I'm trying to solve quite a tough analytical problem and hoping I can find a way to do it in QGIS. Basically, I have developed a map of Australia with an overlay of agricultural production by zone (polygons linked to a dataset of tonnes of grain produced per zone). In addition, I have mapped all of Australia's railway lines in a vector layer. What I need to do is calculate what fraction of the grain production is within X km of a railway line.

The mathematical formulation means I effectively need to draw a polygon of radius X km around all my railway lines and then calculate the area of overlap with the agricultural polygons multiplied by the production intensity of those overlap zones.

Is it at all possible to do something like this with QGIS?

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Yes this is possible. This question and this question should give you some pointers.

The basic method is to calculate the area of your zone with field calculator > perform the buffer > use intersect tool to get the overlap > calculate area of overlap > join the 2 layers > calculate percentage of overlap

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