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 difficult 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)