Navigating Regional Expressions

Local expressions are so hard to get right. In an article about the long shadows cast by Boston's proposed new buildings, a journalist from the New York Times talks about a couple walking in the Common.

Well. You may be in New York's Central Park or in Boston's Public Garden, but you are on the Boston Common. Like any Bostonian, I get my feathers ruffled by the mistakes New Yorkers make. But if I am honest, there is no reason someone not from New England would know how we inhabit our commons, and where would the journalist have gone to look it up?