|
| PE&RS January 2002VOLUME 68, NUMBER 1PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING JOURNAL OF THE AMERICAN SOCIETY FOR PHOTOGRAMMETRY AND REMOTE SENSING Highlight Article The
National Elevation Dataset
|
||||||||||||||||||||||||||||
This
scenario is hypothetical, but the number of files and processing steps
listed under the tiled database approach are accurate and are based
on the current holdings in the USGS 7.5-minute DEM database. As is
shown, all the DEM preprocessing required for tiled data is eliminated
in the seamless NED approach. The final steps of preparing hydrologic
products from the elevation data as listed are being addressed by an
effort parallel to NED, the USGS Elevation Derivatives for National
Applications (EDNA) project (Verdin, 2000; Kost and Kelly, 2001).
| Tiled DEM Database | Seamless Elevation Dataset |
| User employs EarthExplorer to identify "best available" DEMs. | User employs Web browser to view seamless elevation dataset |
| User downloads 476 30-meter SDTS format DEMs | User interactively selects areas of interest and enters order information |
| User orders 408 10-meter DEMs. User receives 408 files on tape, CD or by ftp | User receives one elevation data file and associated derivative products and metadata files on CD or by ftp |
| User imports 884 DEM files | User imports data |
| User converts elevation units of 12 DEMs from feet to meters | |
| User filters 221 30-meter Level 1 DEMs to remove production artifacts that would affect derivative products | |
| User projects 234 30-meter DEMs from UTM zone 13 to UTM zone 12 | |
| User mosaics 476 30-meter DEMs | |
| User mosaics 408 10-meter DEMs | |
| User resamples 10-meter DEM mosaic to 30-meter grid spacing | |
| User merges to 30-meter mosaics to produce uniform elevation data coverage of the study site | |
| User smooths data discontinuities along quadrangle boundary seams with various image processing tools | |
| User projects merged elevation dataset to an equal-area projection in NAD83 horizontal datum | |
| User generates derivative products: slope, aspect, drainage basins, subwatersheds, streamlines | |
| User edits elevation dataset and regenerates derivative products. User repeats process as necessary until proper hydrological conditions are represented | |
| User begins hydrologic modelling study | User begins hydrologic modelling study |
The USGS began the development of seamless elevation datasets in the early 1990s, first working on continental and global datasets at a resolution of 1 kilometer. The experience gained in assembling multisource, multiresolution elevation data into a global seamless dataset proved to be valuable as the methods were adapted, refined, and further developed for use in producing regional U.S. elevation framework prototypes in 1996. The first complete seamless coverage of the continental United States was finished in 1997 and was based on 10-meter, 30-meter, 2-arc-second, and 3-arc-second resolution source data. In 1999, for the first time, the NED was assembled completely (for the continental United States) from 7.5-minute DEM source data (10-meter and 30-meter data).
NED
Specifications and Production
To be a truly seamless dataset, the NED is assembled using a raster data model
cast in a geographic coordinate system (horizontal locations are referenced
in decimal degrees of latitude and longitude). A consistent grid spacing of
1-arc-second (approximately 30 meters) is used, except for Alaska, where lower
resolution source data warrant the use of a 2-arc-second spacing. Elevation
units are standardized to decimal meters. The horizontal datum for NED is the
North American Datum of 1983 (NAD 83), and the vertical datum is the North
American Vertical Datum of 1988 (NAVD 88). FGDC-compliant metadata are provided
for efficient documentation. Additionally, spatially referenced metadata are
supplied as an attributed geospatial data layer that captures all metadata
from the source DEMs and NED processing stages on a quadrangle basis.
The NED is a logically seamless dataset, as computer-processing considerations require that the data be assembled on a tiled basis, and the tiles collectively form the virtual national mosaic. Currently, NED production uses a 1- by 1-degree tile as the unit for assembly and processing of source DEMs. Adjacent tiles are edge matched to ensure the seamless nature of the entire national dataset. In its entirety, the NED comprises almost 1,400 1- by 1-degree tiles, with over 900 tiles covering the continental United States, nearly 400 covering Alaska and Hawaii, and the remainder covering the island territories. The total data volume for the NED currently approaches 60 gigabytes.
The NED is assembled from approximately 57,000 files of quadrangle-based source DEMs. As source data for NED production, nearly 54,000 DEMs are used for the continental United States and about 3,000 DEMs for Alaska, Hawaii, and the island territories. Production of 7.5-minute DEMs, especially at the 10-meter posting interval, is an ongoing activity at the USGS, so the NED is updated on a regular basis to incorporate all new DEM production, thus retaining the “best available” framework concept. A new version of the NED is released every two months, containing updated areas for which new source DEMs became available since the previous release. On average, approximately 20% of the 1- by 1-degree tiles are updated at each release. An update is required for a tile if even one new DEM is available, as edge matching must be done to ensure the seamless quality of NED. The spatially referenced (quadrangle-based) metadata provided with each NED release indicate the specific areas where new source DEMs have been incorporated since the previous release.
