Dot density calculations

A dot density map shows concentrations of subjects within a given area. If an area is divided into polygons containing statistical information, you can model that information using randomly distributed dots within that area using a fixed ratio across the dataset. This type of map is commonly used for population density maps. The cat map in Chapter 1, Learning Geospatial Analysis with Python, is a dot density map. Let's create a dot density map from scratch using pure Python. Pure Python allows you to work with much lighter weight libraries that are generally easier to install and are more portable. For this example, we'll use a U.S. Census Bureau Tract shapefile along the U.S. Gulf Coast, which contains population data. We'll also use the point in polygon algorithm to ensure that the randomly distributed points are within the proper census tract. Finally, we'll use the PNGCanvas module to write out our image.

The PNGCanvas module is excellent and fast. However, it...