We’ve been working with crime stats for YouthExplorer.org.za recently and needed population estimates for Police Districts across the country. That allows us to calculate a crime rate in a district which makes it easier to compare different police districts.
- You can download the estimates from our Data Portal at https://data.code4sa.org/Government/Police-District-population-estimates/vxz2-4rs2
We used the 2011 Census Small Area data to build the estimates and we’ll explain how we did it below.
How we calculated the estimates
We estimated Police District populations using the populations of the 84 000 or so Small Areas across the country. Small Areas are the smallest shapes for which Census 2011 data is publicly available and they’re small enough that it’s reasonable to assume that the population is evenly spread within them.
We found the Small Areas that overlap with each District using the shapefiles for the Police Districts and the Small Areas. We then estimated the population of a District by adding up the populations of the Small Areas that overlap it, taking into account that some Small Areas only partially overlap a district. For instance, if only half a Small Area overlaps with a Police District, then we only count half of that Small Area’s population towards the population of the Police District.
You can read through the code on GitHub.
Mapping crimes to other shapes
Now that we have Police District population estimates, we can calculate a crime rate for each District. We can then map those back to Small Areas using the same technique as above, giving us an estimate of crimes per Small Area that takes into account the population of each Small Area.
We can use the technique again to map Small Area crime estimates to other areas such as wards, municipalities and provinces. This is really useful because it lets us compare crimes alongside all the Census data that we already have at ward and municipality level.
In fact, that’s exactly what we’ve done for the youth-related crime indicators on YouthExplorer.org.za.
It’s only an estimate
These numbers are only an estimate and they are based on Census data from 2011.
There is a small amount of error that creeps in during these calculations, mostly because the two sets of shapefiles don’t align perfectly. About 0.16% of the population is not accounted for in the Police District population estimates (about 85 000 people out of 51 million).