[GRASS-user] Dissolve and sum field

GRASS GRASS user
1

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?

Example table:

Dissolve Column | Sum column
1 | 0.5
1 | 0.5
1 | 0.5
2 | 0.25
2 | 0.25

into:

Diss C | Sum C
1 | 1.5
2 | 0.5

Cheers Phil.

2

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?

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:

lets assume: a vector named ‘v’,

the dissolve column is called ‘id’ and the column to be summed is called ‘val’

CREATE TABLE v_sum AS SELECT id, SUM(val) AS sum_by_id FROM v GROUP BY id;

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:

v.db.connect map=v driver=sqlite
database=$GISDBASE/$LOCATION_NAME/$MAPSET/sqlite.db
table=v_sum layer=2
v.db.select v layer=2

should show the summed values for each dissolve value

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:
v.dissolve in=v out=v_diss column=id

check resulting cats:

v.category v_diss option=report

Connect to the summed table

v.db.connect v_diss driver=sqlite database=$GISDBASE/$LOCATION_NAME/$MAPSET/sqlite.db table=v_sum key=id

I think that should do it.

··· 
-- 
Micha Silver
GIS Consultant, Arava Development Co.
[http://www.surfaces.co.il](http://www.surfaces.co.il)