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On 10/30/2012 03:31 AM, Phil Donovan wrote:
<blockquote
cite="mid:CAHOEYY5N7YUh8a+FH_cH4QNdXqhDFsQMxB+GO0-=skSBGghtsg@mail.gmail.com"
type="cite">How does one dissolve and sum a field in GRASS? I
guess one way would be to create a separate table of summed values
and then join them back to the new dissolved layer? How would you
create this table of summed values?
<div>
<br>
</div>
</blockquote>
<br>
This problem can be split (no pun intended) into two parts. First,
to group the values in one column and sum the values in another
column could be done in a database. If your vector is in sqlite,
then:<br>
# lets assume: a vector named 'v', <br>
# the dissolve column is called 'id' and the column to be summed is
called 'val'<br>
sqlite> CREATE TABLE v_sum AS SELECT id, SUM(val) AS sum_by_id
FROM v GROUP BY id;<br>
<br>
Now you have an additional sqlite table with the aggregated values
of the sum column for each dissolve value. At this point you can
actually just connect your vector to this table thru a second layer
and you'll see the summed values for each id:<br>
<br>
v.db.connect map=v driver=sqlite \<br>
database=$GISDBASE/$LOCATION_NAME/$MAPSET/sqlite.db \<br>
table=v_sum layer=2<br>
v.db.select v layer=2<br>
# should show the summed values for each dissolve value<br>
<br>
But we have not yet done any dissolve. The v.dissolve module will
dissolve only polygons, and only those that share a common boundary.
If that's the case you can run v.dissolve, then attach the result to
the table created above:<br>
v.dissolve in=v out=v_diss column=id<br>
# check resulting cats:<br>
v.category v_diss option=report<br>
# Connect to the summed table<br>
v.db.connect v_diss driver=sqlite
database=$GISDBASE/$LOCATION_NAME/$MAPSET/sqlite.db table=v_sum
key=id<br>
<br>
I think that should do it.<br>
<br>
-- <br>
Micha<br>
<br>
<blockquote
cite="mid:CAHOEYY5N7YUh8a+FH_cH4QNdXqhDFsQMxB+GO0-=skSBGghtsg@mail.gmail.com"
type="cite">
<div>______________________________________</div>
<div><b>Example table:</b></div>
<div><br>
</div>
<div>Dissolve Column | Sum column
<div>1 | 0.5</div>
<div>1 | 0.5</div>
<div>1 | 0.5</div>
<div>2 | 0.25</div>
<div>2 | 0.25</div>
<div><br>
</div>
<div><b>into:</b></div>
<div><br>
</div>
<div>Diss C | Sum C</div>
<div>1 | 1.5</div>
<div>2 | 0.5</div>
<div><br>
</div>
<div>________________________________________</div>
<div><br>
</div>
<div>Cheers Phil.</div>
</div>
<br>
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</pre>
</blockquote>
<br>
<br>
<pre class="moz-signature" cols="72">--
Micha Silver
GIS Consultant, Arava Development Co.
<a class="moz-txt-link-freetext" href="http://www.surfaces.co.il">http://www.surfaces.co.il</a></pre>
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