[GRASSLIST:3908] Re: How to generate merge key in Grass to enable merging data in R?

[Craig Aumann]

On Wed, 2004-07-14 at 01:55, Radim Blazek wrote:
> On Tuesday 13 July 2004 23:27, Craig Aumann wrote:
> > What I then do is read the text file "seismic.txt" into R and find the
> > center of the line.  What I want to do now is link these centers to the
> > attribute data which is sitting in the attribute database.  Reading the
> > attribute database into R is easy:
> 
> You can generate centers with v.segment (calculate length/2 first).
> 
> Radim 

I don't think I completely I understand your approach here.   Why do you
want length/2 and secondly, how would one calculate/get at this?  I
suspect under your approach I'd also need either the begining or ending
vertex for the vector...

Given that I didn't see how to do the above, I started playing around
with v.extract.  Given the centroid, I should be able to add this to the
attribute table with v.to.db after some more futzing.  

---------------------------------

SHPFILE="seismic"
DBFFILE="seismic"
ATT=_att
ATTNAME="$SHPFILE$ATT"

v.in.ogr -o dsn="$SHPDIR" output="$SHPFILE" layer="$SHPFILE"

db.copy from_driver=dbf from_database=$DBFDIR \
    from_table=$DBFFILE \
    to_driver=pg to_database="dbname=alpac_netdown,user=caumann" \
    to_table=$ATTNAME

v.db.connect -o  map="$SHPFILE" table="$ATTNAME" key=OID \
    driver=pg database="dbname=alpac_netdown,user=caumann"

v.extract input=seismic output=seismiccenter type=centroid field=1
where="(width >= '0')"

-----------------------

I (incorrectly??) thought that v.extract would extract the centroid of
the line.  The output of v.extract above is distressing:

----------
GRASS 5.7.-cvs:~/grassalpac > v.extract input=seismic
output=seismiccenter type=centroid field=1 where="(width >= '0')"
WARNING: Vector 'seismiccenter' already exists and will be overwritten.
WARNING: coor files of vector 'seismiccenter at caumann' is larger than it
         should be (14 bytes excess).
WARNING: Table 'seismiccenter' linked to vector did not exist.
Load cats from the database (table = seismic_att, db =
dbname=alpac_netdown,user=caumann).
147488 cats loaded from the database
ERROR: Column 'OID' not found
---------------

Why is OID not found??  Its my link key (see above).  Have I screwed
something up here?  

When I modify v.extract to (added "new=-1") :

-------------------
v.extract input=seismic output=seismiccenter type=centroid field=1
new=-1 where="(width >= 0)"
WARNING: Vector 'seismiccenter' already exists and will be overwritten.
Load cats from the database (table = seismic_att, db =
dbname=alpac_netdown,user=caumann).
147488 cats loaded from the database
0 primitives registered
0 areas built
0 isles built
Number of nodes     :   0
Number of primitives:   0
Number of points    :   0
Number of lines     :   0
Number of boundaries:   0
Number of centroids :   0
Number of areas     :   0
Number of isles     :   0
 
Extracted vector map <seismiccenter> has been created.

---------------

I end up with an empty vector map!