1. What is the NED?
2. What is the coverage of the NED?
3. Where can I get data?
4. What formats are available?
5. What are the NED data sources?
6. How often is the NED updated?
7. How can a user get updates of the NED?
8. Where can a user find a graphic or shapefile showing the updated areas for the NED?
9. What is meant by "best available" elevation data?
10. What is the vertical accuracy of NED data?
11. What are the projection, horizontal and vertical datum, units, and cell size for NED?
12. What metadata is available for the NED?
13. Where can I find the NED data dictionary?
14. What are the NED Release Notes and where can they be found?
15. How do I convert Z (elevation height) unit of the NED data from meters to feet?
16. What if I am having problems creating shaded relief images?
17. What software imports and converts NED data?
18. Can the NED be changed to a different datum or projection?
19. When resampling NED, should I use Cubic Convolution, Bilinear Interpolation, or Nearest Neighbor?
20. Are elevations referenced to pixel centers or to a pixel corner?
21. What is the general process for the NED?
22. How does NED fit into The National Map?
23. What is the data volume of the NED 1, 1/3 and 1/9 arc second dataset?
24. Are there restrictions on the use of the data?
25. How does NED compare to SRTM?
26. What is the difference between NED and CLICK?
27. Is there a difference between NED and LIDAR?
28. What is the difference between bare earth and top of canopy?
29. How should the NED be cited?

Q: What is the NED?

A: The National Elevation Dataset (NED) serves the elevation layer of The National Map, and provides basic elevation information for earth science studies and mapping applications in the United States. Scientists and resource managers use NED data for global change research, hydrologic modeling, resource monitoring, mapping and visualization, and many other applications.

The NED is a seamless dataset composed of the best available raster elevation data of the conterminous United States, Alaska, Hawaii, and territorial islands. The NED is updated on a nominal two month cycle to integrate newly available, improved elevation source data. All NED data are in the public domain. The NED are derived from diverse source data that are processed to a common coordinate system and unit of vertical measure. NED data are distributed in geographic coordinates in units of decimal degrees, and in conformance with the North American Datum of 1983 (NAD 83). All elevation values are in meters and, over the conterminous United States, are referenced to the North American Vertical Datum of 1988 (NAVD 88). The vertical reference will vary in other areas. NED data are available nationally (except for Alaska) at resolutions of 1 arc-second (approx. 30 meters) and 1/3 arc-second (approx. 10 meters), and in limited areas at 1/9 arc-second (approx. 3 meters). In most of Alaska, only lower resolution source data are available. As a result, most NED data for Alaska are at 2-arc-second (approx. 60 meters) grid spacing. Part of Alaska is available at the 1- and 1/3-arc-second resolution, and plans are in development for a significant improvement in elevation data coverage of the state.

Q: What is the coverage of the NED?

A:

• The NED 1-arc-second dataset covers the conterminous United States, Hawaii, small areas of Alaska, Puerto Rico, Territorial Islands of the United States, and Mexico.
• The NED 1/3-arc-second dataset covers the conterminous United States, Hawaii, small areas of Alaska, and Territorial Islands of the United States.
• The NED 1/9-arc-second dataset covers areas of the conterminous United States and small areas of Alaska.
• The NED 2-arc-second dataset covers the state of Alaska.

Q: Where can I get data?

A: The elevation data are available from the NED in a seamless, nationally consistent form through the USGS The National Map Seamless Server. The Seamless Server allows users to interactively select and download an area of interest by drawing a box, defining coordinates, or using available templates to extract areas. NED data are also available through a bulk data delivery system (Email: bulkdatainfo@usgs.gov for information) and in the future will be available through the Tiled Data Distribution System accessible through The National Map Seamless Server.

Q: What formats are available?

A: NED data are available in ArcGRID, GeoTIFF, BIL and GridFloat (floating point binary raster with external header files) formats with accompanying textual metadata delivered as HTML, XML, TXT, FAQ, or SGML depending on the customer’s selections. NED raster data distributed through the bulk data delivery system are available in ArcGrid or GridFloat with textual metadata provided in both XML and TXT.

Q: What are the NED data sources?

