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 standard 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 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=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)