how to extract overlap from non-dissolved buffer zones in qgis 1.7.4

Question

I have a shapefile with the inventory of all limetree in a region. I made a buffer around each tree. So, the place where buffers overlap each other is the best place to put beehives.

Is there a way to extract the overlap between non-dissolved buffers in the same layer? Or is there another way?

Thanks for the help. Pieter

Answer 1

In QGis this is not so easy. Since you work with only one table. All the tools I know which perform intersection always need two input tables.

General idea is to intersect all buffers with each other. You can do that easily in PostGIS. Do you have experience?

SELECT st_intersection(t1.geom, t2.geom) 
 FROM mytable AS t1, mytable AS t2 
 WHERE t1.gid <> t2.gid AND t1.geom && t2.geom

Another approach is to convert the buffers into raster and count the polygons per pixel.

Answer 2

You can use QGIS with OpenJUMP to achieve the result you're looking for.

Here's my related question:

Extracting Overlapping Parts of Single Polygon Layer using QGIS?

Hope this helps.