When a NED tile is assembled, the best available source data are selected according to the following criteria (ordered from first to last): 10-meter DEM, 30-meter Level 2 DEM, 30-meter Level 1 DEM, 2-arc-second DEM, 3-arc-second DEM. For the continental United States, the number of 10-meter source DEMs is increasing continually, so the number of 30-meter DEMs used in NED production has been decreasing correspondingly. Currently, for the continental United States, over 30% of the NED is derived from 10-meter source DEMs, and less than 10% is derived from the older 30-meter Level 1 DEMs, with the remaining 60% of the area based on the 30-meter Level 2 DEMs. Figure 2 shows the source data by DEM type for recent NED releases.
An interactive map server on the NED Web site (Figure 3) allows a user to display the NED data source index, which indicates the date of the most recent update, the resolution of the source data, and the production method of the source data for specific areas. The user can also query the spatially referenced metadata to examine additional information about each quadrangle-based DEM used to assemble the NED. The NED Web site also contains documentation on the NED assembly process, accuracy, metadata, standards, and data distribution.
NED assembly and updating are accomplished with a highly automated production process that was developed specifically for this seamless elevation dataset. The main steps in the process are outlined in Figure 4. The selection of source data is accomplished by an automated query of the USGS National Mapping Program’s Sales Database (SDB), known to the public as USGS GeoData. The “best available” data criteria outlined above are applied to select specific quadrangle-based DEMs from the SDB. The headers of the selected DEMs are read to create an “index,” which controls production and triggers subsequent processing steps. Industry standard tools from the National Geodetic Survey (NGS), NADCON and VERTCON, are used to perform horizontal and vertical datum transformations, respectively.
The 7.5-minute source DEMs are reprojected from their native UTM coordinate
system to a geographic coordinate system (decimal degrees of latitude
and longitude) using standard cartographic transformation software.
Resampling of the original elevation values is done with an implementation
of cubic convolution that is optimized for elevation data by maintaining
the integrity of shorelines and water bodies. Artifact removal is performed
on the older Level 1 DEMs that were produced with photogrammetric methods.
The “mean profile filter” (Oimoen, 2000) was specifically designed
to process these 7.5-minute DEMs. The filter process uses a series
of directional filters to isolate the high-frequency artifacts, which
are then subtracted from the DEM. The magnitude of these artifacts
is small, typically less than 1 meter, so the change to the DEM is
negligible, but the removal results in significant improvements in
derivative elevation products, such as shaded relief, slope, and aspect
(Figure 5).
|
|
|
|
|
The final steps in the production process include paneling the DEMs to fill the 1- by 1-degree tile, filling slivers of missing data along DEM boundaries as necessary by interpolating adjacent values, and edge matching the seams along the DEM boundaries and tile edges. The edge match algorithm uses a feathering approach that maintains local slope continuity across the seam. Finally, a shaded-relief image of the tile is generated for inspection by an analyst to verify successful processing, especially artifact filtering and edge matching. In some cases, DEM header information, which automatically triggered certain processing options, was incorrect, and the result is data that are not acceptable for inclusion into the NED. These cases are detected in the visual inspection step, the DEM header is corrected, and the tile is submitted for reprocessing. Tiles passing inspection are transferred to the next release version, and the spatially referenced metadata are updated accordingly.
Data
Distribution
In keeping with the framework concept of easily accessible data, NED products
are available in several common formats through a Web-based seamless data distribution
system (Figure 6) linked to the NED home page. The system uses interactive
map server technology to provide users the capability for viewing shaded-relief
images derived from the NED. The map server also includes numerous reference
layers, including the NED spatially referenced metadata, to help users define
their study areas. The system incorporates a geographic names capability wherein
users can automatically pan and zoom to specific feature locations. The user
enters customer and order information interactively, and orders for products
resulting in network-compatible file sizes can be delivered electronically
to the user. Orders for larger areas are produced on hard media for the cost
of reproduction and shipped to the customer.
Current
Development
Several NED development activities are currently underway to enhance the seamless
elevation data available to users. The NED production system has been modified
to produce 1/3-arc-second data, thereby maintaining the full information content
of the 10-meter source DEMs. Currently, 1/3-arc-second data are being produced
only where there is full coverage of 10-meter source DEMs for an entire 1-
by 1-degree NED tile. This results in multiresolution NED products being available
over certain areas. In these areas, both 1-arc-second and 1/3-arc-second data
are produced with the same characteristics and are “nested” spatially to facilitate
easy transition between resolutions.
A second area of current research and development is the integration of source data other than standard USGS DEMs into the NED. Researchers in the USGS elevation program are actively evaluating new elevation data collection technologies, including LIDAR, IFSAR, and softcopy photogrammetry (Osborn and others, 2001). DEMs resulting from these newer production methods generally have higher resolution and accuracy than existing DEMs. To maintain the “best available” characteristic of the NED, the USGS must exploit and incorporate these new sources into the NED, especially in those areas where applications are limited by existing source data, such as low-relief coastal areas.
