### What's all this?

It's a website that compares the size and population density of different metro areas.

### Why?

For fun.

Data for these maps was produced by the European Commission, Joint Research Centre (JRC) in association with Columbia University, Center for International Earth Science Information Network, and is available here. This map uses the 2015, 250-meter variant.

The map and website design are mine. I'm Brandon Martin-Anderson.

The same process is used for each city to convert the world-wide population density geotiff into city maps. Given a coordinate of a metro area center:

1. Reproject the map to a UTM projection centered on the city.
2. Blur the raster, with a gaussian kernel with sigma of 1500 meters.
3. Threshold the image at the "non-rural" density; around half an acre per person.
4. Execute a binary erosion with a radius of about 1500 meters.
5. Select the component connected to the given metro center. This is the boundary of the metro area.
6. Blur the original raster with a smaller sigma (to capture more detail), and then apply different thresholds for different colors.

### How did you choose the different density thresholds?

I'm glad you asked. The lowest threshold - 480/km²; about half an acre per person - corresponds to the highest density of subsistence agriculture. It's difficult to feed someone on less than half an acre of land, which means that people living at a higher density than that are not growing all their own food on the land where they live. At this density, there's usually a person within eyeshot.

The highest population density threshold - 24000/km² - corresponds to the density at which the nearest person, on average, is about 7 meters away, or close enough to easily and quickly recognize their face.

The middle density - 4800/km² - is in the middle.

### The map of [city X] is actually map of several other cities.

These maps don't show the administrative or cultural districts of particular cities, but of all connected non-rural regions around those cities. In the most extreme cases, such as the area around Cairo or Beijing, the high-density farmland around the cities means that you can travel from the center of the city for several hundred kilometers without crossing significant gaps of open countryside.