How to remove polygon under other one?

Question

Let's 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 ;)

Answer 1

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.

Answer 2

You can use ST_Intersects() in PostGIS,

ST_Intersects — Returns TRUE if the Geometries/Geography "spatially intersect in 2D" - (share any portion of space) and FALSE if they don't (they are Disjoint). For geography -- tolerance is 0.00001 meters (so any points that close are considered to intersect)

This will show you all the Polygons that touch the red.

SELECT ST_Union(geom) WHERE ST_Intersects( 2ndpoly, geom );