1. Does NED replace any of the existing USGS DEM products?


2. How often will NED be updated? What areas are updated?


3. Will the stripping evident in normal DEMs be eliminated?


4. What is the projection, datum, units, cell size?


5. Can I get the data in units of feet?


6. Where can I get data?


7. Is there a description of the formats available?


8. How does NED compare to SRTM?


9. What if I have having problems with applying hillshades?


10. What is the general process for NED and how does this fit into The National Map?


11. What is the byte order for the .BIL?


12. Can the data be produced in a different datum or projection?


13. Is there any conversion software available?


14. What software reads the data?


15. How large is the NED dataset?


16. Is there a data dictionary associated with the NED?


17. What is the average vertical accuracy of NED?


18. Is 10m resolution data available as a seamless product?


19. Are there restrictions on the use of the data after it is acquired?


20. Where will the announcement be that more 10m data is available as a seamless product?

Q: Does 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 and those of its business partners that resell digital data.

Q: How often will NED be updated? What areas are updated? How can I get updates?

A: The NED is updated every two months. A visual index is at Data Source Index Map showing the areas that have been updated and when. Information on the updates and shapefiles of updates can be found at ftp://edcftp.cr.usgs.gov/pub/data/ned/. You can setup a standing request 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 customer service or earthexplorer.

Q: Will the striping evident in normal DEMs be eliminated?

A: The filtering process removes as much as possible, but not all of the striping is eliminated. As more source DEMs are produced they will eventually replace those areas. The striping show in NED is usually where the DEMs are still level 1 and the GPMII DEM creation method was used. This is an old DEM process that is not used anymore.

Q: What is the projection, datum, units, cell size?

A: There is no projection. The coordinates systems 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 to software dependent.

The horizontal datum for all areas, except Alaska, is NAD83. Alaska is NAD27. The vertical datum for all areas, except Alaska, is NAVD88. Alaska is NAVD29.

The z units in meters

The cell size for 30m NED is 0.00027778 and the cell size for 10m NED is 0.000092593.

Q: Can I get the data in units of feet?

A: No, the z units is only available in meters.

Q: Where can I get data?

A: You can get data at SDDS. Free downloads are available for areas up to 30 square degrees latitude and longitude in 100mb files. Any size area is available for $32 per CD, plus $45 base charge and $5 handling. The formats available are ArcGrid, BIL, GridFloat, and TIFF.

Bulk or Box Sets of CD's are also available for 30m NED. The cost is $3520, plus $45 base charge and $5 handling. The formats available are ArcGrid and GridFloat.You can search and order at http://earthexplorer.usgs.gov for the full dataset or single CD's.

Q: Is there a description of the formats available?

A: The ArcGrid format is an Arc/Info proprietary format. Customers who specify ArcGrid will be delivered a workspace. Software that can read this format are ArcGis, ArcView, and Arc/Info with spatial analyst type packages.


The BIL format is non-proprietary made by running the GRIDIMAGE command in ARC. The NED in 16- bit (2 byte) 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.). The byte order is I or Intel byte order.

http://grouper.ieee.org/groups/754/
http://www.math.grin.edu/~stone/courses/fundamentals/IEEE-reals.html
http://www.psc.edu/general/software/packages/ieee/ieee.html

The GridFloat format is non-proprietary made by running the GRIDFLOAT command in ARC. The format is a 32- bit (4 byte) 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.).


TIFF is a non-proprietary format. It is a 32 bit floating point grid format.
http://partners.adobe.com/asn/developer/graphics/graphics.html

Q: How does NED compare to SRTM?

A: The chart 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 whereas SRTM is canopy based. To read more about the SRTM go to the home page at http://srtm.usgs.gov.

Resolution	1-arc-sec (30-m)	1-arc-sec (30-m)
Source Data	Maps/aerial photos	Radar images
Source Resolution	10-m & 30-m DEMs	30-m
Source Dates	1925 - 1999	Feb. 2000
Source Type	"Bare earth"	"First return"
Accuracy Spec.	7 m RMSE	10 m RMSE

Q: What if I have having problems with applying hillshades?

A: The difficulty in producing 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 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 hill shade does not have this factor so the grid need 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. Here are some instructions for doing this process.

Making Hillshade from NED data

The process is:

1. Download the Grid Projector 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.
2. Start ArcView
3. Select extensions: File > Extensions> Select Grid Projector, Spatial Anmalyst
4. Set Working Directory: File > Set Working Directory use a directory which has enough room
5. Add grid to the View: View > Add Theme or "+" button :be sure to specify Grid Data Source in the Data Source Types drop down list
6. Set View properties: Map units should be decimal degrees
7. Select Grid Projector button ( the new one- furthrest right on the button bar) Output units - meters Category: UTM - 1983 from drop down list Type: select zone from drop down list Add Theme to
8. In New View make the projected grid active. Save the grid with: Theme > Save Data set and specify the directory and name to save the grid into.
9. Compute the Hillshade: Surface > Azimuth: 315 Altitude: 45 (the standard northwest view) The values will range from 0 to 254.

For large areas a connection to the GeograpyNetwork will ArcMap allow the viewing of data with a hill shade which has already been calculated.

Start ArcMap8.2

1. File > Add Data from Geographynetwork > Browse > Select Publisher to View > U.S. Geological Survey from drop down list > use arrow to see map services > scroll to "National Elevation Dataset - Shaded Relief" Add to ArcMap
2. Additional layers in decimal degree coordinates can be laid over this view

Q: What is the general process for NED and how does this fit into The National Map?

Example header file:

Steps to see what areas of 1/3 arc second NED are available:

1. Go to the United States viewer on SDDS
2. At the map interface, on the lower right, under Layer, scroll to 1/3 NED Index
3. Click on the layer, then scroll to the bottom and click Refresh Map
4. This will display the areas where 1/3 arc second NED is available