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National Elevation Dataset

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Frequently Asked Questions

  1. What is the NED?
  2. What is the coverage of the NED?
  3. Where can I get the 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. How are the highest quality NED source datasets selected for deriving the NED seamless layers?
  10. What is the vertical accuracy of NED data?
  11. What are the projection, horizontal and vertical datum, units, and cell size for the NED?
  12. What metadata are 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 NED elevations (z coordinates) 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 NED data be changed to a different datum or projection?
  19. Which resampling methods are best for preserving NED data accuracy and terrain characteristics?
  20. How are elevations referenced to the NED datasets?
  21. How does the NED fit into The National Map?
  22. What is the data volume of the NED 2, 1, 1/3 and 1/9 arc second dataset?
  23. Are there restrictions on the use of the data?
  24. How do NED data compare to SRTM data?
  25. What is the difference between a DEM and a DSM?
  26. Is there a difference between a DEM and a DTM?
  27. Is there a difference between NED data and LIDAR data?
  28. What is the difference between bare-earth and first return lidar?
  29. Where can I find lidar data?
  30. 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 an elevation dataset that consists of seamless layers and a high resolution layer. Each of these layers is derived from the highest quality DEMs available in the NED source database for any geographic location within the conterminous United States, Alaska, Hawaii, territorial islands, Mexico and Canada. The NED is updated continually as new data become available. All NED data are in the public domain. The NED data layers are derived from diverse source data that are processed to a common coordinate system and unit of vertical measure. These data data are distributed in geographic coordinates in units of decimal degrees, and referenced to 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 over about one-third of the Nation 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 from IFSAR collections starting in 2010. Plans are in place to complete statewide collection of IFSAR by 2016. Very limited areas in Alaska are covered with 1/9 arc-second data.


Q: What is the coverage of the NED?

A: The area of coverage of any particular NED layer is generally determined by the availability of source elevation data of similar or higher resolution. For example, the 1/3 arc-second layer is available for only the portion of Alaska where higher resolution lidar or IfSAR elevation sources exist. The 1/9 arc-second layer is available in the US only where high-resolution, high-accuracy lidar or photogrammetric collections have been performed. The coverage of these higher resolution layers continually increases as new elevation data are collected and added to the NED. The NED spatial metadata is the most current and reliable source for information regarding available coverage for each of the NED layers described below. The spatial metadata may be downloaded from the NED Spatial Metadata page.

  • The NED 1-arc-second dataset covers the conterminous United States, Mexico and Canada. It also covers Hawaii, areas of Alaska, Puerto Rico and the Territorial Islands of the United States.

  • The NED 1/3-arc-second dataset covers the conterminous United States, Hawaii, 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 limited areas of Alaska.

  • The NED 2-arc-second dataset covers the state of Alaska.


Q: Where can I get the data?

A: The elevation data are available from the NED through the USGS The National Map. Access to the viewer can be found here: http://viewer.nationalmap.gov/viewer/.

NED data are also available through a bulk data delivery system.

To download data from the Viewer:

  1. Zoom to your area of interest.

  2. Turn on the Elevation Availability layer to see coverage of best available NED resolution.

  3. Click the Download Data tool near the upper right corner of the Viewer banner.

  4. Use a bounding box, or choose a reference area in the Download options menu, or select by current map extent or coordinate input.

  5. Select the Elevation theme.

  6. Select available NED products, such as 2 arc-second (Alaska only), 1 arc-second (conterminous U.S. and Territorial Islands, Hawaii, Puerto Rico, portions of Alaska, most of Canada, and all of Mexico), 1/3 arc-second (conterminous U.S. and Territorial Islands, Hawaii, and portions of Alaska), or 1/9 arc-second (in limited areas of U.S. only).

  7. Add selected products to the Cart.

  8. Click on the product in the Cart to view footprints and to initiate download of selected individual products.

  9. Checkout and enter your e-mail address twice to place your order to receive the complete list of download links in one message.


