University of Chicago scholars Robert Park and Charles Newcomb used newspaper circulation data for the major daily newspaper in forty-one major cities across the country. They examined the circulation of these major dailies in small towns located between the 41 urban areas to determine where each daily had the strongest influence. They mapped the circulation of the dailies and found many interesting features.
For example, the boundary between the influence of the San Francisco and Los Angeles daily papers was found to be approximately halfway between the two cities. Other instances were not so clean-cut. While southern Montana and northern Wyoming are hundreds of miles from Chicago, it is the Chicago newspaper that is most often received in this area. An island of Chicago newspaper readers is surrounded by areas that subscribe to the Denver and Helena newspapers. Most of northwestern Montana received a daily from Helena while the southern half of Wyoming and almost all of Colorado was served by a Denver newspaper.
Other examples include Minneapolis-St.Paul, which had a large distribution area in the late 1920s that stretched through almost all of Minnesota through North Dakota and into eastern Montana. The researchers excluded papers like the New York Times because their influence extended far beyond the New York City metropolitan area. Therefore, the influence of New York on the surrounding area is limited to most of the state of New York, western Connecticut, and northern New Jersey. At the time of the study, western papers from cities like Salt Lake City extended for thousands of square miles in the vast west. Salt Lake City's daily not only covered all of Utah, it also extended through southeastern Idaho, eastern Nevada, and even northern New Mexico.
This study, from two prominent researchers in the Chicago School (and Park is most famous for his collaboration with Burgess in their model of urban structure), is still often referred to when defining large metropolitan areas. It's quite an interesting concept and study, though one I think would yield much different results today in light of CNN, national newspapers, and the Internet.
Source: Park, R.E. and Charles Newcomb. "Newspaper Circulation and Metropolitan Regions." The Metropolitan Community. R.D. McKenzie. Russell & Russell, 1933.