National Elevation Data (NED) FAQ
Q. What are the "NED Release Notes" and where can they be found? A. The NED Release Notes provide details about the current release version of NED, the source data, and, noteworthy characteristics of the dataset, including accuracy, data distribution statistics, and processing notes. The NED Release Notes are available as PDF files via anonymous ftp from: ftp://edcftp.cr.usgs.gov/pub/data/ned/documents Q. How large is the NED 1 arc second dataset? A. The full NED 1 arc second dataset, including conterminous US, HI, AK, PR, and the Pacific Islands, is approximately 60 gbytes of data. Q. How often is the NED 1 arc second and 1/3 arc second data updated? A. The NED 1 arc second is updated every other month with any new "best available" elevation data. The NED 1/3 arc second is updated every other month, but the opposite of the NED 1 arc second. An example of the schedule would be NED 1 arc second updated in October 2004 and NED 1/3 arc second November 2004 and so on. Q. How can I get updates for the NED 1 arc second data and NED 1/3 arc second? A. A visual index is at Data Source Index map showing the areas that have been updated and when for both the NED 1 arc second and NED 1/3 arc second. Metadata and shapefiles of updates can be found at ftp://edcftp.cr.usgs.gov/pub/data/ned For NED 1 arc second, a standing request can be setup within Earth Explorer, http://earthexplorer.usgs.gov to receive notifications of updates. You would have to be a registered user to enable this service. You can order them through earthexplorer or customer service. For NED 1/3 arc second, the best way is to download the metadata and shapefiles for the NED 1/3 arc second from ftp://edcftp.cr.usgs.gov/pub/data/ned Then compare dates with what was received and what may have been updated. Q. Is there a status graphic or shapefile showing the updated areas for NED 1 arc second and 1/3 arc second? A. The Data Source Index map interface displays source resolution, production method of source, and date added to NED. Metadata and shapefiles of updates can be found at ftp://edcftp.cr.usgs.gov/pub/data/ned Q. Will the striping, evident in level 1 DEM's, be eliminated in the NED processing? A. The striping is in the original source DEM's that were processed as Level 1 using the GPMII or Manual Profile methods. Any DEMs that are produced by photogrammatic methods are considered to be Level 1. Level 1 Methods are:
This method produced the "stripping" seen in some level 1 DEMs. GPM2 (Gestalt Photo Mapper Model 2)--- This was an instrument that was semi automated to produce orthophotos and DEMs at the same time from photography. GPM2 DEMs can have an appearance of a grid pattern when a shaded relief graphic is produced from the DEM. Q. Are the USGS DEM's the only source used in NED processing? A. Source data for the NED are selected from the available DEMs according to the following ranking (highest priority listed first): high-resolution elevation data, 10-meter USGS DEMs, 30-meter Level 2 USGS DEMs, 30-meter Level 1 USGS DEMs, 2-arc-second USGS DEMs, 3-arc-second USGS DEMs. Note that the 2-arc-second DEMs are used only in Alaska, and the 3-arc-second DEMs are used only to fill in values over some large water bodies. The composition of the source data used in NED has a trend with 10-meter DEMs increasing and the corresponding decrease in 30-meter DEMs. Thus, the ongoing production of USGS 10-meter DEMs is reflected in each NED release. The total for 10-meter DEM source includes 777 7.5-minute quadrangle-based DEMs derived from 1:24,000-scale hypsography data over Kentucky. Although these DEMs are not standard USGS 10-meter DEMs because they were produced in a non-standard projection (State Plane), they were produced with the same method as the 10-meter DEMs and thus have the equivalent information content. The Ohio Elevation Dataset incorporated into the NED 1 and 1/3 arc second database was generated by merging individual DLG Hysography 7.5 minute quadrangle shapefiles into shapefiles 20 - 30 quadrangles in size. These were then converted to ArcInfo coverages and a DEM was created using the ArcInfo "TOPOGRID" command. Each of the ensuing GRIDs were clipped to the actual coverage extent to remove edge artifacts and converted from a floating point to an integer GRID (rounded up). The final procedure was to mosaic all of these individual GRIDs into a single statewide 10-Meter GRID. The high-resolution source includes elevation data derived from photogrammetric mapping over Bexar County in central Texas (first used in the August 2003 release) and from lidar data over eastern North Carolina (see the December 2003 NED Release Notes for details). The area covered by these high-resolution data sources is equivalent to that covered by 251 7.5-minute quadrangles. Multi-resolution NED In addition to the standard 1-arc-second resolution, NED data for a portion of the United States are available in 1/3-arc-second resolution (approximately 10 meters). These higher resolution data have been produced where 10-meter DEMs and other high-resolution DEMs are available as NED source data. As with 1-arc-second NED, some of the 1/3-arc-second NED is derived from "non-standard" source data (data other than standard USGS 7.5-minute DEMs). The two areas derived from non-USGS source data are Bexar County in south central Texas (photogrammetric data), and eastern North Carolina (lidar data). As new source data (either high-resolution data or USGS 10-meter DEMs) become available, production of 1/3-arc-second NED will continue, and additional areas will be made available as they are completed. More information about NED and data sources can be found within the NED Release Notes Q. What is the projection, datum, units, and cell size for NED 1, 1/3, and 1/9 arc second A. There is no projection. The coordinates system is geographic. To keep the dataset consistent the geographic coordinate system can be used across the dataset. Once the data is received it can be reprojected into UTM zones. This conversion is software dependent. The X, Y units are in decimal degree units.
