Atlanta's two largest airlines are Delta Air Lines and Southwest Airlines. Southwest, the second-largest carrier, does not operate a hub-and-spoke network, and at Hartsfield-Jackson focuses primarily on travelers from Atlanta or headed to Atlanta and not connecting.
Chicago has an advantage for domestic connectivity based on its central U.S. location, which makes it a particularly attractive place for a hub connecting domestic travelers going from east to west or vice-versa.
In contrast, airports in coastal locations "limit domestic connectivity options," and are more beneficial for international connectivity, according to OAG.
Atlanta serves as a hub for the Southeast and as a gateway to international destinations in Latin America and Europe.
When it comes to international connectivity overall, London Heathrow was tops and Hartsfield-Jackson ranked 8th.
In the domestic ranking, behind O'Hare and Hartsfield-Jackson came Charlotte, Dallas/Fort Worth and Denver.
Megahubs "are critical to global airline efficiency and play an essential role in providing travelers with more options for ticket costs, flight frequencies and available destinations,” said OAG chief operating officer Mike Benjamin in a written statement.
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Hartsfield-Jackson ranked 2nd for domestic connectivity between Chicago O'Hare, even though Atlanta has more domestic flying.