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There are obviously more airports in the world than this. What are the criteria for inclusion in the visualization?

Inclusion by http://ourairports.com/data/ in the categories "medium" or "large".

> 2,980 large and medium airports with scheduled services from OurAirports.

It's at the very top of the description, just below the globe.

"2,980 large and medium airports with scheduled services from OurAirports."