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Hi, lets say that green polygon is under red and part of blue with yellow stripes is also under red polygon. how can i make one polygon from every polygons that intersects my main ( red ) polygon and also remove polygons ( all green and part of blue with yellow stripes ) under red polygon? i'm using quantumgis and postgis.

i did this (in qgis): 1) dissolve, 2) multipart to singleparts, please tell me that i'm right ;)

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Yes, that's the way. The only thing you have to take care is choosing the correct dissolve field (or avoid using it).

You can also try the Sextante plugin. It includes another three dissolve tools, from SAGA, GRASS and fTools. But it's experimental right now.

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ok, thank You :) – simon t May 5 '12 at 7:42

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