4

I want to calculate the values by hand to know which parameters effect the result in the image, but i couldn't able to do that because i didn't understand it. I tried with 500 mt and 1000 mt radius and here are the results and i want to know the correlation between radius, cell size and the result.

This is 500 mt radius. enter image description here

and this is the 1000 mt radius results. enter image description here

4

Good thing about open-source is you can see the source code of the heatmap plugin and see how it is calculating the value for each pixel

https://github.com/qgis/QGIS/blob/master/src/plugins/heatmap/heatmap.cpp

From a quick study of the code this is what the plugin is doing

  • For each point in the vector,
    • Define the area of the output it will affect
    • For each pixel in the area
      • calculate the distance to the original point
      • calculate the pixel value of based on distance (formula depends on the algorithm chosen)
    • After computing the pixel values for all pixels in the region, write that data for that portion of the raster

The formulae for different algorithms are given in the code. You can plug the values in and manually perform the calculation.

3

Here is an older post about "How does QGIS calculate heatmap values?", but may it helps: How does QGIS calculate heatmap values?

  • 1
    i already checked it but it's not clear enough. – Joy Jul 9 '15 at 8:34
2

The QGIS 2.2 User Guide Documentation at page 274 contains a section detailing how the different parameters operate such as the cell size, Kernel shape, Decay ratio etc. The following quote from the manual is a basic introduction to using the plugin:

"The Heatmap plugin uses Kernel Density Estimation to create a density (heatmap) raster of an input point vector layer. The density is calculated based on the number of points in a location, with larger numbers of clustered points resulting in larger values. Heatmaps allow easy identification of “hotspots” and clustering of points."

The latest QGIS User Guide is under testing but the same details can be found on page 297, if you want a more up-to-date manual. Note that this is subject to change, particularly the page number.

Hope this helps!

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