All flight data is downloaded via the ICAO through their API. Connections were excluded in the counts.
Flights to Cities: You can find the mapping of cities with multiple airports: /analysis_data/airport-city-aggregations.csv For cities to airports, we used this reference file: /analysis_data/city-mappings.csv
Population Data: you can download this directly from the UN https://esa.un.org/unpd/wup/General/FAQs.aspx
The routes depicted in the first map were copy/pasted from https://www.flightstats.com/ on July 14, 2018.
Code to create this map is here: https://beta.observablehq.com/@matthewfdaniels/untitled
The code to create the remaining maps are here: https://beta.observablehq.com/@matthewfdaniels/untitled/2