Accuracy assessment and reporting for the NED is another topic being actively pursued. The accuracy of the NED varies spatially because of the variable quality of the source DEMs. As such, the NED “inherits” the accuracy of the source DEMs. Some accuracy statistics are available in the source DEM headers, and this information is captured in the spatially referenced metadata. Many times this accuracy information has limited usefulness because it is a relative measure of how well the DEM fits the source material from which it was generated. In an effort to provide more information to users on the absolute vertical accuracy of the NED, the USGS is testing the NED by comparing it with an independent reference source of very high accuracy. The reference data are the geodetic control points that form the High Accuracy Reference Network (HARN) maintained and distributed by the NGS. Nationally, almost 6,000 HARN points are available, so they are an excellent high-quality reference dataset for assessing the NED. Use of the NED spatially referenced metadata also allows the calculation of accuracy statistics by source DEM characteristics, including resolution, level, age, contour interval, and production method. Land surface characteristics derived directly from the NED, including elevation, slope, aspect, and local relief, allow for examining accuracy as a function of specific site conditions. The use of other independent reference data for accuracy assessment, including control points from digital orthophoto production projects, is also being investigated. Future plans call for updating overall accuracy statistics with each NED release and for providing accuracy estimates for specific areas ordered by the customer as part of the delivered metadata.
Summary
The NED is a seamless raster dataset from the USGS that fulfills many of the
concepts of framework geospatial data as envisioned for the NSDI, allowing
users to focus on analysis rather than data preparation. It is regularly
maintained and updated, and it provides basic elevation data for many GIS
applications. The NED is one of several seamless datasets that the USGS is
making available through the Web. The techniques and approaches developed
for producing, maintaining, and distributing the NED are the type that will
be used for implementing the USGS National Map (http://nationalmap.usgs.gov/).
Authors
D. Gesch, M. Oimoen, C. Nelson, M. Steuck, and D. Tyler are employed by Raytheon
ITSS at the USGS EROS Data Center under U.S. Geological Survey contract 1434-CR-97-CN-40274.
S. Greenlee is employed by the U.S. Geological Survey.
References
Brown, D.G., and Bara, T.J., 1994. Recognition and reduction of systematic
error in elevation and derivative surfaces from 7.5-minute DEMs: Photogrammetric
Engineering and Remote Sensing, v. 60, no. 2, p. 189-194.
Brunson, E.B., and Olsen, R.W., 1978. Data digital elevation model collection systems, in Proceedings of the Digital Terrain Models (DTM) Symposium, St. Louis, Missouri, May 9-11, 1978: Falls Church, Virginia, American Society of Photogrammetry, p. 72-99.
Garbrecht, J., and Starks, P., 1995. Note on the use of USGS Level 1 7.5-minute DEM coverages for landscape drainage analyses: Photogrammetric Engineering and Remote Sensing, v. 61, no. 5, p. 519-522.
Kelly, R.E., McConnell, P.R., and Mildenberger, S.J., 1978. The Gestalt photomapping system: Photogrammetric Engineering and Remote Sensing, v. 43, no. 11, p. 1407-1417.
Kost, J., and Kelly, G., 2001. Watershed delineation using the National Elevation Dataset and semiautomated techniques, in Twenty-First Annual ESRI International User Conference, San Diego, California, July 9-13, 2001, Proceedings: Redlands, California, Environmental Systems Research Institute, Inc. (CD-ROM).
Oimoen, M.J., 2000. An effective filter for removal of production artifacts in U.S. Geological Survey 7.5-minute digital elevation models, in Fourteenth International Conference on Applied Geologic Remote Sensing, Las Vegas, Nevada, November 6-8, 2000, Proceedings: Ann Arbor, Michigan, Veridian ERIM International, p. 311-319.
Osborn, K., List, J., Gesch, D., Crowe, J., Merrill, G., Constance, E., Mauck, J., Lund, C., Caruso, V., and Kosovich, J., 2001. Chapter 4 – National digital elevation program (NDEP), in Maune, D., ed., Digital Elevation Model Technologies and Applications: The DEM Users Manual: Bethesda, Maryland, American Society for Photogrammetry and Remote Sensing, p. 83-120.
Verdin, K., 2000. Development of the National Elevation Dataset-Hydrologic Derivatives (NED-H), in Twentieth Annual ESRI International User Conference, San Diego, California, July 10-14, 2000, Proceedings: Redlands, California, Environmental Systems Research Institute, Inc. (CD-ROM).
|
advertisement
 |
|
advertisement
 |
|
advertisement Trucking Companies / Canadian Trucking Companies
|
|
| 5410 Grosvenor Lane, Suite 210, Bethesda, Maryland 20814-2160 Tel (301) 493-0290 -- |
Copyright© 2009
ASPRS Last Update: |