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Background: I am plotting car crashes along a road and creating a heatmap of crash frequency. I'm using multiple layers that were pulled from the same source data. The layers just sort things by whether they were northbound or southbound on this particular stretch of road.

One layer:

The other:

Issue: When I create a heatmap with the heatmap plugin, different layers return different sized circles dispite the radius setting in the heatmap setup being identical in each of these layers. Oddly, the cell size changes for these different layers as well even though they come from the same source.

  • Why wouldn't the accident rates/occurrences be different between the layers? Unless you've got the same distribution of accidents for both directions, I would expect the heat maps to differ. – mkennedy Jan 9 '17 at 20:51
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    @mkennedy, Thank you for your reply. The rates and occurrences are different, but that is not the issue. I meant that when the circles are drawn for each occurrence, some circles are smaller than others. That is, for example, when you plot the northbound and southbound layers at the same time, you can see that the southbound occurrences appear smaller on the map. This happens despite the radius being assigned the same value (as you can see in the images above - 1300) – Alexis Biddle Jan 9 '17 at 21:23
  • Is there a difference in the total number of incidences in the two data sets? – ragnvald Mar 18 '17 at 23:08
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Have you checked to make sure the CRS is identical for both layers?

The cellsize will differ because both layers have different extents - so a 2000 pixel wide image will have a different resolution for the two heatmaps which cover an extent with different width.

But the radius should indeed be constant so long as the CRS matches.

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