Q. What is the Null value or nodata value for NED? A. The value is -9999. Q. Is the NED data available in feet, instead of meters? A. No, the Z units is only available in meters. Q. What if I am having problems applying hillshades? A. The difficulty in hillshades is the Z units. They should be the same as the X, Y units. In the NED the X, Y are in decimal degree units and the Z is in meters. With ArcInfo or Arc GIS hillshade there is a Z-factor parameter which can be set to 0.00003 to scale the meters to decimal degrees. In the ArcView Spatial Analyst hillshade does not have this factorr so the grid needs to be projected to a projection (UTM, Albers) which has meter units. The Grid Projector by Kenneth R. McKay at (http://gis.esri.com/arcscripts/details.cfm?CFGRIDKEY=1846870825) will project the grid so that hillshade can be run on the new grid and a usable dataset produced. Making Hillshade from NED data The process is:
For large areas a connection to the GeograpyNetwork with ArcMap allow the viewing of data with a hill shade which has already been calculated. Start ArcMap8.2
Q. What is the difference between NED and SRTM? A. The table below shows the differences. Basically, the advantage of getting the SRTM over the NED is that SRTM is more current. This would especially be important over mining and quarry areas. The elevation from the NED is a bare ground reading where as SRTM is canopy based. To read more about the SRTM go to the home page at http://srtm.usgs.gov
Q. What is the general process for NED and how does this fit into The National Map? A. National Elevation Dataset (NED) Process: The NED is in fact the elevation data theme of The National Map. Internally, we produce new DEMs, and fix bad DEMs (from customer reports) for archival into the GDA. The Sales Data Base retrieves all new uploads from the GDA, whether they are new or replacements. But more importantly, the NED also retrieves new uploads from the GDA. These uploads contribute to maintaining a current version of the NED in a seamless environment for The National Map . In basic terms, the NED upload process converts archive DEM data into the geographic format of arc-seconds, cast on the datums of NAD83 and NAVD88. The conversion process takes into account all horizontal and vertical datums, as well as many map projections that our DEM data holdings represent for both the past and present. Q. Is there a data dictionary associated with NED? A: Yes. The NED Data Dictionary can be found at ftp://edcftp.cr.usgs.gov/pub/data/ned/documents Q. Will NED replace any of the existing USGS DEM products? A: No. The USGS DEMs and other digital elevations data that are used as source data in building NED will continue to be available from the USGS EarthExplorer and USGS business partners that resell digital data. Q. Is there any conversion software available? A: There is some conversion software at the website http://www.ngs.noaa.gov/TOOLS/ . If this does not meet your needs, then run a search online with a search engine. Q. What software imports the NED data? A: The data is a raster dataset. Some known software that read NED are: ArcView (spatial analyst package), ArcGIS (spatial extensions), Arc/Info, ER Mapper, ERDAS, Microdem, Global Mapper, Rivertools, and more. We do not recommend one software over another. The best thing is to download a small area and try it to see which format best fits your software. These are just a few. You can run search online to see what software is able to import NED. Q. What is meant by "best available" elevation data? A. "Best Available" refers to the highest resolution elevation data available to be incorporated into the NED database. These would vary from area to area. As stated in the NED Release Notes, the source data for the NED are selected from the available DEMs according to the following ranking (highest priority listed first): high-resolution elevation data, 10-meter USGS DEMs, 30-meter Level 2 USGS DEMs, 30-meter Level 1 USGS DEMs, 2-arc-second USGS DEMs, 3-arc-second USGS DEMs. Note that the 2-arc-second DEMs are used only in Alaska, and the 3-arc-second DEMs are used only to fill in values over some large water bodies. Q. What is the vertical accuracy of NED data? A. The vertical accuracy is basically +/- 7 to 15 meters. It alls depends on the original source DEM and if it was level 1, level 2, or 10m resolution. Q. What is NED 1 arc second? A. NED is designed to provide National elevation data in a seamless form with a consistent datum, elevation unit, and projection. Data