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I think there must be a straight forward answer to this but I haven't managed yet:

I have two shapefiles for the same watershed they are both divided into different areas with different erosion factors. I am trying to get a new shapefile with, say, the sum of both factors (a column in each file). The problem is that a polygon from one file isn't necessarily covering the same area as a polygon from the other file.

What would be the best approach to this task?

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Make an intersection or union on the two layers (Vector/Geoprocessing from the menu). After it you will get a new layer with the attributes of the two input layers and the polygons of one layer will be divided by the borders of the other layer. Using the field calculator you can sum up the two columns in the result layer.

  • Will intersection leave parts that belong only to one layer out of the new layer? I guess union is the right option here. I don't know how I missed them. Thank you. – asher Feb 19 '16 at 6:12
  • Union and intersection are different in the extent of the result layer. In case of intersection only the common part (where they overlap) of the two input layers will be considered. In case of union all elements from both input layers will be considered. – Zoltan Feb 19 '16 at 6:51

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