UNBLOCK_DLG(1)						      General Commands Manual						    UNBLOCK_DLG(1)

NAME

       unblock_dlg - Insert linefeeds into an optional-format Digital Line Graph (DLG) file

SYNOPSIS

       unblock_dlg [-L]

DESCRIPTION

       The  drawmap  program processes USGS optional-format Digital Line Graph (DLG) 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
       difficult  for a human to read.	Unblock_dlg filters a DLG file and inserts linefeeds for human readability.  It takes input from the stan-
       dard input and writes to the standard output.  The input data must be in uncompressed form, not in gzip-compressed form.  Also, unblock_dlg
       does not work with SDTS DLG files.

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

       Some  DLG  files  already  have	embedded linefeeds.  Unblock_dlg 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_dlg 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_dlg 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 80 characters long.  Drawmap may also become confused if you make any significant changes to the format of the file.

       You can use this capability to remove selected items from DLG files, when those items cause problems.  You  may	even  be  able	to  repair
       faulty  data.  However, you need to be familiar with the content and structure of DLG files in order to do this effectively.  You also need
       to remember that drawmap depends on DLG files having a predictable structure, and it won't properly parse  files  that  violate	this  pre-
       dictability.  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=8000 cbs=80 conv=unblock

       I  prefer  the  unblock_dlg  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_dlg or dd can be undone by typing:

       dd if=inputfilename of=outputfilename ibs=8000 cbs=80 conv=block

       You may come across files that have a linefeed in the 81st byte.  Drawmap doesn't like these, both because it can't tell (within the  space
       of  the	first  80-byte	record) whether the file contains linefeeds or not, and because the records exceed the maximum record length of 80
       bytes.  You may be able to repair such files by blocking them with the above dd command.

SEE ALSO

       unblock_dem(1), drawmap(1)

								   Jul 24, 2001 						    UNBLOCK_DLG(1)