heat map of density ratio of 2 layers of points

Question

I have 2 vector layers of points:

• one with a point for each house in the city (thanks to osm.org), roughly equivalent to population
• one with a point for each case of cholera in the city

I'm trying to define in which locations I should site new water supply points based on a local attack rate (ie: number of cholera cases / local population). I've built separate heat maps for each layer (using QGIS heat map plugin), but it's hard to visually figure out hot spots.

So I'm looking for a way to calculate this local attack rate, ideally something like (number of cases within X meters radius / number of houses within same radius) and spit that out as a raster layer. Basically just like a heat map, but using a ratio rather than a simple count.

I'm guessing there's some magic keyword I don't know that will open a swath of solutions. Can anyone point me in the right direction? I'm using QGIS.

Answer 1

Here's what I found:

• make a heatmap of each layer using the Raster->Heatmap plugin.
• use raster->raster calculator to compute the ratio

this seems to give me what I want, the only caveat being that I'm not 100% sure how the calculation is done, considering that my two heatmaps pixels don't perfectly overlap. I read something hinting at "nearest neighbour" algorithm being used in such situation, but I'm not sure how that would affect my result.