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I need to split several polygons (census blocks) that contains numerical attributes (population) into smaller blocks, but assigning that data proportionally.

For example: if the splitting is made by 40/60%, and the population value is 1000, i want QGIS 3.2 to assign 400/600 proportionally to each polygon.

With the Split features function, the data is duplicated (QGIS replicates ALL the original values). I would love if there was a plugin that allows it to be made using the Split Polygon function "by hand", but I know that it is practically impossible.

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The calculated field does not work.

1st step: create a calculated field ($area)

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2nd step: cut the “one” polygon:

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3rd step: create a calculated field (“population” * $area / “old_area”) as Virtual Field:

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Result: the first result is OK (on the the polygon that was cut first), the “new_pop” value was calculated fine:

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BUT the next split does not work (polygon "two"), the “new_pop” values are: 100 and 40, when should be 60 and 40. The first row is not updated.

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Is this a bug?

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  • Does the virtual field update after saving the edits?
    – csk
    Oct 11, 2018 at 19:42
  • Yes, it does. Your instructions are ok. I'm new to GIS, and until now I thought that the commands of the field calculator operated as Excel functions (updated automatically). Thank you for you help!
    – facu pedemonte
    Oct 16, 2018 at 13:21
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    Sometimes there's a bit of a lag between when you edit a feature and when the virtual field updates. It's best to save edits to be sure the $area function is calculated using the updated geometry.
    – csk
    Oct 17, 2018 at 16:41

1 Answer 1

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You can achieve this with Field Calculator.

  • Use Field Calculator to add area of original polygon as an attribute, called "old_area".
  • Split all the polygons you want to divide.
  • Add a new population attribute, called "pop_new". Use Field Calculator to calculate "pop_new" as old population multiplied by the ratio of the current area to the old area.

    "population" * $area / "old_area"

If you use a virtual field or a default field value for "pop_new" it will be automatically calculated anytime you split another polygon.

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  • Great, easier than I thought! Thanks! The only thing that does not work is the automated calculation. When I continue splitting (after creating a virtual field as indicated), the "population" value of the original polygon is not updated, but the new polygon does.
    – facu pedemonte
    Oct 4, 2018 at 23:38
  • Are you saying that when you split one polygon into two polygons, one of the two smaller polygons is updated, but one of them is not? That seems like a bug, since the $area value is changed. Please report it at issues.qgis.org. In the mean time, you should be able to force it to update by moving one of the vertices a little bit.
    – csk
    Oct 5, 2018 at 15:05
  • I've updated the question, can you take a look, please? To confirm if I should report it as a bug or i'm doing something wrong.
    – facu pedemonte
    Oct 11, 2018 at 14:19

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