NGDC Development of Coastal DEMs
The National Geophysical Data Center (NGDC), an office of the National Oceanic and Atmospheric Administration (NOAA), builds high-resolution coastal digital elevation models (DEMs) to support NOAA's mission to understand and predict changes in Earth's environment and conserve and manage coastal and marine resources to meet our Nation's economic, social and environmental needs. The DEMs seamlessly integrate ocean bathymetry data with topographic elevation data to visualize surface relief in coastal zones and to model the behavior of natural processes.
Table 1: Examples of specifications for coastal inundation DEMs of Mobile, Alabama
DEM Development Process
NGDC follows these basic steps when building coastal DEMS:
Steps three and four are repeated iteratively numerous times as anomalies or inaccuracies are found in preliminary DEMS, their cause determined, data corrected, and a new version of the DEM is created.
1) Gather Elevation Data
DEMs are developed using all of the best available digital elevation data. Shoreline, topographic, bathymetric, and shoreline-crossing data are obtained from many different government agencies, academic institutions, and private companies (e.g., Figure 2). Many of the datasets retrieved by NGDC for use in DEM development are already archived at NGDC. The original data are collected and compiled in numerous ways (e.g. multibeam swath sonar, lidar, satellite altimetry, USGS quadrangle digitization, etc.), in different terrestrial environments, throughout several time periods, and at various scales and resolutions (e.g., Table 2).
Sometimes, important bathymetric or topographic features are not represented by any existing digital data. This necessitates the hand digitization of features for inclusion in DEMs (e.g, Figure 3). Satellite imagery, available from ESRI, NASA, and/or Google is often used to help assess the current morphology of features.
2) Convert Data
Before generating a DEM, NGDC converts all original datasets into viewable file types and common horizontal (NAD 83 or WGS 84) and vertical (NAVD 88 or MHW) datums. Horizontal datum shifts and file conversions are accomplished using the Feature Manipulation Engine (FME), a software package developed by Safe Software. The relationships between vertical datums (e.g. Mean High Water, Mean Lower Low Water, Mean Sea Level) in some gridding regions have been established and incorporated into the VDatum software tool developed jointly by NOAA's Office of Coast Survey and National Geodetic Survey. VDatum is often used to shift the vertical datum of original datasets. In areas where the VDatum software tool does not provide coverage, NGDC calculates vertical datum relationships based on local tide station values.
3) Visually Evaluate and Edit Data
After the original datasets have been converted, NGDC assesses the datasets for quality and accuracy. It is important that each dataset is accurate both within itself and when compared to and overlaid with other datasets. Datasets must be consistent in order to transition smoothly when their edges meet or overlay. If a dataset contains errors or is overlaid by a more accurate dataset, NGDC edits and clips the data as necessary. NGDC visually assesses data using ESRI's ArcGIS, Applied Imagery's Quick Terrain Modeler, IVS 3D's Fledermaus, and other software products.
4) Build and Evaluate DEMs
DEMs are generated using the shareware package MB-System, a National Science Foundation (NSF)-funded software package designed for manipulating multibeam swath sonar data. The edited, clipped, and interpolated digital elevation datasets are converted to ASCII xyz format and assigned a relative gridding weight in a DEM data 'hierarchy' (e.g., Table 3). This ensures that datasets with the highest quality and resolution have the greatest impact in determining DEM elevation values. MB-System is used to generate the final DEM ASCII grid, using the weighted xyz datasets. A tight spline tension gridding method is used to interpolate values for DEM grid cells with no real data available; this ensures every cell in the DEM is assigned an elevation value.
Once generated, the DEM has a vertical datum that corresponds to that of its input xyz data. The DEM can be transformed to a new vertical datum, however, to meet the specifications of individual users. The vertical transformation of a DEM can be accomplished by, (a) adding a conversion grid of the same extent to the DEM, or (b) re-transforming the original data and repeating the DEM generation process in MB-System.
NGDC assesses the horizontal and vertical accuracy of final DEMs, based on the metadata of the original datasets. The final DEM ASCII grid cell values are directly compared with the original data (e.g., Figure 6).
For a more realistic comparison with true elevation values, NOAA's National Geodetic Survey (NGS) monument locations are extracted from datasheets(e.g., Figure 7). Values from tide stations within the DEM region or values from USGS topographic data can also be used as known elevations that can be compared with the DEM values.
5) Document DEM Development
After a coastal DEM is developed by NGDC, the processing procedures, data sources, and analyses information are thoroughly documented in a technical report. The technical report allows DEM users to understand the quality and accuracy of the DEM; it also allows anyone to replicate the DEM development process. NGDC creates a detailed metadata record for the DEM, which meets Federal Geographic Data Committee (FGDC) standards.
6) Distribute DEMs
When the DEM development process has been completed, the DEM is posted online for public access and use. Documentation and metadata accompany each of the DEMs. Additional Information on specific DEM projects, along with links to completed DEMs, can be found on the Integrated Models of U.S. Coastal Relief page.