UNBLOCK_DEM(1)						      General Commands Manual						    UNBLOCK_DEM(1)

NAME

       unblock_dem - Insert linefeeds into a Digital Elevation Model (DEM) file

SYNOPSIS

       unblock_dem [-L]

DESCRIPTION

       The  drawmap  program  processes  USGS  Digital	Elevation  Model (DEM) files (along with other types of files) to produce customized maps.
       Drawmap is able to read these files in their native format.  However, since the files usually don't contain any linefeeds, they are  diffi-
       cult  for  a  human to read.  Unblock_dem filters a DEM file and inserts linefeeds for human readability.  It takes input from the standard
       input and writes to the standard output.  The input data must be in uncompressed form, not in gzip-compressed form.  Also, unblock_dem does
       not work with SDTS DEM files or GTOPO30 files.

       Unblock_dem normally takes no options, but, if you use the "-L" option, the program will print out some license information and exit.

       Some  DEM  files  already  have	embedded linefeeds.  Unblock_dem makes a feeble attempt to detect these files and abort.  If the detection
       fails, the results are likely to be garbled, so it is wise to check the format of each file first.  The	drawmap  program,  with  the  "-i"
       option, will attempt to tell you whether a file contains linefeeds or not.

       The  intended  use  of  unblock_dem is simply to let you conveniently examine the files to see what is in them.	However, if you are in the
       mood for adventure, or if you need to repair a defective file, you can use unblock_dem to make the file easy to edit, then you can edit the
       file  as  desired.   Drawmap  can still process the files after linefeeds are inserted, but will become confused if any line (including the
       linefeed) is more than 1024 characters long.  Drawmap may also become confused if you make any significant changes to  the  format  of  the
       file.

       You may be able to use this capability to repair faulty data.  However, you need to be familiar with the content and structure of DEM files
       in order to do this effectively.  You also need to remember that drawmap depends on DEM files having a predictable structure, and it  won't
       properly  parse files that violate this predictability.	Do this sort of thing only if you know what you are doing, or if you have a lot of
       free time on your hands.

       It should be noted that you can obtain a similar effect by using the command recommended by the USGS:

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

       I prefer the unblock_dem command because it simply replaces the last byte of each record with a linefeed.  This lets  me  easily  tell  how
       long the original records were.	The effect of unblock_dem or dd can be undone by typing:

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

       You  may  come  across  files  that have a linefeed in the 1025th byte.	Drawmap doesn't like these, both because it can't tell (within the
       space of the first 1024-byte record) whether the file contains linefeeds or not, and because the records exceed the maximum  record  length
       of 1024 bytes.  You may be able to repair such files by blocking them with the above dd command.

SEE ALSO

       unblock_dlg(1), drawmap(1)

								   Jul 24, 2001 						    UNBLOCK_DEM(1)