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I am trying to generate some Voronoi/Theissen polygons from points across the UK. From my understanding, any point within a Voronoi polygon should be closer to that polygons origin point than any other origin point; however, this isn't whats happening. The most obvious example is below:

enter image description here

In this example the Swansea (the bay which is close to the centre point of my measuring line) is about 33km closer to the Pembrokeshire point than the Devon point, but that region is still covered by the Devon Voronoi polygon.

Am I setting this task up incorrectly? I have since tried to use GRASS's function which yields the same result, SAGAs which I can't get working, and MMQGIS's function, which also yields the same result. Or is this working correctly, and I'm misunderstanding the purpose of these tools/Voronoi diagrams?

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It can be a distortion due the projection. Check the points's CRS and the CRS of the project. I think this is the problem because the Voronoi's line between these two points is not orthogonal in relation to connect line of the points.

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    Thanks, Ithink this is it - it was all in WGS84, I reprojected everything to 27700 and reran it, and it's now working - thanks!
    – TC95
    Commented Jun 24, 2020 at 11:44

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