Digital elevation models (DEMs) are digital representations of the topographic landscape of a given area. High resolution DEMs, particularly those that include both topography and bathymetry, are an invaluable resource to coastal and scientific applications. If the data used as input to the DEM is from multiple sources, there is a possibility that the data may be referenced to different vertical datums. This is particularly true when merging data across the land-sea interface, where data may be collected referenced to a range of tidal, orthometric and ellipsoidal vertical datums. The VDatum vertical datum transformation tool was developed to address this issue by enabling users to properly transform elevation data among approximately 30 different vertical datums.

The pilot project of VDatum in Tampa Bay facilitated the building of a DEM using NOAA hydrographic survey data and USGS topographic data. Bathymetry from NOAA’s GEOphysical DAta System (GEODAS) database were filtered according to survey dates, giving the more recent surveys higher priority when assembling the DEM. The vertical datums of these data were a mix of Mean Low Water and Mean Lower Low Water, all of which was converted to NAD83 through VDatum. Elevation data from the USGS National Elevation Dataset (NED) were quality-controlled at a 1-arc-second horizontal resolution, and the vertical datums were converted from NAVD88 to NAD83 using VDatum transformations. Using the NED shoreline as the dividing line between land and water, the resulting bathymetric and topographic points were assembled in a 1-arc-second mosaic DEM using a spline interpolation technique. An image of the Tampa Bay DEM is displayed below:

Combined bathymetric and topographic DEM of Tampa Bay, FL.

A VDatum application more recently developed for North Carolina was also used to generate a high resolution DEM referenced to NAVD88. Bathymetric data from both NOAA and the U.S. Army Corps of Engineers was used as input for the water depths, while topographic heights were extracted from the USGS NED and LIDAR data derived from the Federal Emergency Management Agency (FEMA). The lidar data are available in raw point format, as bare-earth points, and regular gridded data. The bare-earth data represent the true ground surface, as the original data set was filtered to remove vegetation, buildings, and other obstructions. The bare-earth data was primarily used in the construction of the topographic portion of the DEM. In areas not covered by lidar data, the USGS NED was used.

The resulting North Carolina DEM has a horizontal resolution of approximately 6 meters. The vertical accuracy is about 20cm in coastal counties and around 25cm in inland regions. The coverage of this DEM is shown below.

Combined bathymetric and topographic DEM of central coastal North Carolina.

DEMs such as those prepared for Tampa Bay and North Carolina using VDatum provide a valuable resource in linking data from multiple sources. VDatum thus enables the production of seamless DEMs across the land-water interface. As VDatum expands to national coverage, these bathy/topo DEMs will likewise become a seamless product available throughout the nation’s coastal regions. NOAA and the USGS are coordinating their efforts with respect to DEM development using VDatum and have agreed to make these DEMs available through the National Geophysical Data Center (NGDC) in the future.