A: NED source data are selected from an ever-growing inventory of standard production USGS Digital Elevation Model (DEM’s), and also from an increasing number of datasets that are project- or agency-specific. The first consideration is always given to quality. Selections are made according to the following ranking, listed in order of descending priority:

1. High-resolution data, typically derived from lidar or digital photogrammetry, and often with edited water bodies. If collected at a ground sample distance no coarser than 5 meters, such data may also be offered within the NED at a resolution of 1/9th arc-second.
   
2. Moderate-resolution data, other than that compiled from cartographic contours. These data may also be derived from lidar or digital photogrammetry, or less often by Interferometric Synthetic Aperture Radar IFSAR. A typical ground sample distance is 10 meters, though it is commonly called “1/3 arc-second data”.
   
3. 10-meter DEM’s derived from cartographic contours and mapped hydrography. Most often, such data are produced by or for the USGS as a standard elevation product, and they currently account for the bulk of the NED.
   
4. 30-meter cartographically derived DEM’s. Similar in most respects to their 10-meter counterparts, though usually of lower overall quality.
   
5. 30-meter photogrammetrically derived DEM’s. These are the oldest DEM’s in the 7.5-minute series. These data were derived directly from stereo photography, either by a human operator or by an early form of electronic image correlation. They are badly marred by production artifacts that are addressed to the greatest practical extent by digital filtering within the NED production process.
   
6. 2-arc-second DEMs are a standard USGS product. They are derived from cartographic contours at a scale of 1:63,360 over the state of Alaska, and a scale of 1:100,000 elsewhere.
   
7. 1-arc-second Shuttle Radar Topography Mission (SRTM) data, to date, are only used in preference to 3 arc-second data in the Aleutian Islands.
   
8. 3-arc-second DEMs are another standard USGS product, and are generally only used within the NED as a source of fill values over large water bodies.
   

Q: How often is the NED updated?

A: The NED 1- and 1/3-arc-second data sets are updated on a nominal two month cycle to integrate newly “best available”, improved elevation source data. Bi-monthly updates may be skipped due to budget or data constraints.

New datasets are being released into the NED 1/9-arc-second collection on a monthly basis and in conjunction with the bi-monthly updates when possible.

Q: How can a user get updates of the NED?

A: The Data Source Index map shows areas which have been updated for all resolution of the NED collectively and is viewable at the NED - Data Source Index. Spatial metadata shapefiles showing these areas of updates for each resolution are also available to download from the NED Metadata download page. Once the customer identifies a specific area of interest has been updated the data can be reacquired through the USGS National Map Seamless Server or through a bulk data delivery request. Email: bulkdatainfo@usgs.gov for information.

Q: Where can a user find a graphic or shapefile showing the updated areas for the NED?

A: Shapefiles delineating update areas can be found at the NED Metadata download page.

Q: What is meant by "best available" elevation data?

A: There are frequently multiple sources of elevation data that cover a given location. Preference is given to that which is the most detailed and up-to-date, unless there are overriding quality concerns. An important qualification is that any candidate source data be freely redistributable. Thus “best available” should not be confused with “best obtainable”.

Q: What is the vertical accuracy of NED data?

A: The accuracy of the National Elevation Dataset (NED) varies spatially because of the variable quality of the source digital elevation models (DEMs). As such, the NED inherits the accuracy of the source DEMs. The most recently published figure of overall absolute vertical accuracy expressed as the root mean square error (RMSE) is 2.44 meters. Details of this analysis are explained in the Vertical Accuracy of the National Elevation Dataset paper, and are published in the Digital Elevation Model Technologies and Applications: The DEM Users Manual 2nd Edition.

Q: What are the projection, horizontal and vertical datum, units, and cell size for NED?

A: The NED data are not projected, but are provided in geographic coordinates (latitude and longitude) in units of decimal degrees, horizontally referenced to the North American Datum of 1983. All elevation values are in units of meters typically referenced to the North American Vertical Datum of 1988, although the National Geodetic Vertical Datum of 1929 and local reference surfaces are used in some areas outside of the conterminous United States. Most areas within the NED are available at a cell spacing (resolution) of 1 arc-second (approx. 30 meters), and 1/3 arc-second (approx. 10 meters). Some areas from specific sources are also available at a cell spacing of 1/9 arc-second (approx. 3 meters). The bulk of Alaska is, at this writing, only available at a point spacing of 2 arc-seconds (approx. 60 meters).

Q: What metadata is available for the NED?

