from the NOS Hydrographic Survey Database.  Fifteen spatial and temporal filters were applied to the 47 digital NOS hydrographic surveys, dating from 1945 to 1996, which cover the Tampa Bay region.  Because the hydrographic surveys overlap in many areas, the soundings were subjected to a spatial-temporal filtering process to select the most recent bathymetric data for all areas in the Tampa Bay region.  Spatial survey polygons, or indexes, were created for each of the hydrographic surveys. The survey polygons were sorted by date, merged with other survey polygons for the same year, and clipped on the basis of the survey date.  Almost 99% of the project area is covered by digital sounding data at variable data densities. The soundings identified as the most recent from the polygon processing were merged into a single file and sorted on the basis of the vertical datum.  For Tampa Bay, approximately half of the soundings were collected in reference to a mean low water (MLW) vertical datum, and the other half were collected for surveys using a mean lower low water (MLLW) vertical datum.  Approximately 800,000 soundings were transformed from multiple vertical tidal datums to a common vertical reference, the NAD 83 ellipsoid (GRS 80), using the VDatum Transformation Tool developed jointly by NOAA's Office of Coast Survey and National Geodetic Survey specifically for converting coastal elevations from one datum to another.

The best available topographic data were selected from the USGS National Elevation Dataset (NED).  NED is an implementation of the National Spatial Data Infrastructure (NSDI) concept of “framework” data.  Framework data are defined as those spatial datasets that are fundamental to many applications.  In this case, land elevation and bathymetry are basic layers of information upon which other data layers can be appended. NED was produced by processing standard USGS tiled (map-based) DEM products.

NED was produced by processing standard USGS tiled (map-based) DEM products.  NED is a seamless raster elevation dataset that provides national U.S. coverage at a grid spacing of 1-arc-second (approximately 30 m). NED is derived from source DEM’s that have a resolution of either 10 or 30 m. NED production includes the following processing steps performed on the individual source 7.5-minute DEM files:  datum and coordinate unit conversion (horizontal and vertical);  projection transformation and resampling; filtering (for removal of production artifacts), mosaicing, edge matching; and metadata generation. The resulting 50-gigabyte dataset includes an elevation value (expressed in decimal meters referenced to NAVD 88) posted every 1-arc-second on a latitude/longitude grid (referenced to the NAD 83 horizontal datum).

Data for the Tampa Bay region was extracted from NED. Standard tools and datasets (VERTCON and GEOID99) from the National Geodetic Survey were used to transform the elevation data into the common ellipsoid vertical reference frame.  The NED “shoreline” (interface of zero/non-zero elevations) was used to make the final selection of bathymetry and topography points for merging.  Land elevations within 600 m of the shoreline were converted from raster format to ASCII XYZ point data.  All bathymetry points coinciding with areas of zero elevation in NED were selected.  Because of the age of some of the hydrographic surveys, some of the soundings were located on areas that had been filled and are now represented as land in the DEM.  These points were withheld from further processing.

The selected topography points within the shoreline buffer zone and the bathymetry points were gridded to produce a raster surface model with a 1-arc-second grid spacing to match the resolution of NED. The points were input to an implementation of the Australian National University Digital Elevation Modeling program, ANUDEM, thin plate spline interpolation algorithm, which is optimized for generation of topographic surfaces.  The bathymetry points could have been gridded independently of the topographic data, but the shoreline zone land elevations were included in the interpolation to ensure a better match of the bathymetric and topographic surfaces for the subsequent mosaicing step.  To avoid introduction of any interpolation edge effects into the merged elevation model, the output grid from the interpolation was clipped to include only land elevations within 300 meters of the shoreline.  The final processing step involved the mosaicing of the bathymetry grid and the NED elevation grid.  The values in the 300-meter overlap area were blended by weighted averaging, where the weights for each grid are determined on a cell-by-cell basis according to the cell's proximity to the edges of the overlap area.  The resulting final merged product is a seamless bathymetric/topographic model covering the Tampa Bay region at a grid spacing of 1-arc-second. The vertical coordinates represent elevation in decimal meters relative to the GRS 80 ellipsoid, and the horizontal coordinates are decimal degrees of latitude and longitude referenced to the NAD 83 datum.