To obtain elevation data in bulk:

Send an email request to tnm_help@usgs.gov with your contact information and a general product description. You will receive a response email with procedures for bulk delivery and instructions for specifying the product area and data format. To receive bulk data, we require you to supply the external hard drive(s), within our specifications, and pay for return shipping.


Q: What formats are available?

A: Seamless NED data (2, 1 and 1/3 arc second) are available in ArcGRID, GridFloat and IMG formats. The 1/9 arc second data are available in IMG format. Accompanying metadata are available in TXT, XML and HTML with each download.


Q: What are the NED data sources?

A: NED source data are selected from an inventory of standard production USGS digital elevation model (DEM’s), and from an increasing number of datasets that are project- or agency-specific. Quality and currency are primary factors in determining if a dataset will be ingested into the NED. General categories of NED source data are characterized below.

  1. High-resolution data (3 meters or better), is typically derived from lidar or digital photogrammetry, and often with edited water bodies

  2. Moderate-resolution raster data (4 to 10 meters) is typically derived from digital photogrammetry and IFSAR

  3. 10-meter DEM’s derived from 1:24,000-scale cartographic contours and hydrography - Such data were produced by the USGS as a standard elevation product, and though they are presently the predominant source in the NED, they continue to be replaced as better source data are acquired.

  4. 30-meter DEM’s from 1:24,000-scale cartographic contours - Similar in most respects to their 10-meter counterparts, though usually of lower overall quality. These DEMs are only used as source for Puerto Rico and as void fill around the coterminous U.S.

  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. These DEMs are the source for most of Canada and all of Mexico.

  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.

  7. 1-arc-second Shuttle Radar Topography Mission (SRTM) data are the NED source over 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 and small slivers at the borders with US and Canada/Mexico.

  9. Bathymetric data for some coastal areas are collected using lidar, sonar, and other methods of subsurface terrain measurement, transformed to a common datum, and resampled to only 1/9 arc-second resolution within the NED.


Q: How often is the NED updated?

A: The NED is updated continually as new data become available.


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

A: The Data Source Index map shows areas which have been updated for all resolutions of the NED and is viewable at the NED Data Source Index within the National Map viewer Spatial metadata shapefiles showing the updated areas for each resolution are also available for download from the NED Metadata download page. Once the customer identifies a specific area of interest that has been updated, the data can be acquired through the USGS The National Map or through a bulk data delivery request. Contact National Map Help for more 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: How are the highest quality NED source datasets selected for deriving the NED seamless layers?

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. Data source requirements for the NED seamless layers are listed below.

  • Bare-earth elevation surface

  • Highest resolution USGS source for an area

  • Most recent USGS source for an area

  • Most accurate USGS source for an area

  • Freely redistributable

  • Hydro flattened*

  • Must not be hydro-conditioned or hydro-enforced.

* NED seamless layers represent a topographic surface. Hydro-flattening is a must for any dataset to be considered for the NED seamless layers. Currently, non hydro-flattened data are accepted into the 1/9th layer only if they are in a location where there are no other data available to the public. 1/9 arc-second data will not migrate to the seamless layers unless they are hydro-flattened. Hydro-enforced DEM data and hydro-conditioned DEM data create surfaces that are not suitable for topographic mapping, and are not considered suitable for seamless NED layers.


Q: What is the vertical accuracy of NED data?

A: The most recently published figure of overall absolute vertical accuracy of the NED within the conterminous US, expressed as the root mean square error (RMSE), is 2.44 meters. It is important to note, however, that the vertical accuracy actually varies significantly across the US because of differences in source quality, terrain relief, land cover, and other factors. 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 datums are used in some areas outside of the conterminous United States. Most areas within the NED are available at a horizontal 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 resolution of 1/9 arc-second (approx. 3 meters).

Q: What metadata is available for the NED?

A: There are two types of metadata available for the NED, spatial metadata and FGDC metadata.

  • Spatial metadata in the form of a shapefile is supplied as a part of each NED data download, or may be obtained at the NED Metadata download page. Polygon 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. Spatial metadata may be obtained at the NED Metadata download page.
  • FGDC metadata for NED products are also included with each data download formatted as HTML, XML, TXT, FAQ or per user selection. The FGDC Metadata contains a general description about the NED layer from which a product is derived, resolution of the product, geographic extents of the product, and contact information regarding the NED.


