[postgis-users] Area of non-intersecting parts of layers

[Hugues François]

Hello,

 

I'm not sure to understand what you are trying to do. Do you want to find the area of polygons from layer 1 which don't contain any polygon from layer 2 or the difference between l1 and l2  (l1 - intersection(l1,l2)) ?

 

Hugues.

 

 

De : postgis-users-bounces at lists.osgeo.org [mailto:postgis-users-bounces at lists.osgeo.org] De la part de Cedric Duprez
Envoyé : mardi 21 mai 2013 18:55
À : PostGIS Users Discussion
Objet : [postgis-users] Area of non-intersecting parts of layers

 

Hi all,

 

I have 2 layers composed of multipolygons in Postgis 2.0. Each layer contains polygons with a name that is sometime common to several polygons.

When I try to find area of intersections between the 2 layers, no problem with the following query:

SELECT l1.name1, l2.name2, SUM(ST_Area(ST_Intersection(l1.geom, l2.geom))) / 10000 AS surface_ha

FROM layer1 l1

INNER JOIN layer2 l2 ON ST_Intersects(l1.geom, l2.geom)

GROUP BY 1, 2

ORDER BY 1, 2;

 

OK.

But I also try to get the area of my layer1 (grouped by name1) that does not contain elements of my layer2.

 

I tried with LEFT JOIN, with ST_CONTAINS, without success.

 

Does anyone have an idea on how to write this query?

 

Thanks in advance,

 

Cedric Duprez

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