Using alpha values to show data density

Thematic maps often use a color ramp based on a single color to show data density. Darker colors show higher concentration of objects, while lighter colors show lower concentrations. You can use a transparency ramp instead of a color ramp to show density as well. This technique is useful if you want to overlay the density layer on imagery or other vector layers. In this recipe, you use some bear-sighting data to show concentrations of bears over an area. We'll use alpha values to show the density. We'll use an unusual hexagon grid to divide the area and a rule-based renderer to build the display.

Getting ready

You will need to install the MMQGIS plugin used to build the hexagon grid using the QGIS plugin manager.

You also need to download the bear data from the following URL, unzip the shapefile, and put it in the ms directory of your qgis_data directory:

https://github.com/GeospatialPython/Learn/raw/master/bear-data.zip

How to do it...

We will...