If you don't like MSA and CSA, you can use Census-defined Urbanized Areas that measure physical spread. One picks up where the other leaves off. I think in a nation as auto-centric as our own, commuting patterns are definitely important when measuring the influnce of a central urban area.
I believe that MSA/CSA, when combined with UA's, give a pretty comprehensive and accurate picture of the social and physical spread of a city.
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Where the trees are the right height
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