I appreciate the notion that "explicit is better than implicit", so as to minimize assumptions and miscommunications. But that said, if the service is providing details for a region using the center-and-radius approach, that sounds perfectly reasonable. It explicitly states what is being returned, and the fact that it isn't a single point. For responsible clients who are using the API correctly, this data-response is the easiest to parse and make sense of.

I would hate to see a world where services are responsible for generating and clients are responsible for parsing data in an overly convoluted and cumbersome format, just to minimize the risk of irresponsible clients misinterpreting it.