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: http://ned.usgs.gov/metadata.html.


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

A: NED Release Notes were previously produced to provide details about the current and previous releases of the NED, its source data, and noteworthy characteristics of the dataset including accuracy, data distribution statistics, and processing notes. These legacy Release Notes are available at the NED Processing Notes page.
Currently, Release Notes are not being produced as a stand-alone product; however, this information is still provided and can be found under NED Processing Notes.


Q: How do I convert NED elevations (z coordinates) 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 data from meters to feet, use this formula.

feet = meters / .3048

This can be done in a variety of software packages. It is important to note that the above formula must be used because the actual pixel values (elevation) must be changed, not just the spatial referencing information. After converting, make sure to change the spatial referencing information.


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 hill-shading functions require extra care in specifying an exaggeration (or "Z") factor. Depending on the software used to generate a hillshade raster, the data may need to be projected before building the hillshade.
There are two common methods for creating a hillshade of NED data. The first method requires projecting the data to a rectangular coordinate system and converting the elevation units to the match the horizontal units. The second method requires the application of a “Z factor” when the hillshade is created.
Method 1: First, project the NED data to the desired coordinate system. If the horizontal units (x,y) of the desired coordinate system are meters (for example, UTM), no further processing is needed before a hillshade can be created. If the horizontal units of the desired coordinate system are not meters (for example, State Plane (US Feet)), the elevation values (z) must be converted to match the horizontal units. This requires conversion of the actual pixel values of the DEM after the projection is completed. In other words, the projection alone will not provide a DEM suitable for creating a hillshade if the x,y units do not match the z units.
Method 2: Using the NED data directly to create a hillshade can be done by applying a Z factor. The Z factor can usually be applied as a parameter in the software being used to create the hillshade. The purpose of applying the Z factor is to properly scale the horizontal units (decimal degrees) to match the elevation units (meters). Although the factor for converting decimal degrees to meters varies for different latitudes, a Z factor of approximately 0.00003 should produce reasonable results for most applications.

Q: What software imports and converts NED data?

A: NED data can be ingested by most commercial GIS software.


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. Users can project the NED data to a different datum or projection after downloading the data, using most GIS software. Frequently, software providers have tutorials or forums that will provide you with the assistance you need to produce the required results.


Q: Which resampling methods are best for preserving NED data accuracy and terrain characteristics?

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 stair-stepping 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 rasters.


Q: How are elevations referenced to the NED datasets?

A: The NED datasets are centered to each pixel in raster dataset which are used by most GIS applications


Q: How does the NED fit into The National Map?

A: The NED serves as the elevation layer of The National Map, and provides elevation information for earth science studies and mapping applications in the United States.


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

A: The volume of the NED for each resolution and format are listed in the table below.

Format 2 arc-second 1 arc-second 1/3rd arc-second 1/9th arc-second
float 6 GB 175 GB 507 GB Not Available
grid 7 GB 187 GB 518 GB Not Available
img 5 GB 135 GB 431 GB 793 GB


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

A: All NED data are in public domain and may be used without restriction. We do request that proper credit be given when used for redistribution, resale, presentation or publication. Please refer to Acknowledging or Crediting USGS as Information Source for additional citation information.


Q: How does NED compare to SRTM?

A: The Shuttle Radar Topography Mission (SRTM) digital elevation dataset was collected by a modified radar system that flew on board 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 dataset of the Earth. SRTM data were collected for approximately 80% of the Earth's surface. Read more about the mission at the SRTM homepage: http://srtm.usgs.gov/index.php.

