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. Older DEM's produced by methods that are now obsolete have been filtered during the NED assembly process to minimize artifacts that are commonly found in data produced by these methods. Artifact removal greatly improves the quality of the slope, shaded-relief, and synthetic drainage information that can be derived from the elevation data. The figure below illustrates the results of this artifact removal filtering. NED processing also includes steps to adjust values where adjacent DEM's do not match well, and to fill sliver areas of missing data between DEM's. These processing steps ensure that NED has no void areas and artificial discontinuities have been minimized.

Figure. A shaded-relief representation of the Rockypoint, Wyoming 7.5-minute DEM is shown above on the left. The same area is shown on the right after NED artifact filtering has been performed. The super-imposed red lines are synthetic drainage networks derived from each elevation dataset.

• Horizontal Resolution
  10 Meter resolution for Continental US
  Also know as 1/3 Arc Second Resolution
  30 Meter resolution for Continental US, HI, PR and VI
  Also know as 1 Arc Second Resolution
  60 Meter resolution for Alaska
  Also know as 2 Arc Second Resolution
  
  
• Vertical Resolution
  1 DeciMeter or 10 CM or approximately 4 Inches.
   
• Vertical Datum
  NAVD88
   
• Vertical Accuracy
• +/-  7 to 15 Meters. 
  Based on the original DEMs digitized from.
  
  



• Individual Quads available in:
  USGS Standard DEM file format
  
  ESRI Grid Float Format
  
  ASCII Text, X,Y,Z format
      XYZ text file format is a simple text format
      where each elevation point is described by x, y, and z coordinates.
      The x value defines the east/west coordinate, 
             y defines the  north/south coordinate, and 
             z defines the elevation.
      User can select Tab, Comma or Space delimited file.
      The XYZ text format does not contain any header information:
  
             x1,y1,z1
             x2,y2,z2
             x3,y3,z3
             .....
             xn,yn,zn
  
  PLS-CADD ASCII Text, X,Y,Z format
      XYZ text file format specifically for PLS CADD Software
  
  C Tech File Format
      in C-Tech GMF and EFF format which is directly read by C-Tech's software
  
   
• Optional Shaded Elevation Models can be added as an additional
  file to your current Elevation Data selections.
            Click Here for Additional Information.
  
  
• Available Projections
  UTM - User Selectable Zones - NAD27 / NAD83 / WGS72 /  WGS84
  State Plane - User Selectable Zones - NAD27 / NAD83
  Latitude / Longitude - NAD27 / NAD83 / WGS72 /  WGS84
  
  
• Downloading a 30 Meter 1 Degree file.
  File Size for a 1 Degree area is approximately 50MB +
  Select Zip files during Check Out to reduce file to approximately 20MB
  Available as 64 individual quads in available formats.
  
  
• Individual 30 meter NED Quads are available as 1 Free Download
• Individual 10 meter NED Quads are NOT available as 1 Free Download
• 1 Degree NED blocks are NOT available as 1 Free Download

• Available via Download and Custom CD 
  
• Currently only the Continental US quads available online.
  
• Available Projections
  UTM - User Selectable Zones - NAD27 / NAD83 / WGS72 /  WGS84
  State Plane - User Selectable Zones - NAD27 / NAD83
  Latitude / Longitude - NAD27 / NAD83 / WGS72 /  WGS84
  
• Available Formats
  USGS Standard DEM file format
  ESRI Grid Float Format
  ASCII Text, X,Y,Z format
  PLS-CADD ASCII Text, X,Y,Z format
  C Tech File Format

• 1 Degree by 1 Degree Mosaiced Areas
• Available via Internet Download or Custom CD

• Up to 1 Degree by 1 Degree Mosaiced Areas
• Available via Internet Download or Custom CD

• 30 Meter Individual Quads
• Available via CD

• 1 Degree by 1 Degree - 30 Meter - Mosaiced Areas
• Available via CD

• Individual 30 Meter Quads
• Data Sets are available on other media such as:
  CD, DVD, or Window Firewire Disks

• 1 Degree by 1 Degree - 30 Meter - Mosaiced Areas
• Data Sets are available on other media such as:
  CD, DVD, Window Firewire Disks, Window Servers 
  and for the ESRI Professional -- ESRI ArcSDE Server.