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