|Image source and source notes: Counties are based on 2010 geography and historical populations are based on population estimates derived from decennial census data (1790-2010).|
Point locations are based on population-weighted county centroids.
Why are the two measures so different?
The answer is in how you define "coast."
The Census Bureau analysis, pictured above, defines a "coastal county" as any county in which a portion of the county's boundary is adjacent to an ocean. NOAA's analysis includes the Great Lakes in the definition of coastal. Densely populated areas, like Chicago, account for the difference in the two measurements.