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 U.S. Geological Survey has developed the National Elevation Dataset (NED). The NED is a seamless mosaic of best-available elevation data drawn from a variety of sources. While much of the NED is derived from USGS Digital Elevation Models (DEM’s) in the 7.5-minute series, increasingly large areas are being obtained from active remote sensing technologies, such as LIDAR and IFSAR, and also by digital photogrammetric processes. Efficient processing methods were developed to filter production artifacts in the source data, convert to the NAD83 datum, edge-match, and fill slivers of missing data at quadrangle seams. NED is available in spatial resolutions of 1 arc-second (roughly 30 meters), 1/3 arc-second (roughly 10 meters), and 1/9 arc-second (roughly 3 meters). The dataset is updated with "best available" elevation data on a two month cycle. These digital elevation datasets are essential in understanding the Earth’s landscape: elevation, slope, and aspect (direction a slope faces.) NED is critical to identifying and modeling geologic features such as water drainage channels and basins, watersheds, peaks and pits, and land movements such as avalanches. NED is used to create relief maps, 3-D visualizations, to classify land cover and to geometrically correct data from satellite or aircraft sensors (orthorectification). The fire community, natural resource managers, urban planners, conservationist, emergency responders, communication companies to name a few all rely on these elevation datasets. This data also supports The National Map. 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 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 Arc Second Resolution OR floatn40w110_13 = data is GRIDFLOAT format within the bounding coordinates and is 1 Arc Second Resolution, includes the .prj and .hdr files. n40w110_13_meta.txt = metadata for 1 x 1 degree area. PART 4: INDEX INFORMATION Indices provided include: The complete data source information of the areas that were processed into NED. example: ned_13_meta0812_shape The source data information of the updated areas that were processed into NED example: ned_13_update_200812_shape The 1 x 1 degree tiles updated. example: ned_13_1x1_0812_shape 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