National Elevation Dataset (NED) 1/3 Arc Second Readme
Table of Contents
INTRODUCTION
Part 1: DATA INFORMATION
Part 2: DATA SPECIFICATIONS
PART 3: CONTENTS OF FOLDERS
PART 4: INDEX INFORMATION
PART 5: RESOURCE INFORMATION
INTRODUCTION:
The National Elevation Dataset (NED) is the primary elevation data
product produced and distributed by the USGS. The NED provides the
best available public domain raster elevation data of the
conterminous United States, Alaska, Hawaii, and territorial islands
in a seamless format. The NED is derived from diverse source data,
processed to a common coordinate system and unit of vertical
measure. All 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 provided
in units of meters, and are referenced to the North American
Vertical Datum of 1988 (NAVD 88) over the conterminous United
States. The vertical reference will vary in other areas. NED data
are available nationally 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). At present, the bulk
of Alaska is only available at a 2 arc-second (approx. 60 meters)
resolution, owing to a lack of higher resolution source data,
though some areas are available at resolutions of 1 and 1/3
arc-second with plans for significant upgrades of the state over
the next five years. The NED is updated on a nominal two month
cycle to integrate newly available, improved elevation source data.
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. The data are utilized by
the scientific and resource management communities for global change
research, hydrologic modeling, resource monitoring, mapping and
visualization applications.
Part 1: DATA INFORMATION
Development of the NED required the merging of over 50,000 different
DEM data files. A processing system was designed to assemble a
seamless dataset from multiple data sources, resolutions, and
production methods. Procedures were developed to maintain the
database with periodic updates and to insure the integration of
higher resolution elevation data as they become available. A raster
data model referenced to a geographic grid was used for NED. The
data model is logically seamless but uses an internal tile structure
initially selected as a 1- by 1-degree area. The NED dataset
currently achieves complete national coverage by integrating the
"best" available data. Even with the "best" available, there could
be a wide range of source dates and some artifacts in the source
data, such as Level 1 30 DEM's. The system filters production
artifacts, and performs any necessary datum conversions and
coordinate transformations. The NED data is only as good as the
orginal source data. Individual files are appended together into
the larger tile structure specified for the database. Edge
matching, a 6 pixel overlap to ensure no gaps or issues when users
perform functions like reprojection to the data, and metadata
generation are applied lastly in assembling each NED tile.
NED Homepage is http://ned.usgs.gov
Part 2: DATA SPECIFICATIONS
Cell size: one-third arc-second
Data type: Floating Point
Number of rows: 10812
Number of columns: 10812
Projection: Geographic
Datum: NAD83
Units: Decimal Degrees
Zunits: Meters
Spheroid: GRS1980
PART 3: CONTENTS OF FOLDERS
The NED data is stored in 1 x 1 degree tiles in ArcGRID or
GRIDFLOAT format. Each tile covers a 1 x 1 degree area of the
earth's surface. The naming convention of the folders utilize the
latitude and longitude coordinates. The coordinate represents the
upper left (northwest) corner of the grid.
Directory Name = The coordinate of the upper left corner
of the grid.
Sub Directory = The format, the upper left corner
coordinate, and resolution
Sub Directory = info
Metadata file
Metadata Shapefile
Example:
n40w110 = North Latitude of 40 degrees and West
Longitude of 110 degrees. This is the upper left
corner coordinate. Area within this tile covers N
40 degrees top boundary, N 39 degrees bottom
boundary, W 110 degrees left boundary, W 109
degrees right boundary.
grdn40w110_13 = data is ARCGRID format within the
bounding coordinates and is 1/3 Arc Second
Resolution
OR
floatn40w110_13 = data is GRIDFLOAT format within
the bounding coordinates and is 1/3 Arc Second
Resolution, includes the .prj and .hdr files.
n40w110_13as_meta (.dbf, .shp, .shp.xml, .shx) =
data source information of the specific 1 x 1
degree area.
n40w110_13_meta.txt = metadata for 1 x 1 degree area.
PART 4: INDEX INFORMATION
Indices provided in shape files include:
The complete data source information of the areas that were
processed into NED.
example: ned_13_meta_200902
The source data information of the most recent updated areas
that were processed into NED.
example: ned_13_update_200902
The 1 x 1 degree tiles updated.
example: ned_13_1x1_200902
Indices are available providing data source information of the areas
that was processed into NED, giving "best available" elevation data.
The indices can be found at:
http://ned.usgs.gov/downloads.asp
The attributes include data source quadname, production method,
original creation date, resolution, etc. Descriptions of the
attribute information is in the NED Data Dictionary. This can be
downloaded from:
http://ned.usgs.gov/downloads.asp
PART 5: RESOURCE INFORMATION
NED Homepage is:
http://ned.usgs.gov
Metadata indices are available at:
http://ned.usgs.gov/downloads.asp
NED Data Dictionary available at:
http://ned.usgs.gov/downloads.asp
NED Release Notes available at:
http://ned.usgs.gov/downloads.asp
To acquire custom downloads go to:
http://seamless.usgs.gov
To acquire entire datasets via Bulk Data Distribution:
email bulkdatainfo@usgs.gov
Disclaimer: Any use of trade, product, or firm names is for
descriptive purposes only and does not imply endorsement by
the U. S. Government.
Publication Date: March 2009