NCEI Development of Coastal DEMs

The National Centers for Environmental Information (NCEI) 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.

Coastal DEMs serve as a base layer for a variety of uses including, (a) modeling of coastal processes (storms, tsunamis, ocean currents, sediment transport, sea-level rise, etc.), (b) ecosystems management and habitat research, (c) coastal and marine spatial planning, and (d) community hazard preparedness and disaster mitigation. DEMs are constructed for specific geographic areas, provided that there are digital elevation data available. NCEI built high-resolution coastal DEMs of communities along the U.S. East and West Coasts, the Gulf of Mexico, Alaska, Hawaii, Puerto Rico, and the Pacific Islands, along with lower resolution regional and global DEMS.

DEM specifications (e.g., Table 1) vary and are determined by the intended final use of the DEM.

DEM Development Process

NCEI follows these basic steps when building coastal DEMS:

1. Gather elevation data from multiple sources
2. Convert into common file formats and common horizontal and vertical datums (reference frames)
3. Visually evaluate and edit data
4. Build and evaluate DEMs
5. Document DEM development
6. Distribute DEMs on the Internet for public access

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.

Figure 1: Diagram illustrating the basic sequence of DEM development procedures.

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 NCEI for use in DEM development are already archived at NCEI. 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).

Figure 2: Spatial coverage of datasets used in DEM of Arena Cove, California. Many of the datasets were obtained from government organizations, including NOAA [Coastal Services Center (CSC), Electronic Navigational Charts (ENC), and National Ocean Service (NOS)] and the United States Geological Survey [USGS lidar, National Elevation Dataset (NED)]. California State University at Monterey Bay also provided data for the Arena Cove DEM.

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.

Figure 3: Example of hand-digitized breakwaters near Ocean City, Maryland DEM, shown in red. These breakwaters were not represented in any available topographic datasets.

2) Convert Data

Before generating a DEM, NCEI 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, NCEI calculates vertical datum relationships based on local tide station values.

3) Visually Evaluate and Edit Data

After the original datasets have been converted, NCEI 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, NCEI edits and clips the data as necessary. NCEI visually assesses data using ESRI's ArcGIS, Applied Imagery's Quick Terrain Modeler, IVS 3D's Fledermaus, and other software products.

Figure 4: Illustration of errors in a lidar dataset.

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.

In general, NCEI builds DEMs in the North American Vertical Datum of 1988 (NAVD 88), and transforms them into new DEMs with a Mean High Water (MHW) vertical datum by adding a conversion grid to the NAVD 88 DEMs. Conversion grids are developed by using the datum relationships from the VDatum software tool, with a thin plate spline interpolation method to estimate inland values. In areas where the VDatum software tool does not provide coverage, (e.g., Alaska), NCEI creates a conversion grid using the datum relationships from local tide stations with a Kriging interpolation method to estimate offshore and inland values. All DEMs and conversion grids are thoroughly reviewed and assessed for errors throughout the process.

DEMs are evaluated using several different methods. First, the DEMs are visually inspected for anomalous "spikes" and "wells" using ESRI's ArcGIS, Applied Imagery's Quick Terrain Modeler, IVS 3D's Fledermaus. These software programs render three-dimensional views of the DEM grids that can be rotated, color-coded by depth, and vertically exaggerated.

A "slope" map is also generated from the DEM (e.g., Figure 5). A slope map visually highlights changes in slope throughout the DEM, and should reflect natural morphology. Artificial features, often located at the edge of datasets, can be easily spotted on a slope map. A close visual inspection of DEMs reveals errors, which may necessitate reevaluation of the original data and the re-gridding of the DEM.

Figure 5: Slope Map of a DEM of Craig, Alaska

NCEI 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).

Figure 6: Histogram comparison an original bathymetric dataset (U.S. Army Corps of Engineers Alabama Hydrographic Surveys) and the Mobile, Alabama DEM

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.

Figure 7: Locations of NGS monuments within the Portland, Maine DEM boundaries

5) Document DEM Development

After a coastal DEM is developed by NCEI, 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. NCEI creates a detailed metadata record for the DEM, which meets Federal Geographic Data Committee (FGDC) standards.

6) Distribute DEMs

When the DEM development process is complete, the DEM is posted online. Documentation and metadata accompany each of the DEMs. Information on specific DEM projects: Integrated Models of U.S. Coastal Relief.

Figure 8: 3D image of the 3 arc-second Pago Pago DEM, American Samoa