[postgis-users] clip polygons

[Marta Debolini]

Hi,
I am a new postgis user, so sorry if my request seems trivial for someone.

I have to clip a polygon with another polygon file. I was used to do this operation with the simple comand "clip" in GRASS or QGIS.
How can I do the same operation in postgis? I tried with sy_intersect, but it takes also the polygons outside my area, which simply intersect the border.
The request I wrote is:


CREATE TABLE w_md.x
AS (SELECT area, a.the_geom
    FROM oc a, co b
    WHERE st_intersects(a.the_geom, b.the_geom));
   
    ALTER TABLE w_md.x_1999 ADD column ggid1 serial;
     ALTER TABLE w_mdi.x_1999 ADD PRIMARY KEY (ggID1);

Thank you for your help,

Marta

[Nicolas Ribot]

Hi,

You also have to perform the actual intersection if you want to clip your data, not only the intersect test:

SELECT area, st_intersection(a.the_geom, b.geom) as geom

    FROM oc a, co b
    WHERE st_intersects(a.the_geom, b.the_geom)

Nicolas

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[Francisco Salas]

Martha ,

I think that you must use in the select sentence st_intersects(a.the_geom, b.the_geom) as the_geom , instead of a.the_geom

 

Francisco Salas

[Tim-Hinnerk Heuer]

I think Nicolas is more correct. St_Intersection sounds like a function that gets a geometry while St_Intersects returns a Boolean.

[Martin Davis]

Correct.  See http://postgis.net/docs/manual-2.0/ST_Intersection.html

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[Francois Hugues]

Hello,

To complete these answers about the usage st_intersection in combination with st_intersects, if you want to avoid to clip features "which simply intersect the border" you should try to add st_touches(a.geom, b.geom) = 'f' in the where clause.

Hugues



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Hugues FRANÇOIS

IR - UR DTM



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________________________________

[Marta Debolini]

Thank you all!
I followed your instructions and fixed the problem.

Marta