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this is strange: I intersected 2 shape-files and the resulting shape does not contain the Shape_Area and Shape_Length of the intersected polygons. Instead the field Shape_Area contains the polygon-area of one of the 2 original Layers.

Recalc of the geometries with vector->geometry->add/export column seems to add the areas. That is quite a drag and very slow for my 350.000 polygons. So I am hoping for a systematic approach that adds the areas while generating the intersect.

Is intersect done by QGIS (qgis.bin) or by a gdal-tool? Either way: is there a "calculate areas"-option hidden somewhere?

THX, stn

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The area values in attribute table are just random attributes in the eyes of QGIS. It is therefore perfectly normal that these attribute values are not manipulated when running Intersect.

Intersect is provided by ftools plugin (a Python plugin). You can expand the tool's code to calculate the area of the resulting intersecting areas.

A non-programming solution would be to load the Shapefiles into PostGIS or Spatialite and calculate the intersections and areas there.

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