Hans Kurath did the initial work on a linguistic atlas of the United States in the 1930's. It turned out to be much more involved than expected, and he only finished the section from the Appalachian Mountains eatward. He recorded not only pronunciation - do you say aunt or ant, pecahn or pecan -but word-frequency: whether people said "cellar" or "basement" or used both, observing a distinction between the two. Whether one says "firefly" or "lightning bug." This is how it looked in 1939.
This is how the whole country looks today, with explanation here, including dialect maps in higher resolution. For a one-man show, Kurath's work was apparently pretty good.
One can see the settlement pattern of Americans from the eastern regions moving pretty much WSW across half the country, until it all fragments around the Mississippi River, becoming less distinct.
My personal favorite is bubbler, the eastern New England word for a water fountain. It is not used in the rest of the country, except one small patch of Wisconsin. I have no idea why. Make up your own theory on that.