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I'm working on a project involving a ratio of frequent travellers / population divided in Forward Sortation Areas from Canada Post (FSAs are the first three characters of a postal code: A1A).

I created an 8km buffer around stations and then Intersected the FSA data with the 8km buffer, only to realize that the attributes were completely imported.

It seems to me that there is an option in ArcMap that scales the attributes proportionaly to their size when clipped, is there such a thing in QGIS or do I have to do this manually?

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I don't think there is a tool/option in QGIS which automatically scales attributes during the clipping/intersecting process. There is something called a virtual field which, when an expression is entered such as returning an area, gets automatically recalculated when the associated polygon is edited.

What I would probably do in your case, assuming there is an attribute field with the original population (let's call it old_pop), is to create a new field to calculate the new population (e.g. new_pop) and use an expression like:

($area * "old_pop") / (pi() * 8000)

where $area = new intersected area.

This is all from cross-multiplying your 3 attributes and trying to determine the 4th: ("old_pop" / 8km) = ("new_pop" / $area).

Hope this helps!

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  • Thanks a lot Joseph for the info, I'll do this manually then!
    – Dams
    Oct 6, 2015 at 15:40

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