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Found this super-precise map of US ethnic groups and... low and behold... it looks absolutely realistic (corresponds with surnames stats: British surnames being dominant almost everywhere in US)!!!
Source:
http://censusviewer.com/2013/12/us-ethnicity-map/
UPDATE
The map is indeed based on surnames (not on bogus self-reported ethnicity):
https://ethnicity.censusviewer.com/client
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