This directory contains the 1:250,000-scale Digital Elevation Models (DEM).

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
For a complete explanation of the 1:250,000-scale DEM format see the USGS 
National Mapping Program, Technical Instructions, Data Users Guide 5, "Digital
Elevation Models".  This booklet can be ordered, for a small fee, from the 
Earth Science Information Center at the following address:

Earth Science Information Center
U.S. Geological Survey
507 National Center
Reston, VA  22092
Tel:  783-648-6045 or 800-USA-MAPS
Fax:  783-648-5548
 
Information about the DEM data set can also be found through the World Wide 
Webb (WWW) at the following URL address: 
http://edcwww.cr.usgs.gov/glis/hyper/guide/1_dgr_dem 

The Global Land Information System (GLIS) also provides on-line data set
documentation.

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
basic elevation model is produced by the Defense Mapping Agency (DMA) using
cartographic and photographic sources.

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 exception 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.