A: Spatial metadata in the form of an ESRI shapefile is supplied as a part of each NED data download, or may be obtained at the NED Metadata download page. Polygonal footprints of the contributing area of each source dataset provides spatial context, with various attached attributes describing the source data’s characteristics, such as its original resolution and production method. A NED user might, for example, use this metadata to identify portions of a study area that were produced with now-obsolete methods.

Textual metadata for NED products is also included with each data download formatted as HTML, XML, TXT, FAQ, or SGML per user selection.

Q: Where can I find the NED data dictionary?

A: The NED Data Dictionary, which provides textual definitions of the fields within the NED spatially referenced metadata, may be downloaded from the NED website at the NED Metadata download page.

Q: What are the NED Release Notes and where can they be found?

A: The NED Release Notes provide details about the current and previous releases of NED, its source data, and noteworthy characteristics of the dataset including accuracy, data distribution statistics, and processing notes. The NED Release Notes are available at the NED Metadata download page.

Q: How do I convert Z (elevation height) unit of the NED data from meters to feet?

A: The NED is derived from diverse source data that are processed to a common coordinate system and unit of vertical measure. All NED elevation values are in meters. To convert the Z unit of downloaded NED from meters to feet, use this formula.

feet = meters / .3048

Q: What if I am having problems creating shaded relief images?

A: Creating shaded relief directly from NED data can be problematic, because the horizontal units are decimal degrees but vertical units are meters. When horizontal and vertical units aren't the same, most hillshading functions require extra care In specifying an exaggeration (or "Z") factor.

In the ArcInfo or ArcGIS hillshade function, the Z-factor parameter can be set to a value to scale the meters to decimal degrees. A Z-factor value of approximately .00003 should produce reasonable results.

Other Commercial Off The Shelf (COTS) tools may not have an equivalent parameter, making it necessary to project the data into a projection (UTM or Albers, for example) which represents the x,y's in meter units.

Q: What software imports and converts NED data?

A: NED data can be ingested by most commercial GIS software. However, as a government entity, the USGS cannot endorse a specific software or conversion tool.

Q: Can the NED be changed to a different datum or projection?

A: Yes. To maintain consistency, the NED data distributed from USGS are only produced in NAD83 and a geographic coordinate system. But users can project the NED data to a different datum or projection on their own, using third party software. Please check with your software provider. Frequently, software providers have tutorials or forums that will provide you with the assistance you need to produce the required results.

Q: When resampling NED, should I use Cubic Convolution, Bilinear interpolation or Nearest Neighbor?

A: Cubic Convolution and Bilinear Interpolation are the preferred methods of resampling digital elevation data, and will result in a smoother appearance. Nearest Neighbor has a tendency to leave artifacts such as stairstepping and periodic striping in the data which may not be apparent when viewing the elevation data but might affect the derivatives, such as shaded relief or slope.

Q: Are elevations referenced to pixel centers or to a pixel corner?

A: The NED datasets are pixel centered, as are most raster datasets used in GIS applications. This is in contrast to older elevation-specific formats, such as USGS DEM, that are logically formatted as a series of equally spaced postings in variable length profiles.

Q: What is the generation process for NED data?

A: The source DEMs are projected from their native coordinate system to the geographic coordinate system (decimal degrees of latitude and longitude) using standard cartographic transformation software. Both horizontal and vertical datums are addressed as well during NED processing. Resampling of the original elevation values is done with implementations of cubic convolution and bilinear interpolation in order to retain the integrity of shorelines and water bodies. All NED vertical elevations are reported in meters, so if the data are in other units a conversion is applied. The data are then mosaicked into a seamless layer and made available for user view and download.

Q: How does NED fit into The National Map?

A: The NED serves as the elevation layer of The National Map, and provides basic elevation information for earth science studies and mapping applications in the United States. Scientists and resource managers use NED data for global change research, hydrologic modeling, resource monitoring, mapping and visualization, and many other applications.

Q: What is the data volume of the NED 1, 1/3 and 1/9 arc second dataset?

A: The volume of the NED 1- and 1/3-arc-second collection are approximately 65 and 450 Gbytes respectively. The NED 1/9-arc-second layer continues to grow rapidly with the regular addition of new datasets to the collection.

Q: Are there restrictions on the use of the data?

A: All NED data are in public domain. We do request that the following statement be used when copying or reprinting data:
"Data available from U.S. Geological Survey.”

