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://geospatialpython.googlecode.com/files/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 = "/Users/joellawhead/qgis_data/census/\
   GIS_CensusTract_poly.shp"
   tractLyr = QgsVectorLayer(src, "Census Tracts", "ogr")
   

3. Then, we'll create our memory layer...