The SRTM data was captured at a nominal 30 meter posting over the United States and provided in 1x1 degree blocks. The SRTM data were later edited to remove significant spikes and delineate water bodies and coastlines, but certain issues persist. Water features depicted in the SRTM data were derived from ancillary source imagery resulting in significant temporal differences between the water features and the SRTM data. Significant voids and above-ground elevations exist in areas such as tree canopy due to the sensors measurement of reflective surfaces. A key difference between NED data and SRTM data, is that source data used to produce the NED are processed to represent bare ground readings and do not contain elevated surfaces. The NED datasets are developed from a variety of source data, including photogrammetrically derived terrain data, digitized contours, lidar, and ifsar. Source data undergo rigorous quality assurance to qualify as input to the NED layers (see the FAQ How are the highest quality NED source datasets determined for deriving the seamless NED layers?). Many of these source data that contribute to the NED are collected at much higher resolutions and have much greater inherent accuracies than the SRTM data. While source data are continually collected and used to update the NED layers, some source datasets pre-date the SRTM. Therefore, localized variations in elevations and geographic features are evident when comparing the NED data and SRTM data.

The chart below shows differences between the NED and SRTM collections.

  NED SRTM
Source Data Photogrammetry/Aerial Data/Digitized Contours/Lidar/Ifsar Data Ifsar datatd>
Source Resolution High resolution, 10m & 30m DEMs 30-m
Source Dates 1925 - Present February, 2000 Space Shuttle Endeavour
Source Type Digital Elevation Model Digital Surface Model
Vertical Accuracy 2.44m RMSE (root mean square error) Measured 10-m RMSE (Mission Specification)


Q: What is the difference between a DEM and a DSM?

A: A digital elevation model, or DEM, is a representation of the terrain (bare-earth) with elevations on the ground at regularly spaced intervals. A digital surface model, or DSM, also contains elevations at regularly spaced intervals. However, the elevations represent only the first returns from each laser pulse. These first returns may be reflected by bare ground or by surface features such as trees and structures.


Q: What is the difference between a DEM and a DTM?

A: A digital terrain model (DTM) may be a different type of model from a DEM, such as an irregularly spaced vector model of bare earth points. The USGS distributes Alaska elevation datasets that are labeled as DTM in the file names; however, these files are raster models with elevations at regularly spaced intervals. Therefore the Alaska datasets labeled as DTMs are are equivalent to USGS DEMs in format and surface representation.


Q: Is there a difference between NED data and LIDAR data?

A: Yes. Lidar data are highly dense point data representing an 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, and powerlines.
The NED data layers are bare-earth raster surfaces in raster digital elevation model (DEM) format. While many currently generated DEMs and much of the NED are produced from bare-earth lidar point data, the NED layers are not lidar.


Q: What is the difference between bare-earth and first return lidar?

A: The laser pulses emitted from terrestrial lidar systems can be reflected from man made structures, vegetation or the earthen surface. A single pulse may reflect from upper, middle, and lower tree canopy as well as the ground beneath, resulting in multiple measurable returns from that pulse. Very simply stated, these returns are processed, with time and positional information, to derive elevations for each of these returns. The full set of discrete returns and their corresponding x, y , z coordinates are stored in what’s referred to as a ‘point cloud’ dataset. The USGS requires the points to be classified into one of several general categories including vegetation, buildings, water, ground, and others.
Many types of information can be extracted from a classified lidar point cloud. Two common products are bare-earth DEMs and first return digital surface models (DSMs). First return is created by identifying those points which are the first reflection from each laser pulse. These can be extracted and delivered as a separate point dataset, or converted into a raster surface. In open areas, a single point is returned. Together, the first return and bare-earth allow vegetation height to be modeled; this can in turn, support biomass and carbon estimates. Datasets of first return points, only, are referred to as “first-return surfaces” (raster or vector) or Digital Surface Models (DSM) which are typically raster”


Q: Where can I find lidar data?

A: Lidar acquired by the USGS through contracts, partnerships, and other opportunities are available through EarthExplorer These data are discrete-return, classified point-cloud data provided in LAS format. Please visit the USGS Long Term Archive website for more information about the available lidar products and data limitations.


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.

To cite the NED data for research or other purposes, please use the following reference:
Data Credit: National Elevation Dataset http://ned.usgs.gov/


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Last modified: Tuesday, 15-Apr-2014 17:38:26 EDT