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I have several data sets that represent a type of activity within a specific geographic boundary. Each heatmap I create shows the relative activity spread nicely, but I have been frustrated in my attempts to calibrate absolute levels between each of the heatmaps. I am wanting to reveal not only the shifting in relative activity spatially, but also in terms of absolute activity overall.

For example, I'd like to create two heatmaps, say day versus night, and show how the activity moves from, say, city center to suburbia. But I also want to see if the activity is half as much or twice as much during the daytime peaks as compared to the nighttime peaks.

The data I have are a series of lat/lon pairs with a value ranging from 0 to 50, one value per dot:

Reference raw lat/lon/value data

Here is the second set of data, same idea, except that the maximum values only reach 30, not 50. I am trying to know how to make sure that the color for maximum in the second show that the activity not only moved, but is half the amount (quantitatively) than the first data set:

Comparison raw lat/lon/value data

I am also having some confusion about the differences bewteen heatmap plugin and the heatmap style: how do they differ? I am on QGIS 2.8 (Gentoo Linux). The style seems to work, but then gets corrupted, and the heatmap plugin seems kaput (it worked in 2.6...)

Here are the settings I used in 2.6 that worked (but do not at all in 2.8):

Heatmap plugin dialog

Here is a dialog from the Heatmap style I tried to use. This works in 2.8 but gets corrupted if I add other layers:

heatmap style dialog

If it's not obvious yet, GIS is not my specialty.

  • Please add details about how you computed your heatmaps. Screenshots of the layer style settings will help understand the issue as well. – underdark Feb 27 '15 at 19:40
  • See the above screenshots and additional explanations and details. please note that in QGIS 2.8 I cannot get heatmapping plugin to work at all, and the Heatmap style works if it is the only layer. QGIS 2.6 worked fine for the plugin (I didn't see the heatmap style until 2.8). I do not intend for this to be a bug report, but rather how to use the heatmap function to have a consistent maximum value that makes sense per-layer and then also comparatively across data sets taken at different times. – Capt. Frito Feb 28 '15 at 1:25
  • Which heatmap 'plugin', the one available under the "Raster|Heatmap|Heatmap..." menu option (which I think is now part of the QGIS code)? Or are you using the old plugin from the repository? – Simbamangu Feb 28 '15 at 7:12
  • Have you tried setting the Maximum value in the heatmap style to 50 for both layers? That should give you the result you are looking for and if you only need the visualization rather than the actual heatmap values for further computation then the style is the easiest way to go. – underdark Feb 28 '15 at 9:17
  • underdark: I have tried putting in the maximum value as you suggest, but it is the definition of "maximum" that is proving elusive. Apparently 'radius' is the inclusion area surrounding the point being evaluated (a circle?). The point's rendered value is computed according to some formula (quartic, linear, etc). Where are these formulae defined? The cell size definition escapes my understanding entirely -- how does this relate to radius? Units? Neither do I have a good grasp on the mutual influence of adjacent "radius areas". Because of this I can't predict what the maximum value should be. – Capt. Frito Feb 28 '15 at 14:01

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