A dot density map shows concentrations of subjects within a given area. If an area is divided up into poylgons containing statistical information, you can model that information using randomly distributed dots within that area using a fixed ratio across the data set. 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. For this example, we'll use a US Census Bureau Tract shapefile along the US Gulf Coast which contains population data. We'll also use the point in polygon algorithm to ensure the randomly distributed points are with 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 doesn't have the ability to fill in polygons beyond simple rectangles. You can implement a fill algorithm but it is very slow in pure...