Creating a dot density map

A dot density map uses point density to illustrate a field value within a polygon. We'll use this technique to illustrate population density in some US census bureau tracts.

Getting ready

You will need to download the census tract layer and extract it to a directory named census in your qgis_data directory from https://github.com/GeospatialPython/Learn/raw/master/GIS_CensusTract.zip.

How to do it...

We will load the census layer, create a memory layer, loop through the features in the census layer, calculate a random point within the feature for every 100 people, and finally add the point to the memory layer. To do this, we need to perform the following steps:

1. In the QGIS Python console, we'll import the random module:

           import random 
   

2. Next, we'll load the census layer:

           src = "/qgis_data/qgis_data/census/GIS_CensusTract_poly.shp" 
           tractLyr = QgsVectorLayer(src, "Census Tracts", "ogr") 
   

3. Then, we'll...