Obtaining and processing DEMs from the USGS ftp site:

Background information may be found at the URL address:
    
    http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/ndcdb.html

1) FTP the file.  Files are organized alphabetically by the first letter in the
   name of the quad sheet for the DEM (i.e. not alphabetically by state)
   
FTP Instructions 

To access the account:

    FTP to edcftp.cr.usgs.gov 
    Enter "anonymous" at the Name prompt. 
    Enter your complete e-mail address at the Password prompt. 
    Change (cd) to the "pub/data/DEM/250" subdirectory. 
    Set the file transfer mode to binary by typing the word "binary". 
    Use the "get" and "mget" commands to download the 00README or data files. 
    The 00README file located under the "/pub/data/DEM/250" subdirectory 
    contains an explanation of the directory structure and instructions for
    uncompressing data files. 

The files are also available sorted by state, or can be selected from an index 
map.

Set the transfer mode to binary

    ftp> binary

cd to the letter of the quad sheet that you want: (e.g. subdirectory "R" for
Russellville)

    ftp> cd /pub/data/DEM/250/R

get the file

    ftp> get russellville-w.gz

Quad files are typically divided into east and west halves.  The "w" above
stands for west.

2) The file you get will be *.gz so you need to gunzip it.

  unix-prompt> gunzip russellville-w.gz
  
  The output will be simply a file called "russellville-w".
  
3) You need to add record delimiters.

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

   unix-prompt> dd if=russellville-w of=russwest.dat ibs=4096 cbs=1024 conv=unblock

where the text immediately following the "if=" and "of=" are the input (non-
delimited) file and the output (delimited) file.  The remainder of the 
information is typed verbatim as shown above.

4) This resulting outputfile is now suitable for input to the program 
   
       readdemraw.e

which reads an input datafile called readdemraw.in.  This program produces the
"demfile" compatible with the other dem programs at the website:

    http://www.ence.umd.edu/~moglen/geo/