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I am trying to understand the output of the heatmap tool of QGIS (kernel density). For this reason I created a kernel density map for a single point that has a projected coordinate system (epsg: 21781). I used the following parameters:

  • radius: 200m
  • Cell size X and Y: 5m
  • kernel shape: "Quadratic (biweight)"
  • output value: "Raw values" and "Scaled by kernel size" => I tried both options

On first sight the outputs of the heatmap generated with the output value "Raw values" and "Scaled by kernal size" seem identical as there is no visual difference between them. However, at second sight I realized that the legends differ.

My questions are:

  • Am I right to assume that "Raw values" refers to the total quantity of sand in each cell and that "Scaled by kernal size" refers to the amount of sand per unit area? (See this link)

  • What is the unit of the output values "Raw values" and "Scaled by kernal size" assuming that my points are burglary incidents in a city? Am I right to assume the following:

"Raw values" = number of burglary per cell ?

"Scaled by kernal size" = number of burglary per m2 (my layer unit is meter)

  • Is the term "kernal size" a synonym for the chosen radius (in my case 200m) or does it refer to the cell area (in my case 25m2)?

  • As described here I tried to multiply the density values by the cell area and added up these values over the grid in order to get the total sum of the original data. My cell area is 25m2 and I firstly multiplied this value with all cell values of the heatmap produced with the "Scaled by kernal size" output value. Secondly I summed up all multiplication results: value of cell 1 *25m2 + value of cell 2 * 25m2, + ... + value of cell n * 25m2. I expected to get a total sum of one as I created a kernel density map for a single burglary incident (In case my points represented 50 burglary incidents, I expected the total sum to be 50). However, I got a much higher value (41887). Maybe I misunderstood the meaning of "total sum of the original data"?

  • I expected the sum of all cell values of the heatmap produced with the "Raw values" also to equal one (= total amount of sand = in my case one burglary?). However, this is not the result I got (1675). Why?

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