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I'm using QGIS 1.8 with a PostGIS database and I have three postgis-layer in qgis a lines layer, a points layer and a polygon layer.

What I'm trying to do is to select a polygon in the polygon layer and mark all points within this polygon.

For me the only way to do this is to use the button "select by polygon" but in this way i cannot use an existing polygon for selection.

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Two possible solutions:

  • You can use the "Vector->Research tools->Select by Location" and tick the "Use selected features only" checkbox.
  • If you want to actually export the points which intersect your selected polygon to a new shapefile then "Vector->Geopressing tools->Intersect" and again tick the "Use only selected features" checkbox. The input layer will be your points layer, and the select layer will be your polygon layer.
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Furthermore, am I correct in adding that for the Vector processing to work, the following two conditions need to be met: [1.] The layers need to have the same CRS; and [2.] The layer needs to be selected in the 'Layers' panel. – Gonja Apr 5 at 14:26

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