This directory contains the 1:250,000-scale Digital Elevation Models (DEM)
Users are encouraged to obtain DEM’s via the National Eelevation Dataset (NED) server at:
	http://ned.usgs.gov/

The NED server provides a seamless service that is not inherent is the DEM tiles.  And NED provides other data sources to compliment the USGS data.

DIRECTORY STRUCTURE
Contiguous United States:
Cd to the directory (A-Z) that represents the first character of the
1:250,000-scale map name you wish to download.  Data files under these directories are named after the maps with the east or west portion specified.

Alaska
Cd to the Alaska directory then move to the directory (A-Z) that represents the first character of the 1:250,000-scale map name you wish to download.  Data files under these directories are named after the maps with the 1-degree portion specified.

RECORD DELIMITERS:
The DEM files do not contain record delimiters.  You can add delimiters by using the following UNIX command:

dd if=inputfilename of=outputfilename ibs=4096 cbs=1024 conv=unblock

FILE COMPRESSION
The files have been compressed with the GNU "gzip" utility.  If you do not have access to gzip, the FTP server will uncompress the file as you retrieve it.  To do this, simply leave off the ".gz" extension when retrieving the file (NOTE: This option is not available through MOSAIC).  
For example, to retrieve the file "aberdeen-w.gz" without compression just use "get aberdeen-w".  Note that the uncompressed files are typically five times larger than the compressed versions and so will take five times longer to transmit.  The gzip program is available via anonymous FTP at the following sites:

 UNIX: prep.ai.mit.edu:/pub/gnu
       wuarchive.wustl.edu:/systems/gnu
 MAC:  mac.archive.umich.edu:/mac/util/compression/macgziop0.2.cpt.hqx
 MSDOS: prep.ai.mit.edu:/pub/gnu/gzip-1.2.4.tar

The data files are stored on a robotic mass-storage device so there
may be a short (~10 seconds) delay while the media is retrieved.

DATA FORMAT
A complete explanation of the 1:250,000-scale DEM format is available at:
http://rockyweb.cr.usgs.gov/nmpstds/demstds.html

AdditionaliInformation about the DEM data set can also be found at: 
	http://eros.usgs.gov/#Find_Data/Products_and_Data_Available/DEMs

	http://eros.usgs.gov/#Guides/dem

	http://egsc.usgs.gov/isb/pubs/factsheets/fs04000.html


EXAMPLE data files are available at:
http://edc2.usgs.gov/geodata/examples.php

BACKGROUND INFORMATION
A digital elevation model (DEM) consists of an array of elevations for ground positions that are usually at regularly spaced intervals.

The 1-degree DEM provides coverage in 1- by 1-degree blocks and is available for all of the contiguous United States, Hawaii, and most of Alaska.  

The 1-degree DEM consists of a regular array of elevations referenced horizontally on the geographic coordinate (latitude/longitude) system of the World Geodetic System 1984 Datum.  Elevation data located on the degree lines (all four sides) correspond with the same profiles on adjoining DEM blocks.

Elevations are in meters relative to mean sea level.  Spacing of the elevations along and between each profile is 3 arc-seconds with 1,201 elevations per profile.  The only exceptions are DEMs in Alaska, where the spacing and number of elevations per profile varies depending on the latitudinal location of the DEM.  Latitudes between 50 and 70 degrees north have spacings at 6 arc-seconds with 601 elevations per profile and latitudes greater than 70 degrees north have spacings at 9 arc-seconds with 401 elevations per profile.