Since my original request for assistance regarding DEM tapework
on 3/21 to grassu-list, IDS staff have examined and made a small
modification to the m.dem.extract tool.
This modification permits reading a USGS DEM file format that was
extracted from tape. The solution involves modifying code.
Once the fix is in place, a two step process permits bring
USGS 7.5 min. DEM data in GRASS4.0
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For the programmers:
Modifications to the code:
You need to edit the file main.c in directory
grass4.0/src/misc/m.dem.extract.
In the CERL delivered form, line ~37 of main.c reads:
blocksize = sscanf(opt3->answer, "%d", &blocksize) ;
when we tried in dbx to look at the value of blocksize it always was 1.
The following fix was implemented in main.c.
Add the local variable:
char *blocktemp;
Modify line ~37 with the following two lines of code:
blocktemp = opt3->answer;
sscanf(blocktemp, "%d", &blocksize) ;
Compile the code appropriately.
This modification removes the restriction describe in the man page, that
rmt0 is the only supported driver. I would welcome any experiences
so that CERL might consider this small improvement in pending fixes.
Note this work was done on sun sparc 2 environments, but should be
applicable elsewhere as well. 
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For the non-programmers:
The following sequence of commands were used. You will need to adjust
this for the blocksize provide on your tape. The example illustrated
is documented in detail in grassu-list 3/21/92.from tape
1. Retreive the file from tape using dd and tape block size.
dd if=/dev/<appropriate_tape_driver>
of=file1
ibs=<appropriate number, in our case 8192)
This reads the next file from tape. For reading multiple files from
a single tape you need to use an `n` leading your device driver, e.g.
/dev/nrst17. This avoids rewinding the tape after each read.
2. Read the files into a new structure that m.dem.extract will be able to
access. This new structure is obtained by reading the original
files with dd and converting them to a new format the command is the
following:
dd if=(file name, e.g. file1)
of=<output file name, e.g. file1.out>
ibs=<blocksize of step 1, e.g. 8192>
obs=<blocksize larger than any single record in your dem, e.g.4096>
conv=sync,block
This prepares a file that the modified m.dem.extract can read.
3. You are ready to use the modified m.dem.extract. Set a region large
enough to encompass your data from tape, and execute m.dem.extract.
(see man pages on m.dem.extract)
m.dem.extract
input=<name, e.g. file1.out>
output=<output, dem1 (or actual quad name)>
blocksize=<value, e.g. 4096>
We were successful with four adjacent quads of 7.5 min USGS DEM data
retreived using a SCSI device driver (as described 3/21 grassu).
If you have any questions, pleasecontact us at Integrated Decision
Support group (IDS), Center for Water Resources Engineering and Science.
PS: Thanks Michael, how do we best post this series on inquiries to
grassp-list retroactively
Dr. Luis Garcia and
Steffen Meyer
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Steffen Meyer Integrated Decsion Support Group -- CWRES
phone: (303) 491-7620 Engineering, Colorado State University
fax: (303) 491-2293 Fort Collins, Colorado 80523 USA
email: sm646601.longs.LANCE.ColoState.EDU (129.82.109.16)
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