corrections were made in the NED assembly process to minimize artifacts, perform edge matching, and fill sliver areas of missing data. NED has a resolution of one arc-second (approximately 30 meters) for the conterminous United States, Hawaii, Puerto Rico and the island territories and a resolution of two arc-seconds for Alaska. NED data sources have a variety of elevation units, horizontal datums, and map projections. In the NED assembly process the elevation values are converted to decimal meters as a consistent unit of measure, NAD83 is consistently used as horizontal datum, and all the data are recast in a geographic projection. Q. What is NED 1/3 arc second? A. NED 1/3 arc second (~ 10 meter resolution) is processed and assembled the same as the NED 1 arc second, but the source data is 10 meter elevation data or higher resolution. There is not complete coverage of the United States. Only ~ 70% of the US has source 10 meter coverage. New NED 1/3 arc second data is updated when new 10 meter DEM's or higher resolution data is received. Q. What is NED 1/9 arc second? A. NED 1/9 arc second (~ 3 meter resolution)is processed and assembled the same as the NED 1 and 1/3 arc second, but the source data is using mostly LIDAR elevation data sources. There is very little coverage at this time with LIDAR. As new LIDAR data sources are received and processed, then new NED 1/9 arc second areas will be added to the database. Q. What is the coverage area of NED 1/3 arc second data? A. The coverage area is approximately 70% of the United States. There are several methods to identify NED 1/3 arc second coverage area.
Q. What is the coverage area of NED 1/9 arc second data? A. There is only a small area of coverage for NED 1/9 arc second at this time. As new data is received it will be added to the NED database.
Q. Why does the coverage of NED 1/3 arc second data appear to include a greater area than that covered by the 10 meter DEM's? A. The extent of 1/3-arc-second NED appears greater because some areas are derived from data other than the 10-meter DEMs. The USGS 10-m DEMs are the primary source data for production of 1/3-arc-second NED. Additionally, non-USGS elevation data, derived from lidar and photogrammetric data collections, are used as source data for 1/3-arc-second NED. NED is processed in 1-degree by 1-degree tiles, and when a tile is not fully covered by 10-meter DEMs and/or other high-resolution source data, 30-meter DEMs are oversampled to fill out the tile. The NED spatial metadata delivered with each order can be queried to determine the source data used to produce 1/3-arc-second NED over any given area. As new high-resolution source data become available, either 10-meter DEMs or other sources, the data derived from 30-m DEMs will be replaced. Q. When resampling NED, should I use Cubic Convolution or Nearest Neighbor? A. Cubic Convolution is the better method of resampling. The data will appear more smooth. If Nearest Neighbor is used, a stair step effect will be present and the data does not appear smooth. Q. Does the elevation start in the center of the pixel or one of the corners? A. The of the elevation "posting" depends on the format of the data. ArcGrid format -- the header coordintates are the upper left corner of the upper left pixel. BIL format -- the header coordinates are the center of the upper left pixel. GridFloat format -- the header coordinates are lower lefthand corner of the lowest left-hand pixel. Q. What formats are available? A. ArcGrid is an Arc/Info proprietary format. Some other software packages can read the ArcGrid format. Customers who specify ArcGrid will be delivered a workspace. The workspace is not tarred or gzipped. BIL is a non-proprietary format made by running the GRIDIMAGE command in Arc. The data is a simple binary raster format (signed integer data). There is an accompanying ASCII header file that provides file size information (number of rows and columns). The data are stored in row major order (all the data for row 1, followed by all the data for row 2, etc.). GridFloat is a non-proprietary format made by running the GRIDFLOAT command in Arc. The 32-bit (4 byte) is a simple binary raster format (floating point data). There is an accompanying ASCII header file that provides file size information (number of rows and columns). The data are stored in row major order (all the data for row 1, followed by all the data for row 2, etc.). More information about binary floating point is at:
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