What defines a Class in the Heat map plugin?

As in:

Black holes like that are usually caused by raster values exceeding the range of the color ramp. Try adding another class with a higher associated value. That should fix it.

This quote comes from Why is there a black hole in the Heatmap?

I'm getting all sorts of clunky results and I'm not sure whether I need to find the right settings, alter my data, or both. I'm working on identifying all important parameters: obviously Radius, and cell size, their relation, extents (I'm foggy on), projection (I'm still not positive how it affects the heat map as long as it sit on top of the data. And it does.), projection units,...

I'd like to know better specifically what affects the curve of the histogram.

Also is the color ramp synonymous with the histogram curve?

In addition to lots of other issues, I too have black holes.

I'm starting to find some light with the raster histogram.

  • 1
    Could you please add some context to this question? Perhaps by referencing your original question? Or making this an edit to your original question.
    – Fezter
    Feb 19, 2013 at 2:54

1 Answer 1


The raster histogram shows the distribution of raster cell values in the layer.

The colormap is a user-defined mapping of cell value to display color. Colormaps consist of values and associated colors (and labels) which I called "classes" in the answer you quoted.

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Color ramp is not synonymous with histogram curve. A color ramp in QGIS is simply a pre-defined color sequence which can be reused for different purposes.

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