[Qgis-user] Getting landuse information for several circular polygons

[Micha Silver]

On 16/07/2012 20:22, Manuel Spínola wrote:
> Dear list members,
>
> I have a landuse layer and another layer with 20 circular polygons 
> (buffers) and I want to obtain the amount of each landuse category 
> within each circular polygon.  I tried "clip" from Geoprocessing Tools 
> but I dont get the information individualized for each circular 
> polygon.  Is it possible to get the landuse within each circular 
> polygon in one step?
>

Not exactly "one step" but...
If you don't mind to drop the vectors into spatialite, then a query can 
update the buffers layer with area of each landuse category as follows:
Suppose "buffers" has a label column, and there are landuse categories 
lu1, lu2, lu3, .... then:

SELECT b.label AS Buffer,
         ST_Area(ST_Intersection(b.Geometry, l.Geometry)) AS LandUse1
FROM buffers AS b, landuse AS l
WHERE l.category='lu1' AND
         ST_Intersects(b.Geometry, l.Geometry)
GROUP BY b.buff_id;

If you add columns to the buffers table to hold total area for each 
landuse category::
ALTER TABLE buffers ADD COLUMN lu1_area float;
ALTER TABLE buffers ADD COLUMN lu2_area float;
....

then you'd update each column like so:
UPDATE buffers SET lu1_area=(
SELECT ST_Area(ST_Intersection(b.Geometry, l.Geometry)) AS LandUse1
FROM buffers AS b, landuse AS l
WHERE l.category='lu1' AND
         ST_Intersects(b.Geometry, l.Geometry) AND
         b.label=buffers.label
);

HTH,
Micha

> Best,
>
> Manuel
>
> -- 
> *Manuel Spínola, Ph.D.*
> Instituto Internacional en Conservación y Manejo de Vida Silvestre
> Universidad Nacional
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> Teléfono: (506) 2277-3598
> Fax: (506) 2237-7036
> Personal website: Lobito de río 
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