Please visit the USGS EROS Data Citation website, or the USGS Visual Identity System Guidance website for more information.

Q: How does NED compare to SRTM?

A: The Shuttle Radar Topography Mission (SRTM) digital elevation dataset were collected by a modified radar system that flew onboard the Space Shuttle Endeavor in February 2000, using a technique known as Interferometric Synthetic Aperture Radar to generate the most complete high-resolution digital topographic database of the Earth. SRTM data were collected for approximately 80% of the Earth's surface. Read more about the mission at the SRTM homepage.

The National Elevation Dataset (NED) is a raster product assembled by the U.S. Geological Survey. NED is designed to provide National elevation data in a seamless form with a consistent datum, elevation unit, and projection. Various source data are used in the generation of NED data as well as a wide range of production methods and source dates. The NED is a collaborative effort among the USGS and other Federal, State, and local partners to improve and deliver topographic information for the Nation. Additional information can be found at the NED webpage.

The chart below shows differences between the NED and SRTM collections. The primary difference is the elevation from the NED is a bare ground reading whereas SRTM is canopy based.

	NED	SRTM
Source Data	Maps/Aerial Photos/Lidar	Radar data
Source Resolution	3m, 10-m & 30-m DEMs	30-m
Source Dates	1925 - Present	February, 2000 Space Shuttle Endeavour
Source Type	"Bare-Earth"	"First Return"
Vertical Accuracy	2.44m RMSE (root mean square error) Measured	10-m RMSE (Mission Specification)

Q: What is the difference between NED and CLICK?

A: The National Elevation Dataset (NED) is a National Map dataset that contains bare-earth raster surfaces, maintained at three resolutions: 1 arc-second (~30m), 1/3 arc-second (~10m), and 1/9 arc-second (~3m). The NED layers are derived from a variety of data sources, including cartographic, photogrammetric, auto-correlated, and lidar. The 1 and 1/3 resolutions provide complete CONUS coverage while the 1/9 layer coverage is being expanded as suitable data becomes available. Data is available through The National Map Seamless Server.

The USGS Center for Lidar Information Coordination and Knowledge (CLICK) is a portal for lidar information, discussion, and data. Its primary mission is to support scientific research on lidar point data. Voluntarily contributed lidar point clouds are available for download as tiles. More recent contributions are in LAS format, include all returns, and usually have been classified. Older data are often in ASCII format, may be split into separate files of first and last returns, and are often not classified. The point density, accuracy, and overall quality of these datasets are widely varied.

Q: Is there a difference between NED and LIDAR?

A: Yes, very much! Lidar is point data representing the X-Y-Z location, along with other attributes, of any terrestrial target reflecting the laser pulse. In addition to bare-earth, it typically will include buildings, trees, towers, powerlines … almost anything.

The NED is a bare-earth raster surface, or Digital Elevation Model (DEM). While many currently generated DEMs and much of the NED are produced from bare-earth lidar point data, the NED is not lidar.

Q: What is the difference between bare-earth and top-of-canopy?

A: Because lidar points can be reflected from any object, many types of information can be extracted from a lidar point dataset. The two most common products are Bare Earth and Top-of-Canopy. Bare-earth is created by first classifying each point according to what target it was reflected from (i.e., ground, building, tree, water, etc.). Once classified, the bare-earth points can be extracted and delivered as a separate dataset, or converted into a DEM. Top-of-Canopy is created by first identifying those points which are the first reflection from each laser pulse. In modern lidar systems, this is a standard attribute included in the LAS point file. These can be extracted and delivered as a separate point dataset, or converted into a raster surface. In open areas, the Top-of-Canopy and bare-earth are coincident. Together, the Top-of-Canopy and bare-earth allow vegetation height to be modeled; this can in turn, support biomass and carbon estimates.

Q: How should the NED be cited?

A: To cite the NED in a publication, please use the following literature references:

Gesch, D.B., 2007, The National Elevation Dataset, in Maune, D., ed., Digital Elevation Model Technologies and Applications: The DEM Users Manual, 2nd Edition: Bethesda, Maryland, American Society for Photogrammetry and Remote Sensing, p. 99-118.

Gesch, D., Oimoen, M., Greenlee, S., Nelson, C., Steuck, M., and Tyler, D., 2002, The National Elevation Dataset: Photogrammetric Engineering and Remote Sensing, v. 68, no. 1, p. 5-11.